volume 7 pavement design and … 2004 design manual for roads and bridges volume 7 pavement design...

August 2004

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUME 7 PAVEMENT DESIGN ANDMAINTENANCE

SECTION 3 PAVEMENTMAINTENANCEASSESSMENT

PART 1

HD 28/04

SKID RESISTANCE

SUMMARY

This Standard describes how the provision ofappropriate levels of skid resistance for trunk roads willbe managed. It details how measurements of skidresistance are to be made and interpreted and iscomplemented by HD 36 (DMRB 7.5.1), which sets outadvice on surfacing material characteristics. This latestrevision has changed requirements for settinginvestigatory levels, for annual SCRIM surveys, fordetermining the characteristic SCRIM coefficient andhas further updates in line with current policy.

INSTRUCTIONS FOR USE

This revised Standard is to be incorporated in theManual.

1. This document supersedes HD 28/94, which isnow withdrawn.

2. Remove existing Contents page for Volume 7 andinsert new Contents page for Volume 7 datedAugust 2004.

3. Remove HD 28/94, which is superseded byHD 28/04, and archive as appropriate

4. Insert HD 28/04 in Volume 7, Section 3, Part 1.

5. Please archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

HD 28/04

Skid Resistance

Summary: This Standard describes how the provision of appropriate levels of skidresistance for trunk roads will be managed. It details how measurements of skidresistance are to be made and interpreted and is complemented by HD 36(DMRB 7.5.1), which sets out advice on surfacing material characteristics. Thislatest revision has changed requirements for setting investigatory levels, forannual SCRIM surveys, for determining the characteristic SCRIM coefficientand has further updates in line with current policy.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND

Volume 7 Section 3Part 1 HD 28/04

August 2004

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments

VOLUME 7 PAVEMENT DESIGN ANDMAINTENANCE

SECTION 3 PAVEMENTMAINTENANCEASSESSMENT

PART 1

HD 28/04

SKID RESISTANCE

Contents

Chapter

1. Introduction

2. Operation

3. Measurement of Skid Resistance

4. Setting the Investigatory Level

5. Site Investigation

6. Prioritisation of Treatment

7. Use of Warning Signs

8. References

9. Enquiries

Annex 1 Background Information on theMeasurement and Interpretation of SkidResistance

Annex 2 SCRIM Survey of Operational Procedures

Annex 3 Processing and Computation ofCharacteristic SCRIM Coefficient

Annex 4 Site Investigation

Annex 5 Assessment of Accident Data

Annex 6 Use of Different Types of Test in AccidentInvestigation


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Chapter 1Introduction

1. INTRODUCTION

General

1.1 The purpose of this document is to describe howthe provision of appropriate levels of skid resistance onin-service UK Trunk Roads, i.e. motorways and all-purpose trunk roads, will be managed. This documentdescribes how measurements of skid resistance are tobe made and interpreted and is complemented byHD 36 (DMRB 7.5.1), which sets out advice onsurfacing material characteristics necessary to deliverthe required skid resistance properties.

1.2 In this document, the term “skid resistance”refers to the frictional properties of the road surfacemeasured using a specified device under standardisedconditions. The term always refers to measurementsmade on wet roads, unless specifically stated otherwise.These measurements are used to characterise theroad surface and assess the need for maintenance,but cannot be related directly to the frictionavailable to a road user making a particularmanoeuvre at a particular time.

1.3 The skid resistance of a wet or damp road surfacecan be substantially lower than the same surface whendry, and is more dependent on the condition of thesurfacing material. The objective of this Standard is tomanage the risk of skidding accidents in wet conditionsso that this risk is broadly equalised across the trunkroad network. This is achieved by providing a level ofskid resistance that is appropriate to the nature of theroad environment at each location on the network. Theappropriate level of skid resistance is determined from anetwork accident analysis plus local judgement of site-specific factors.

1.4 In this Standard, the provision of appropriatelevels of skid resistance is treated primarily as an assetmanagement issue rather than one of road safetyengineering, although the accident risk is assessed inorder to determine an appropriate level of skidresistance for each site. Specifically, this Standard doesnot address the identification of locations or routeswhere road safety engineering measures could bebeneficial to reduce accidents.

1.5 This Standard provides advice and guidance toassist the engineer in determining an appropriate levelof skid resistance for each site. It lays down theprocedure to be used for measuring the skid resistance

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and, for cases where the measured skid resistance is ator below a predetermined level, it provides amethodology to assist the engineer in assessing therequirement and priority for remedial works. Remedialworks will be subject to an economic assessment of thecosts and benefits before proceeding, to promote thebest use of maintenance budgets.

Structure

1.6 Chapter 2 summarises the operation of the skidresistance Standard. Chapters 3 to 7 describe keycomponents of the Standard: the measurement of skidresistance, the process of setting Investigatory Levels,site investigation, the prioritisation of treatments anduse of warning signs. These chapters are supported by anumber of Annexes that give more detailed instructionsor advice.

1.7 Annex 1 provides background informationrelevant to the measurement and interpretation of skidresistance. Annex 2 gives operational details formeasuring skid resistance and Annex 3 gives themethods for processing the raw survey data to derivevalues that characterise the skid resistance.

1.8 Annex 4 and Annex 5 describe methodologies forsite investigation and for assessing accident datarespectively.

1.9 Annex 6 discusses the different test methods usedin accident investigation. Police collision investigatorsuse physical evidence (e.g. marks on the road surface)and skid tests at accident sites to estimate the approachspeed of vehicle(s) involved and its relevance to roadsurface condition. In giving evidence in court, policeofficers have, on occasion, used the results of skid tests,often made in dry conditions, to comment on theadequacy or otherwise of the skid resistance of the roadsurface. This has, on occasion, led to claimants usingpolice evidence to try to demonstrate that a road surfacewas unsatisfactory, even though the purpose of theirtests are quite separate from skid resistance tests toassess the surface condition, as explained in Annex 6.

Implementation

1.10 This Standard shall be used forthwith on allschemes for the construction, improvement and

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Chapter 1Introduction

maintenance of trunk roads including motorways,currently being prepared provided that, in the opinionof the Overseeing Organisation this would not result insignificant additional expense or delay. Designorganisations should confirm its application toparticular schemes with the Overseeing Organisation.

Mutual Recognition

1.11 The construction and maintenance of highwaypavements will normally be carried out under contractsincorporating the Overseeing Organisation’sSpecification for Highway Works (MCHW1). In suchcases products conforming to equivalent standards andspecifications of other member states of the EuropeanUnion and tests undertaken in other member states willbe acceptable in accordance with the terms of the 104and 105 Series of Clauses of that Specification. Anycontract not containing these Clauses must containsuitable clauses of mutual recognition having the sameeffect regarding which advice should be sought.

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Chapter 2Operation

2. OPERATION

2.1 This Chapter summarises the procedures formaking and interpreting skid resistance measurementson UK trunk roads.

2.2 Routine measurements of skid resistanceshall be made and processed to deriveCharacteristic SCRIM Coefficient (CSC) valuesin accordance with Chapter 3, supplemented byspecific instructions issued by the OverseeingOrganisation.

2.3 The CSC is an estimate of the underlying skidresistance once the effect of seasonal variation has beentaken into account. This value will be taken to representthe state of polish of the road surface. These terms areexplained in Annex 1.

2.4 On receipt of processed survey data, theCSC values shall be compared with thepredetermined Investigatory Levels, to identifylengths of road where the skid resistance is at orbelow the Investigatory Level.

2.5 Investigatory Levels represent a limit, abovewhich the skid resistance is assumed to be satisfactorybut at or below which the road is subject to a moredetailed investigation of the skid resistancerequirements. Investigatory Levels are assigned basedon broad features of the road type and geometry (thesite category) plus specific features of the individualsite. Investigatory Levels will be reviewed on a rollingprogramme, to ensure that changes in the network areidentified, local experience is applied and consistencyis achieved. The process for setting Investigatory Levelsand the normal range of Investigatory Levels for eachsite category is described in Chapter 4.

2.6 Wherever the CSC is at or below theassigned Investigatory Level a site investigationshall be carried out, to determine whethertreatment to improve the skid resistance isrequired or whether some other action isrequired.

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2.7 A site investigation shall also be carriedout if, in the normal course of accidentinvestigation processes separate from thisStandard, sites are identified where increasedwet or skidding accident levels have beenobserved.

2.8 The process of site investigation is described inChapter 5. The decision of whether treatment isnecessary is unlikely to be clear-cut, but requiresprofessional engineering judgement taking into accountlocal experience, the nature of the site, the condition ofthe road surfacing and the recent accident history. Ifsuccessive site investigations show that treatment is notwarranted at the current level of skid resistance thenconsideration should be given to lowering theInvestigatory Level.

2.9 The processes of setting Investigatory Levels andundertaking site investigations are complementary,since local knowledge and experience gained throughconducting detailed site investigations can be used torefine the criteria for setting Investigatory Levels forsimilar types of site.

2.10 The process of site investigation will result in anumber of lengths being recommended as needingtreatment to improve the skid resistance. The priorityfor treatment will be established taking into account theobserved accident history, the need for othermaintenance works in the vicinity, the cost and thebudget available for the works. This process isdescribed in Chapter 6.

2.11 Signs warning road users that the roadcould be slippery shall be erected, as describedin Chapter 7.

2.12 The decision-making process leading to theidentification and prioritisation of sites for treatment issummarised in Figure 2.1.

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Chapter 2Operation

Figure 2.1: Procedure for Indentification and Prioritisation of Sites

Categorise s ite s (Table 4.1) and assign initial Investigatory Levels

Carry out site investigation in prioritised order

R eview /revise Investigatory

Level

Define/review local criteria for setting I nvestigatory L evel s

Identify and cost suitable treatment strategy

Prioritise and treat sites, taking account of budget and programme

cons iderations

Yes

No

CSC at or

below Investigatory

Level?

Yes

No

Treatment needed ?

Consider revising Investigatory Level

Erect warning signs if required

No

Network changes or >3

years since Investigatory

Level review?

Yes

Site treated ?

Yes

No

Add to n ext year’s programme

Remove warning signs

Other indication of increased skidding

accident risk

Carry out SCRIM survey(s) and calculate

CSC for site

No further action until next CSC meas urement

No further action until next CSC measurement

No further action until next CSC meas urement

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Chapter 3Measurement of Skid Resistance

D RESISTANCE
3. MEASUREMENT OF SKI
Measurement equipment

3.1 Various types of equipment are available formeasuring skid resistance. In different ways, allmeasure the force developed on a rubber tyre or sliderpassing over a wetted road surface and derive a valuethat is related to the coefficient of friction and the stateof polish of the road surface.

3.2 However, the results from the different devicesare not directly interchangeable. For this reason onlyone device is to be used for monitoring the in-serviceskid resistance of UK trunk roads for the purposes ofthis Standard.

3.3 Measurements for monitoring the in-service skid resistance of UK Trunk Roads, inline with this Standard, shall be made with aSideway-force Coefficient Routine InvestigationMachine (SCRIM).

3.4 SCRIM (see Figure 3.1) was introduced in the1970s to provide a method for routinely measuring theskid resistance of the road network. This machine usesthe sideway force principle to measure skid resistance.A freely rotating wheel fitted with a smooth rubber tyre,mounted mid-machine in line with the nearside wheeltrack and angled at 20° to the direction of travel of thevehicle, is applied to the road surface under a knownvertical load. A controlled flow of water wets the roadsurface immediately in front of the test wheel so that,when the vehicle moves forward, the test wheel slidesin the forward direction along the surface.

3.5 The force generated by the resistance to sliding isrelated to the wet road skid resistance of the roadsurface. Measurement of this sideways componentallows the Sideway-Force Coefficient (SFC) to becalculated. SFC is the sideway force divided by thevertical load.

3.6 Depending on the machine, the vertical load iseither assumed to be constant or it is measureddynamically. It has been shown that more reliableresults are obtained when calculating the SFC using themeasured vertical load than when a fixed constant loadis assumed.

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Figure 3.1 SCRIM, test wheel and principle

3.7 Equipment with a vertical loadmeasurement capability shall be used ifspecified by the Overseeing Organisation.

3.8 Measurements are recorded as SCRIM Readings(SR). A SCRIM Reading is the average SFC multipliedby 100 for a pre-determined length of road, normally10m, recorded as an integer.

3.9 Each SCRIM used on Trunk Roads shallhave been accepted by the OverseeingOrganisation. Acceptance will be based uponsatisfactory performance at group correlationexercises arranged through the OverseeingOrganisation.

3.10 A national correlation exercise will usually beheld annually before the start of the testing season, withadditional trials as necessary. Machines that do notperform satisfactorily at the main exercise will berequired to attend and achieve acceptance at anadditional exercise(s). Machines that are unable toattend the main trial will also be required to attend asupplementary correlation exercise.

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Chapter 3Measurement of Skid Resistance

Use of SCRIM

3.11 Skid resistance is not a constant, but isinfluenced by various factors, including temperature,test speed and weather conditions, plus longer-termeffects such as seasonal variation and changes in trafficflow. Further information is given in Annex 2. For thepurpose of this Standard, SCRIM measurements will bemade under standardised conditions to control theseeffects as far as possible, including:

• limiting the testing season to a specific time ofyear;

• specifying a standard test speed;

• specifying the test line to be followed;

• specifying the ambient conditions under whichacceptable measurements may be made.

Further details are given in Annex 2.

3.12 SCRIM operators providingmeasurements under this Standard mustdevelop appropriate procedures to ensure thatmeasurements are carried out safely and to astandard of quality agreed with the OverseeingOrganisation. This must include adhering to theprocedures given in British Standard BS7941-1(1999) for making skid resistance measurementswith a SCRIM and for calibrating and makingregular checks on the equipment, and thefurther instructions given in this Chapter and inAnnex 2.

Survey Strategy

3.13 The surveying strategy is planned so that theeffects of seasonal variation, both within a singleseason and/or between successive years, can be takeninto account in the determination of the CSC for anyparticular length of road.

3.14 Because of the different nature of traffic patternsand road networks in the different UK regions, the wayin which surveys are planned and seasonal variation isaccounted for may be different for the individualOverseeing Organisations. The alternative monitoringstrategies to be followed are explained as part ofAnnex 3.

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3.15 The whole trunk road network will normally betested once during each testing season. However, therate at which skid resistance changes may be slow onsome roads, particularly where traffic volumes are low.In such circumstances it may not be necessary tomonitor annually.

3.16 The Overseeing Organisation will specifythe network to be surveyed, the test lane, thesurvey strategy and the method and/or theaccuracy of location referencing required.

Processing of survey data

3.17 After collection, survey data will be validatedand subject to processing to determine the CSC valuesthat will be used for further analysis. Validation andprocessing will be carried out as specified by theOverseeing Organisation.

3.18 Typically, processing will include:

• application of correction factors, e.g. incircumstances where it was not possible tomaintain the specified standard test speed;

• multiplication by the Index of SFC applicable tothe SCRIM at the time it was making themeasurement;

• calculation of the CSC;

• aggregation of raw data to longer averaginglengths, typically 100m or 50m, for furtheranalysis (see paragraph 4.8).

3.19 Further details of the processing are set out inAnnex 3.

3.20 On receipt of processed survey data, theOverseeing Organisation (through itsmaintaining organisation) shall check that thewhole of the specified network has beensurveyed. Appropriate action must be taken toensure that, as far as possible, valid data isobtained in the following planned survey forlocations where there is missing or invalid datain the current survey.

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Chapter 4Setting the Investigatory Level

ATORY LEVEL
4. SETTING THE INVESTIG
Objective

4.1 The objective of setting the Investigatory Level isto assess the nature of the site and assign an appropriatelevel of skid resistance, at or below which a moredetailed site investigation must be undertaken. It isimportant that the Investigatory Level is not set too low.If, during the site investigation, the Investigatory Levelis found to be too high then it can be lowered. However,if the Investigatory Level is initially set at too low alevel then the need to improve the skid resistance maynot be detected until it has already fallen further than isdesirable.

4.2 The Overseeing Organisation (through itsmaintaining organisation) shall assign a SiteCategory and Investigatory Level to each partof the network, so that the Investigatory Levelcan be compared with the CSC. Thisinformation must be recorded in a formatagreed with the Overseeing Organisation,together with the date of assessment.

Procedure

4.3 Site categories and associated InvestigatoryLevels are defined in Table 4.1. These categories andranges have been developed for trunk roads and maynot be applicable to local authority roads, which aremore diverse in nature.

4.4 The site category most appropriate to the layoutof the site will be selected from the list in Table 4.1. Ifmore than one site category is appropriate then the sitecategory with the higher Investigatory Level will beselected or where the highest Investigatory Levels arethe same then the category highest up the Table will beselected.

4.5 Slip roads will be allocated to categories B, Q, Gor S as appropriate to their length and layout.

4.6 A single site category can vary in length from afew tens of metres to several kilometres, depending onthe nature of the site. The length of site categories Qand K will normally be the 50m approach to the feature,but this shall be extended where justified by site

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August 2004

haracteristics, e.g. if queuing traffic creates andditional risk, to allow for traffic joining the back ofhe queue.

.7 After selecting a site category, the appropriatenvestigatory Level is assigned from the range availableor that site category, following the process described inaragraphs 4.10 to 4.13. The range of Investigatoryevels for each site category has been developed as a

esult of UK research studies on trunk roads andeflects the variation in accident risk within a siteategory. Further details are given in Annex 1. Thelexibility to set different Investigatory Levels forifferent sites within the same category allows for siteshere the risk of skidding accidents is potentiallyigher to have a higher Investigatory Level and possiblye treated to maintain a higher level of skid resistance.

.8 Investigatory Levels are applied to the mean CSCithin 100m averaging lengths (50m lengths for someverseeing Organisations), except that the Investigatoryevels for site category R are based on 10m averaging

engths. Shorter lengths will be necessary where the siteategory is less than 100m long or at the end of a siteategory longer than 100m. Residual lengths less than0% of a complete averaging length may be appendedo the penultimate length, providing the site category ishe same.

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Investigatory Level at 50km/h

Site category and definition0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65

A Motorway

B Dual carriageway non-event

C Single carriageway non-event

Q Approaches to and across minor and majorjunctions, approaches to roundabouts

K Approaches to pedestrian crossings and otherhigh risk situations

R Roundabout

G1 Gradient 5-10% longer than 50m

G2 Gradient >10% longer than 50m

S1 Bend radius <500m – dual carriageway

S2 Bend radius <500m – single carriageway

Notes:

1. Investigatory Levels are for the mean skidding resistance within the appropriate averaging length

2. Investigatory Levels for site categories A, B, and C are based on 100m averaging lengths (50m lengthsfor some Overseeing Organisations) or the length of the feature if it is shorter.

3. Investigatory Levels and averaging lengths for site categories Q, K, G and S are based on the 50mapproach to the feature but this shall be extended when justified by local site characteristics.

4. Investigatory Levels for site category R are based on 10m lengths.

5. Residual lengths less than 50% of a complete averaging length may be attached to the penultimate fullaveraging length, providing the site category is the same.

6. As part of site investigation, individual values within each averaging length should be examined and thesignificance of any values which are substantially lower than the mean value assessed.

Table 4.1 Site categories and Investigatory Levels

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4.9 Dark shading in Table 4.1 indicates the range ofInvestigatory Levels that will generally be used fortrunk roads carrying significant traffic levels. Lightshading indicates a lower Investigatory Level that willbe appropriate in low risk situations, such as low trafficlevels or where the risks present are well mitigated anda low incidence of accidents has been observed.Exceptionally, a higher or lower Investigatory Levelmay be assigned if justified by the observed accidentrecord and local risk assessment.

4.10 On first implementation of this Standard, thelowest Investigatory Level with dark shading in eachsite category, or the middle Investigatory Level forcategory Q, may be assigned. On a rolling programme(see paragraph 4.17), this initial assignment shall besuperseded by the more detailed risk assessmentdescribed below.

4.11 The accident risk for each individual site must beevaluated in comparison with other sites in the samesite category, in order to set an appropriateInvestigatory Level. Criteria for making this assessmentwill be developed at a local level, taking into accountthe features of the local network. Guidelines of thefactors to consider in developing these criteria are givenbelow.

4.12 Roads within the site category with noexceptional risk of skidding accidents will be assignedthe lowest Investigatory Level. It is envisaged that thiswill apply to the majority of sites. The following factorscould influence the choice of a higher InvestigatoryLevel. However, a higher Investigatory Level will notbe appropriate if the overall level of risk is low(e.g. because of low traffic flow or low traffic speed) orif the risk has been mitigated by other means(e.g. through clear signing of a hazard or reduced speedlimit.)

• Notable potential for conflict between road usersat the site, particularly where the outcome islikely to have severe consequences, e.g. ahead-on or side impact at speed.

• Road geometry departing substantially fromcurrent Standards.

• Known incidence of queuing where the trafficspeed is otherwise high.

• Accesses onto the main carriageway, if they arebusy (e.g. for a service station) or have pooradvance visibility, or if the speed of leaving orjoining traffic creates conflict with other trafficon the main carriageway.

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• Two or more events in proximity, e.g. a junctionon a downhill gradient.

• Low texture depth (less than 0.8mm measured asSensor Measured Texture Depth (SMTD)),except for High Friction Surfacing materials.

• Bends where the traffic speed and/or geometry isjudged to give rise to added risk. This couldapply to some sites in non-event categories aswell as sites in the bend categories.

• Sharp left-hand bends, such as on the approach toor exit from a roundabout, where greaterpolishing action by traffic could lead to lowerskid resistance occurring in the right-hand wheelpath than is measured in the left-hand wheel path.

• For non-event categories B and C: approach toslip road leaving main carriageway or mergingarea downstream of slip road joining maincarriageway, except if low traffic flow means thatthe slip road gives rise to little added conflictbetween road users, compared with the mainline.

• For approaches to minor junctions: poor advancevisibility; high risk of head-on collisions; highapproach speed, except if this is mitigated(e.g. by adequate taper length for traffic leavingand joining the main carriageway and/or aseparate lane for right turning traffic).

• For pedestrian crossings: poor advance visibility;high approach speed.

• For roundabouts: high speed of circulatingtraffic; high incidence of cyclists ormotorcyclists; absence of signalised control onroundabouts at grade separated interchanges.

• For all site categories on all-purpose roads,significant numbers of pedestrians crossing at agiven location.

• Known history of accident occurrence beingmore frequent than normal, particularly in wetconditions or where skidding is reported.

4.13 The Overseeing Organisation (through itsmaintaining organisation) shall develop and documentcriteria for assigning Investigatory Levels within eachsite category. This could consist of local examples ofsites assigned different Investigatory Levels. Thesecriteria will be refined on an on-going basis to takeaccount of local experience of detailed site

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investigations.

4.14 The network evolves both through substantialchanges such as the addition or improvement ofjunctions, or the addition of traffic calming measuresand through more subtle changes such as increasingtraffic levels or change of land use. The criteria bywhich Investigatory Levels are assigned may alsoevolve as experience of local conditions is gained, e.g.where treatment to improve skid resistance in somelocations is found to be more effective in reducingaccident rates than in others.

4.15 The Investigatory Levels throughout the networkthat is surveyed for skid resistance must be reviewed ona rolling programme, so that:

• changes in the network are identified and takeninto account;

• local experience is applied;

• consistency is maintained.

4.16 A review of the Investigatory Level shallbe carried out when a significant change to thenetwork is made.

4.17 Notwithstanding the above, a procedureshall be put in place to ensure that theInvestigatory Levels are reviewed at least everythree years unless agreed otherwise with theOverseeing Organisation.

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Chapter 5Site Investigation

5. SITE INVESTIGATION

Objectives and outcomes

5.1 Sites where the CSC is at or below theInvestigatory Level require a detailed investigation. Theobjective is to determine whether a surface treatment isjustified to reduce the risk of accidents, specificallyaccidents in wet conditions or involving skidding,whether some other form of action is required, orwhether the site should be kept under review. Thisinvestigation is an important part of the operation of theskid resistance Standard. In conjunction with theprocess of setting Investigatory Levels, the objective isto promote effective targeting of treatments.

5.2 Treatment will normally be a surface treatment toimprove the skid resistance. However, if the siteinvestigation identifies any characteristic of the site orroad user behaviour that suggests other road safetyengineering measures could be appropriate, then theappropriate specialist dealing with safety schemes mustbe consulted before deciding upon the best course ofaction. If it is found that there is a need for other typesof routine maintenance, for example re-application ofroad markings or additional road sweeping, then thismust also be addressed.

5.3 Some form of treatment will be justified if:

• based on an accident analysis, the number ofaccidents observed is higher than average for thetype of site being considered;

• based on an accident analysis, the site has ahigher than average proportion of accidents inwet conditions or involving skidding for the typeof site being considered;

• the nature of the individual site and the demandsof road users mean that a higher accident risk(compared with other sites in the same category)might be expected with the skid resistance at itscurrent value or if it were to fall further beforethe next measurement. In this case, preventivetreatment is justified to pre-empt a potentialincrease in accident risk.

5.4 If none of the above are true then there iscurrently no justification for treatment to increase theskid resistance. If the site remains below theInvestigatory Level at the next measurement, then it

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will automatically be subject to a further investigation.That is, sites with skid resistance remaining below theInvestigatory Level are automatically kept underreview.

5.5 Further details of making these assessments aregiven in Annex 4 on site investigation and Annex 5 onassessment of accident data.

5.6 If the skid resistance and accident pattern remainstable for an extended period, for example, more than 3years, then lowering the Investigatory Level should beconsidered. However, it is important that stability isobserved before reducing the Investigatory Level,because, unless the skid resistance falls further, regularinvestigation that would detect an increase in accidentswould no longer be prompted by the skid resistanceStandard.

Procedure

5.7 Sites requiring investigation shall beidentified as soon as practicable on receipt ofthe CSC values.

5.8 CSC values should be used in preference tosingle run survey values that have not been adjusted forseasonal effects, as the latter are less reliable. However,if single run survey data indicate that the skid resistanceis significantly below the Investigatory Level (e.g. atleast 0.05 units SCRIM coefficient below) then it islikely that the CSC will also be found to be below theInvestigatory Level. Single run data can therefore beused to give early warning of sites requiringinvestigation, but any decision on the need fortreatment should normally be based on the CSC.

5.9 Site investigations shall be carried out ina prioritised order, by personnel experienced inpavement engineering.

5.10 Site investigations will be prioritised initially onthe basis of the amount by which the skid resistance isbelow the Investigatory Level. This order may berefined to take into account the efficiency of conducting

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Chapter 5Site Investigation

investigations and as a result of other informationgathered during the early part of the investigation, suchas the recent accident history.

5.11 Persons with relevant local experience must beidentified, and should be consulted if appropriate,during the site investigation process. These will includethe person locally responsible for accident investigationand prevention.

5.12 The results of the investigation, includingwhether further action is required, shall bedocumented and retained together with theidentity of the assessor and other partiesconsulted.

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EATMENT

Chapter 6Prioritisation of Treatment

6. PRIORITISATION OF TR

6.1 Site investigation results in the identification oflengths of pavement where treatment is recommendedto improve the skid resistance. This chapter addressesthe prioritisation of these treatments on the assumptionthat budget resources are limited. If other actions areidentified as a result of the site investigation then thesemust be prioritised as appropriate.

6.2 HD 36, HD 37 and HD 38 (DMRB 7.5.1-3) giveadvice about the choice of surfacing materials toprovide the appropriate level of skid resistance andabout the use of re-texturing treatments to provideshort-term improvements to skid resistance and/ortexture depth. Other aspects of pavement conditionmust also be taken into account in selecting the mostappropriate form of treatment.

6.3 The most appropriate form of treatmentshall be identified for each treatment lengthtaking account of current advice.

6.4 Priority must be given to completing treatmentswhere the skid resistance is substantially below theInvestigatory Level (e.g. at least 0.05 units CSC below),or low skid resistance is combined with a low texturedepth, or the accident history shows there to be aclearly increased risk of wet or skidding accidents. Inother cases, treatment will be programmed as a longer-term measure taking into consideration othermaintenance requirements.

6.5 A comparison of the estimated accident savingand the cost-effectiveness of treatments can be used toassist in establishing the relative priority of treatmentsat different locations. The Overseeing Organisation willprovide information that can be used to make thisassessment. Other estimates of accident savings may beused if backed up by local experience or otherinformation.

6.6 The cost effectiveness of treatments at differentlocations can be calculated by dividing the estimatedaccident saving by the anticipated cost of treatment.

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6.7 The priority for treatment should be establishedfor all new treatment lengths and for those lengthspreviously recommended for treatment to improve theskid resistance, but where treatment has not yet beencarried out or definitely programmed. If more than ayear has elapsed since the site investigation was carriedout then the accident history and priority for treatmentmust be re-examined using the most recent dataavailable. This programme should be reviewed andprogress recorded at appropriate intervals.

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7. USE OF WARNING SIGNS

7.1 Warning signs shall be erected at siteswhere the need for treatment to improve skidresistance has been identified following a siteinvestigation. In Scotland, sites identified in thisway shall be referred on an individual basis tothe Overseeing Organisation for a decision onthe provision of warning signs.

7.2 This strategy provides a targeted use of signs andis designed to avoid a proliferation of signs that wouldundermine their effectiveness and would not make bestuse of resources.

7.3 Since warning signs are erected (if required) aftera site investigation, it is particularly important tocomplete site investigations in a prioritised order andwithin a reasonable time period, so that warning signscan be placed where they are needed without unduedelay.

7.4 The slippery roads warning sign(Diagram 557) is to be used in accordance withthe instructions contained in The Traffic SignsRegulations and General Directions (2002).

7.5 A visual inspection of the site shall bemade after the signs are erected to confirm thatthey have been erected and correctly placed anda record of this observation shall be made andretained.

7.6 Depending on the nature of the surface treatment,it may be necessary to leave slippery road warningsigns in place for a period after a new surface is openedto traffic. Additionally, at sites where the risk ofskidding may temporarily increase after surfacetreatment, it may be necessary to erect warning slipperyroad signs if not already in place.

7.7 Warning signs shall be removed as soonas they are no longer required.

Chapter 7Use of Warning Signs

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Chapter 8References

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8. REFERENCES

1976

LR738; Hosking JR and Woodford GC, “Measurementof Skidding Resistance Part ii: Factors Affecting theSlipperiness of a Road Surface”; TRRL.

1999

BS7941; Part1; “Methods for Measuring the SkidResistance of Pavement Surfaces. Side-ways ForceCoefficient Routine Investigation Machine”, BSI.

DMRB

HD 29 (DMRB 7.3.2) Structural Assessment Methods.

HD 36 (DMRB 7.5.1) Surfacing Materials for New andMaintenance Construction.

HD 37 (DMRB 7.5.2) Bituminous Surfacing Materialsand Techniques.

HD 38 (DMRB 7.5.3) Concrete Surfacing andMaterials.


August 2004 9/1

9. ENQUIRIES

All technical enquiries or comments on this Standard should be sent in writing as appropriate to:

Chief Highway EngineerThe Highways Agency123 Buckingham Palace RoadLondon G CLARKESW1W 9HA Chief Highway Engineer

Chief Road EngineerScottish ExecutiveVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Directorate

Director of EngineeringThe Department for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 9Enquiries


FORMATION ON THERPRETATION OF SKID

Annex 1Background Information on the Measurement and Interpretation of Skid Resistance

ANNEX 1: BACKGROUND INMEASUREMENT AND INTERESISTANCEGeneral

A1.1 When a vehicle travels over a road, each part ofthe tyre in contact with the road surface is momentarilyat rest. The frictional forces generated at thesestationary contact areas between the tyre and the roadsurface can allow vehicles to be manoeuvred. However,a vehicle will start to skid whenever the availablefriction between the road surface and the tyre isinsufficient to meet the demands of the driver inwhatever manoeuvre (including braking) they areattempting to make.

A1.2 The friction available to a driver attempting aparticular manoeuvre depends on many differentfactors. The influence of road surface characteristics isdescribed below. Other factors include the vehicle’styres and braking system, the dynamic interaction of thevehicle suspension with the road geometry andenvironmental factors, such as the temperature and thepresence of water or other contaminants. The objectiveof measurements carried out under the operation of thisStandard is to characterise the influence of the roadsurface skid resistance and hence define the skidresistance available to road users.

Road Surface Properties

A1.3 The contribution of the road surface to theoverall friction is known as skid resistance. In practice,it is found that the skid resistance measured on dry, in-service road surfaces is generally high, but that lowerand more variable measurements are obtained when thesame road surfaces are wet or damp. For this reason,measurements of skid resistance for the purpose ofroutine condition monitoring are made on wetted roadsurfaces.

A1.4 The level of (wet road) skid resistance isdependent on two key properties of the surface, themicrotexture and the texture depth. The fine scalemicrotexture, on the surface of aggregate particles andprovided by the fines in the mixture, is the maincontributor to skid resistance at low speeds and themain property measured in wet skid resistance tests.Greater texture depth generates friction by physically

August 2004

deforming the tyre surface and also provides rapiddrainage routes between the tyre and road surface.

A1.5 The effects of microtexture and texture depthcombine to influence the skid resistance at higherspeeds. The standard SCRIM measurement is carriedout at a slip speed less than 20km/h, much lower thanthe slip speed in locked-wheel braking from normaltraffic speed. The typical reduction of skid resistancefrom the 20km/h value at higher speeds, and theinfluence of texture depth, is illustrated in Table A1.1.The effect of texture depth becomes apparent at speedsas low as 50 km/h, but is increasingly significant athigher speeds.

Speed Texture depth (mm SMTD)

Below 0.5 0.5 – 0.8 Above 0.8

50 km/h 40% 30% 25%

120 km/h 70% 60% 50%

Table A1.1 Typical Reduction in Skid ResistanceCompared with 20km/h Value

Effect of Traffic

A1.6 Under the action of traffic, the microtexturebecomes “polished”, leading to a reduction in skidresistance. HD 36 (DMRB 7.5.1) requires thecomponents of the surfacing mixture to satisfy certaincriteria in relation to their resistance to polishing, sothat surfacing materials generally provide adequate skidresistance during their service lifetimes.

A1.7 In combination with the specification ofsurfacing materials, the skid resistance of roads ismonitored to identify areas where the microtexture hasbeen lost as the surface has been polished by traffic andtreatment might, therefore, be needed to improve theskid resistance. This is necessary because theperformance in service cannot be predicted preciselyfrom the properties of the surfacing components andtraffic levels, and the effects of manoeuvring vehicles at

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the location might be greater than was anticipated at thetime the surfacing was designed.

A1.8 Similarly, the texture depth of road surfacingscan reduce with time under the combined influences oftraffic flow, temperature and the nature of the surfaceand is also monitored. The measurement of texturedepth is described in HD 29 (DMRB 7.3.2).

Early Life Skid Resistance of Asphalt Surfacings

A1.9 Asphalt surfacings exhibit different skidresistance properties in the initial period after laying,compared with the same surfacings that have beenexposed to traffic for a period of time. Thisphenomenon is not within the scope of thisdocument, which is concerned with in-servicecondition rather than the properties in the initialperiod.

A1.10 Sections of road that exhibit these different skidresistance properties during this the initial period, mustbe identified, so that they can be excluded from certaintypes of analyses, as described in Chapter 3 andAnnex 4 of this Standard. The duration of this initialphase will depend on local conditions but, for thepurpose of interpreting skid resistance measurements, itis assumed that the surface has reached an equilibriumstate one year after opening to traffic on trunk roads inthe UK.

Seasonal Variation of Skid Resistance

A1.11 After the initial period of wearing in, roadsurfaces reach an equilibrium state of polishing. Forroads where the traffic level is constant, the skidresistance will then fluctuate through seasonalweathering and polishing cycles but will remain atabout a constant level for many years. If the traffic levelsubsequently increases or decreases, the position of theequilibrium will shift so that a lower or higher overalllevel of skid resistance is observed, but with the sameseasonal fluctuation superimposed.

A1.12 An example of long-term variation in skidresistance is shown in Figure A1.1. A suggestedexplanation for the annual variation is that in the winter(October to March) when the roads are wet for much ofthe time, the detritus is mainly gritty so that the roadsurface becomes harsh and the skid resistance rises. Thelowest skid resistance is generally observed in thesummer period, when the roads are wet for a relativelyshort time, the detritus on them is mainly dusty so thatthe road surface becomes polished and the skid

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esistance falls. In practice, the minimum skidesistance varies from year to year and occurs duringifferent periods depending on the prevailing weatheronditions.

Figure A1.1 Example of Long-Term Variation inSkid Resistance (from LR 738)

1.13 Because the skid resistance varies continuously,arious strategies have been developed to provide aeasurement that characterises the state of polish of theicrotexture. Survey strategy and processing

rocedures are designed to reduce the effect of theariation within a year and/or between successive years,o the sites with low skid resistance can be identifiedore accurately. Typically, measurements are made

uring the summer period, when the lowest measuredalues are observed.

1.14 The survey and analysis methods to be used forhe purposes of this Standard are described inhapter 3, Annex 2 and Annex 3 of this Standard.

tandardised Measurements

1.15 To characterise the condition of theicrotexture, measurements of skid resistance are made

nder standardised conditions that restrict the influencef other factors on the measurement as far as possible.or example, measurements are made at a specifiedpeed, using a specified tyre and a controlled amount ofater. Corrections may be applied to the measuredalue where the specified standard conditions were notchieved, e.g. where it was not safe to maintain thepecified speed. Further details of test procedures areiven in Chapter 3.

1.16 The measurements made and interpretedccording to this Standard provide a guide to theeneral condition of the road to assist inaintenance planning. Because they indicate the

eneral level of skid resistance under standardised

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conditions, the values do not relate directly tospecific accident situations, where other factors suchas the tyre condition, vehicle speed and manoeuvreattempted all influence the level of frictiongenerated at that time.

A1.17 In contrast, skid tests carried out by the Policefor the purpose of reconstructing the situation leadingto an accident are intended to recreate the specificconditions of the accident. The results of these differenttypes of test cannot be compared precisely. Furtherdetails of Police skid tests are given in Annex 6.

Relationship to Accident Risk

A1.18 Within normal ranges, low skid resistance doesnot cause accidents on its own although, depending onthe particular circumstances, it may be a significantcontributory factor. The level of skid resistance, evenon a polished surface, will generally be adequate toachieve normal acceleration, deceleration and corneringmanoeuvres on sound surfaces that are wet but freefrom other contamination. However, higher skidresistance can allow manoeuvres that demand higherfriction to be completed, e.g. to stop quickly or cornersharply. Higher skid resistance can therefore reduceaccidents in cases where drivers need to complete amore demanding manoeuvre in order to avoid anaccident. A key part of this Standard is the judgement oflocations where this is more likely to occur, so that theprovision of higher levels of skid resistance can betargeted at these locations.

A1.19 Accident analyses have shown that there arerelationships between measured skid resistance andaccident risk. These relationships are not precise, in thatdifferences in skid resistance may account for only arelatively small part of the difference in accident riskbetween individual sites because of all the other factorsinvolved. Nevertheless, they have allowed generalobservations to be drawn that make it possible toprovide guidance for managing the provision of skidresistance on the network.

A1.20 The influence of skid resistance on accident riskis markedly different for roads with differentcharacteristics. For this reason, site categories havebeen defined to group roads with similar characteristics.

A1.21 For some site categories, no statisticallysignificant relationship, or only a weak relationship, isobserved between skid resistance and accident risk. Agood example of this is motorways, where the roaddesign has effectively reduced the potential for conflict

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etween road users. Although the skid resistance is stillmportant, because of the need to provide uniform roadharacteristics, the level of skid resistance can be lowerhan other categories.

1.22 For other site categories, progressively moreccidents are observed, on average, as the skidesistance falls. For these categories, there are benefitsn maintaining a higher level of skid resistance toontribute to reducing the number of accidents at theseites.

1.23 However, not all sites within a single categoryre equivalent in terms of their accident risk. Figure1.2 illustrates the range in accident risk present for

ndividual sites within a single site category. This ranges not surprising when the range of characteristicsresent within a single nominal site category isonsidered, e.g. in road design and traffic flow. Ithould also be noted that there is no boundary at whichhe skid resistance passes from being “safe” to beingdangerous”.

1.24 Judgement of the relative accident risk andppropriate level of skid resistance for different sitesithin the same category forms a key part of the

ffective operation of this Standard.

Figure A1.2 Accident Risk and Skid Resistance -Variation Within Site Category

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F OPERATIONAL

Annex 2SCRIM Survey of Operational Procedures

ANNEX 2: SCRIM SURVEY OPROCEDURES

A2.1 This Annex lists the standard testing proceduresthat are required to limit the variability of skidresistance measurements resulting from factors otherthan the road surface condition, e.g., the test speed.

Testing Season

A2.2 For standardised tests, measurementsshall be made during the testing season, definedas the summer period 1 May – 30 September.

A2.3 In exceptional circumstances the testing seasonmay be extended but only with the prior agreement ofthe Overseeing Organisation.

Testing Speed

A2.4 On Motorways and Dual Carriageway AllPurpose Trunk Roads where the posted speedlimit is greater than 50mph, the target testvehicle speed is 80km/h. On all other roads, orwhere a SCRIM is being used without dynamicvertical load measurement, the target testvehicle speed is 50km/h.

A2.5 The SCRIM driver shall maintain avehicle speed as close to the target test speed aspossible. This is achievable for most parts of thenetwork.

A2.6 If it is not safe or practical to maintain the targetspeed then, in exceptional circumstances, a differentspeed may be used at the discretion of the SCRIMdriver. The safety of the SCRIM and other roadusers has priority at all times.

A2.7 The Investigatory Levels for the CSC valuesdefined in Chapter 4 have been set in terms of the50km/h standard testing speed. The method forapplying speed corrections is given in Annex 3 of thisStandard.

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Testing Lane and Line

A2.8 For most roads, the leftmost lane will be tested inboth directions of travel. This lane usually carries themost heavy traffic and can, therefore, be expected toshow the lowest skid resistance. In areas where this isnot the case (for example, approaching points whereroutes diverge and a greater proportion of heavyvehicles uses the offside lane) then a different lane, ormore than one lane will be tested.

A2.9 The test lane shall be as specified by theOverseeing Organisation.

A2.10 Measurements shall be carried out withthe test wheel in the nearside (left) wheel pathof the running lane unless an alternative testline has been agreed with the OverseeingOrganisation.

A2.11 If it is necessary for the SCRIM todeviate from the test line (e.g. to avoid aphysical obstruction or surface contamination)the data shall be marked as invalid andeliminated from the standard analysisprocedure.

A2.12 On urban roads where roadside parking inunmarked positions is commonplace, there may be twopairs of wheel paths – one followed at times of daywhen parked cars are mostly absent and another whenthey are largely present. In such circumstances the linethat normally carries the most commercial vehicletraffic must be followed.

A2.13 In situations where the test line is prone tophysical obstruction, for example by parked cars, thenan alternative test line must be agreed with theOverseeing Organisation to avoid recording invalid datafor the same length of road in successive years. Testingthe offside wheel path might be an appropriatealternative in these circumstances, with the aim ofachieving a consistent test path, year on year.

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Annex 2SCRIM Survey of Operational Procedures

Testing on Bends

A2.14 There are no special requirements for testing onbends. At locations where a sharp bend is combinedwith traffic braking or accelerating, the wheel path onthe outside of the bend can become more polished thanthe inside wheel path. This is taken into account insetting the Investigatory Level (See Chapter 4) andduring the site investigation (See Paragraph A4.15).

Testing on Roundabouts

A2.15 Roundabouts can present practical problemsregarding potential traffic conflicts and testing speed.They range from small, mini-roundabouts to largegrade-separated interchanges. Larger roundabouts mayhave free-flowing traffic or traffic light controls atcertain times of day.

A2.16 Mini-roundabouts or small island roundaboutsshould be treated as part of the main carriageway testline and do not need to be tested separately. (Thisapplies to the testing procedure – the roundaboutsection may be assigned to a different Site Category tothat of the main line.)

A2.17 On most roundabouts or interchanges, a lineequivalent to the outermost lane should be tested.

A2.18 On roundabouts with lane markings for specificroutes, a representative line should be chosen broadlyfollowing the most polished path. Usually this will bethat followed by most heavy vehicles, which, becauseof their size, may not be able to keep to the markedlanes.

A2.19 The test line(s) to be followed atroundabouts shall be as agreed by the SCRIMoperator and the Overseeing Organisation.They will take into account the need forconsistency in representative measurements insuccessive surveys and possible variations inSite Category through the intersection.

A2.20 On some smaller roundabouts where the distancebetween the arms is short, it may be appropriate torecord data at a shorter interval than the standard 10mlength. Changes to the recording interval will bespecified by the Overseeing Organisation.

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mbient Conditions During Testing

2.21 The ambient conditions can have an effect both the skid resistance of the road and on theeasurements. The SCRIM operator shall record theeather conditions at the time of the survey as required BS7941-1 (1999).

2.22 Testing in extremely strong side winds must beoided because these can affect the measurements byeating turbulence under the vehicle that causes theater jet to be diverted from the correct line.

2.23 Testing must be avoided in heavy rainfall orhere there is standing water on the road surface.xcess water on the surface can affect the drag forces ate tyre/road interface and influence the measurements.

A2.24 Measurements shall not be undertakenwhere the air temperature is below 5°C.

2.25 The maintaining organisation shall maintain acord of general conditions throughout the testingason. Both the SCRIM operator and the maintainingganisation shall endeavour to record any roadnditions that could affect the results.

2.26 Contamination of the road surface by mud, oil,it, or other contaminants is to be noted and thefected measurements identified (in the same way asr out-of-line testing) so that results are eliminatedom the standard analysis procedure. If thentamination is severe, emergency action may bequired to remove the contamination. In this case theoblem must be reported without delay to theverseeing Organisation, together with the relevantsults if possible. The Overseeing Organisation willen notify the managing organisation for appropriatetion to be carried out.

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COMPUTATION OFOEFFICIENT

Annex 3Processing and Computation of Characteristic SCRIM Coefficient

ANNEX 3: PROCESSING ANDCHARACTERISTIC SCRIM C

A3.1 This Chapter lists the processing options thatmay be required by the Overseeing Organisation.Options that are required must be applied in the orderlisted below.

A3.2 On completion of the survey, thecorrected SCRIM Coefficient (SC) shall bedetermined for each 10m section for which avalid SCRIM Reading is available. Thecorrected SC shall be used to determine theCSC using one of the methods listed below.

Speed Correction

A3.3 The test speed has a significant effect on themeasurements of skid resistance.

A3.4 For the purpose of this Standard,measurements collected within the speed range25 to 85 km/h will be corrected to a speed of50km/h, using the following equation:

SC(50) = SC(s) + (s*2.18*10-3 - 0.109)

Where:

SC(50) is the SC corrected to 50km/h

SC(s) is the SC measured at the test speed, s.

Temperature Correction

A3.5 The temperature of the air or road can have asmall effect on the tyre rubber and the measurementsmade. It has been found that under normal UKconditions, the influence of temperature is not ofpractical significance in comparison with other factorsaffecting the measurements. Temperature correction isnot necessary for surveys carried out under theconditions set out in this Standard.

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ex of SFC

.6 The Index of SFC was originally introduced as ator to relate the values given by SCRIM to the SFCtained from the equipment at TRL during the period63-1972 used to derive information on which to baseposals for specification. From this revision of thendard, it serves as a general correction factor that

ll allow the Overseeing Organisation to maintain the at a consistent general level as future developments made to the equipment or monitoring techniques.

.7 The Index of SFC is defined by the Overseeingganisation. The value currently in force is 78 per cent78) and is applicable to all SCRIMs in current uset it may be amended in future, either for the wholeet or for individual SCRIMs.

A3.8 The SC value shall be multiplied by theIndex of SFC currently in force.

lculation of Characteristic SCRIM Coefficientlues

.9 As noted in Annex 1, the skid resistance of roadfaces can fluctuate within a year and betweencessive years, while maintaining a similar generalel over a long period of time. The basis of thisndard is that skid resistance will be assessed on theis of the overall (summer) level of skid resistance

her than an instantaneous measurement.

.10 By removing the effect of seasonal variation as as possible (both variation within a single year andween successive years) sites exhibiting a lower skidistance can be identified more accurately.

.11 The following paragraphs list different methodsproviding an estimate of the summer skid resistance,erred to as the CSC from the corrected SC values.e choice of which method to apply determines thevey strategy that will be necessary to obtain theuired data.

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Single Annual Survey Method

A3.12 This approach is based upon a single annualsurvey of the network. The method uses measurementsfrom the preceding 3 years to characterise the long-termskid resistance of the network. This value is used withthe mean network skid resistance in the current year, tocalculate a correction factor, which is applied to thecurrent year’s data to make current values consistentwith the long-term average.

A3.13 As the effect of seasonal variation will vary indifferent geographical areas (e.g. due to differentamounts of rainfall), larger networks will be split intosmaller localities and the correction factor will bedetermined and applied separately within each locality.

A3.14 The Single Annual Survey Method isimplemented as follows:

A3.15 The whole network shall be surveyedonce during the Testing Season in each year.Surveys must be planned such that in successiveyears each road length is tested in the early,middle and late parts of the season.

A3.16 The early middle and late parts of the season aredefined, respectively, as: May to mid-June, mid-June tomid-August and mid-August to the end of September.For example, a route tested in the early part of theseason in year 1 could be tested in the late part of theseason in year 2 and in the middle part of the season inyear 3. In year four, it must be tested in the early part ofthe season again, etc.

A3.17 Each site on the network shall beallocated to a locality by the OverseeingOrganisation.

A3.18 A locality is a collection of road sections orroutes for which a Local Equilibrium Correction Factorwill be determined. A locality must be small enough sothat similar weather conditions will normally beexperienced within it, and large enough so that a stablevalue can be calculated to represent the long-term skidresistance. This approach is based on the assumptionthat the climatic effects leading to seasonal variationinfluence all the roads in a local area in a similar way.

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3.19 The Local Equilibrium Correction FactorLECF) is the correction factor determined within eachocality to bring the current year data to a levelonsistent with the long-term average.

3.20 By surveying all road sections within a localityt the same time, this method can remove a componentf the within-year seasonal variation as well as theariation between years.

A3.21 All the road sections within each localityshall be surveyed within the same part of thetest season.

3.22 The LECF is calculated in three stages:

i) The Local Equilibrium SC (LESC) isdetermined to represent the average skidresistance level for the locality over recent years.The LESC is the average SC, calculated for allvalid 10m sub-section measurements in thedefined locality over the 3 years that precede thecurrent testing season. This must contain surveysfrom each of the three parts of the test season.Valid measurements are those that were made inthe required part of the test season, on therequired test line, on road surfaces that were atleast 12 months old at the time of testing.

ii) The Local Mean SC (LMSC) is determined forthe current survey. The LMSC is the average ofall valid 10m sub-sections in the locality in thecurrent year survey.

iii) The LECF is determined by dividing the LESCby the LMSC, i.e.:

LECF = LESC/LMSC

A3.23 The CSC for each 10m sub-section shallbe determined by multiplying the corrected SCby the LECF.

nnual Survey with Benchmark Sites Method

3.24 This alternative approach to the determination ofSC is not to be used in England but has been used in

he past and may be adopted by other Overseeingrganisations.

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A3.25 Historically, when the Investigatory Levels werebased upon MSSC measurements, this approachallowed the survey frequency to be reduced on lightlyused parts of the regional networks. It is based onmonitoring all of a selected network every year andusing the average of the Mean Summer SCRIMCoefficient (MSSC) for selected sites known as“Benchmark sites” spread around the area to indicateseasonal variation.

A3.26 The Annual Survey with Benchmark SitesMethod is implemented as follows:

A3.27 The Overseeing Organisation will agree anumber of Benchmark Sites to cover a relevantgeographical area.

A3.28 The Benchmark Sites shall all be testedthree times with surveys spread through eachtesting season to provide MSSC values for eachBenchmark Site and an overall average MSSCvalue for the area.

A3.29 The whole of the selected network shallbe tested once in each year. It is acceptable tosurvey different parts of the network indifferent parts of the testing season.

A3.30 Whenever a part of the network issurveyed, all the Benchmark Sites shall betested at the same time.

A3.31 The LECF is the correction factor determinedfor the network area to bring the current year data to alevel consistent with the long-term average.

A3.32 With this method it is assumed that the averagebehaviour of the Benchmark Sites is representative ofthe area and that the climatic effects leading to seasonalvariation between years will have influenced all of theBenchmark Sites in an area in a similar way. Bysurveying the benchmark sites three times each season,some account can be taken of the within-year variation.Comparing the sites in successive years allows theeffects of between-year variation to be reduced.

A3.33 The LECF is calculated in five stages:

(i) The Mean Summer Correction Factor (MSCF)is determined to take account of variation in skidresistance between the time of a particular surveyand the average during the testing season. TheMSCF is the overall average of all of the

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Benchmark Sites for the testing season, dividedby the average of all of the Benchmark Sites atthe time of the relevant survey.

i) The MSSC for each 10m section in the survey isestimated by multiplying the SC for each valid10m sub-section by the MSCF.

ii) The LESC is determined to represent the averageskid resistance level in the area over recent years.The LESC is the overall average MSSC for all ofthe Benchmark Sites over the three years thatprecede the current testing season.

v) The LMSC is determined to represent theaverage skid resistance level in the area for thecurrent testing season. The LMSC is the averageMSSC of all Benchmark Sites in the area for thecurrent testing season.

) The LECF is determined by dividing the LESCby the LMSC, i.e.:

LECF = LESC/LMSC

A3.34 The CSC for each 10m sub-section shallbe determined by multiplying the MSSC foreach 10m sub-section by the LECF.

3.35 Because MSSC is used to calculate therrection factors, determination of CSC using thisternative approach will not be possible until after thed of the testing season when the final Benchmark Sitervey has been completed.

ean Summer SCRIM Coefficient Method

3.36 This method uses the MSSC to represent theuilibrium summer level of skid resistance and thiskes the place of the CSC used in the Single Annualurvey method.

3.37 Although the MSSC method takes some accountf within-season variation, it has been found fromperience that the approach is potentially vulnerable to

ifferences between particular years or, with longerpeat cycles, to changes in skid resistance in thetervening period between surveys. Particularly hot oret summers, for example, could give rise to relativelyw or high MSSCs compared with the underlyinguilibrium value. In a “low-MSSC” year, smallanges could give rise to significant lengths of the

etwork requiring investigation and subsequent

A3/3



treatment that may not be necessary. Conversely, in a“high-MSSC” year, sites that should be investigatedmay be missed and not reviewed for another threeyears.

A3.38 Thus, relatively small changes in MSSC betweensurvey years have given rise to large fluctuations in thelengths of road being identified for treatment, withconsequent difficulties for maintenance planning. Also,with a longer repeat cycle, on some sites importantchanges to the skid resistance occurring in theintervening years between tests might pass undetected,with increased accident risk for a time.

A3.39 Using the MSSC method, the networkshall be surveyed three times in the same year,in the early, middle and late parts of the testingseason.

A3.40 The MSSC is determined for each 10m sectionby taking the average of the three SC values from thethree surveys. The MSSC averaged over the relevantsite should be used as the CSC value for comparisonwith Investigatory Levels.

A3.41 In areas where the MSSC method is used,dividing the network into two or three parts and testingthe parts over successive years can reduce theproportion of the network to be surveyed in any year.Thus, half the network is surveyed in alternate years orone-third of the network may be surveyed each year sothat the whole network is covered over a three-yearcycle.

A3.42 This method takes no account of variationbetween years.

Measurements Outside the Normal Testing Season

A3.43 Occasionally, SCRIM measurements may bemade outside the normal testing season. Although datafrom such measurements can be used for comparativepurposes by experienced personnel, such measurementsare subject to the full uncertainty of seasonal variationand do not form part of this Standard.

A3.44 Survey planning should allow for the possibilityof delays, for example, due to a machine breakdown orsevere wet weather, and allow for recovery within thedefined testing season. This also applies to tests madein the early or middle parts of the testing season usingthe main Single Annual Survey method to determineCSC.

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.45 Exceptionally, surveys may be completed up to end of the first week of October with the agreementthe Overseeing Organisation, provided that theeral weather conditions in the area remainparable to those experienced in September and that

frosts or treatments to the road such as gritting haveurred.

.46 Where the Annual Survey with Benchmark Sitesroach is being used, then if measurements are to bede in early October, the relevant Benchmark Sitesuld also be included. The SCs obtained should thenadjusted to a late-season equivalent value byltiplying them by a factor obtained by dividing therage for the benchmark sites in the late part of the

ndard season by the average for the benchmark sitearly October. The corrected results would then bed in the determination of MSSC as described inagraph A3.34.

her Types of SCRIM Survey

.47 Surveys with SCRIM are occasionally carried for special purposes such as research or for localestigations and may not be following one of thenitoring methods set out above.

.48 Such measurements are subject to the fullertainty of the factors affecting test procedures anduire careful interpretation. The data do not form partthis Standard.

.49 Measurements that are made outside the testingson are subject to the full uncertainty of seasonaliation. They do not form part of this Standard.vertheless, a site with low resistance to skiddingasured outside the testing season is likely to have aer resistance to skidding within the testing season.

.50 Although non-standard testing can be of some to experienced personnel who are fully aware of theitations, its main use is only to test roads on aparative basis.

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Annex 4Site Investigation

ANNEX 4: SITE INVESTIGAT

A4.1 This Chapter describes the process of siteinvestigation and should be read in conjunction withAnnex 5 on the assessment of accident data.

Format of the Site Investigation

A4.2 Paragraphs A4.14 to A4.22 provide a list ofheadings under which a structured site investigationmay be carried out. Each heading contains a number ofitems for consideration. Answers are not required toindividual points, but a written assessment of sitecharacteristics must be made under appropriateheadings, taking into account relevant factors from theitems listed plus any other relevant points. Reference tosupporting documents or data should be made asappropriate.

A4.3 The site investigation must include a review ofthe Investigatory Level(s) assigned. If a change isrequired then it should be reported as required by theOverseeing Organisation.

A4.4 The review of the Investigatory Level shouldinclude a check of whether the current criteria forsetting the Investigatory Level have been followed (seeChapter 4). In the event of treatment being requiredthen the Investigatory Level will be needed todetermine the appropriate specification for treatment. If,additionally, issues are identified during the siteinvestigation that have implications for reviewing thecriteria for setting the Investigatory Levels, then thesemust be noted so that they can be acted upon at anappropriate stage.

A4.5 The level of detail appropriate for theinvestigation will depend on the nature of the site andthe time since a detailed investigation was last carriedout. Many of the points listed are only relevant to morecomplex sites. If the site has been investigated recentlythen it will only be necessary to identify the changesthat have occurred since the last investigation.

A4.6 The level of detail used should also beappropriate to the likely cost of the treatment and theamount of disruption likely to be caused to road users.A relatively cheap treatment, that has potential toreduce accident risk and where the works cause littledisruption or added risk to road users, will not warrantan extensive investigation.

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.7 Reviewing the nature of the site and condition of surfacing is an important component of the siteestigation. This can be achieved through a

mbination of the following methods, but it isommended that a physical visit to the site is alwaysde unless the site is a motorway or dual carriageway

on-event” category:

On foot (allows the condition of the road to beobserved in detail but has associated safety risksthat must be controlled).

From a parked or moving vehicle (allows patternof traffic movement and speed to be observedduring the visit).

From recent local knowledge of the site (mayprovide a more general knowledge of the roadusage under a wider range of traffic, weather andlighting conditions).

From video records and maps.

commendation

.8 As a result of the investigation, a clearommendation must be recorded of the actions to been (including if no immediate action is required).

.9 Treatment should be recommended if, taking intocount the nature of the site and the observed accidenttory, it appears that improving the skid resistance of surfacing, or improving the surface condition iner respects is likely to reduce the risk of skidding

cidents.

.10 If the site investigation identified anyaracteristic of the site or road user behaviour thatggests other road safety engineering measures could appropriate, then the appropriate people must betified so that the site is considered for safetyprovements.

.11 If the site investigation identifies requirementsr additional routine highway maintenance, such aseeping, re-application of markings etc thenpropriate action must be taken.

.12 If there is no justification for treatment then norther action is required other than to review the site

after a period of time. If the site remains below the

A4/1



Investigatory Level after the next measurement of skidresistance then it will automatically become subject to afurther investigation.

A4.13 If the skid resistance and accident pattern havebeen stable for more than 3 years then lowering theInvestigatory Level should be considered (see alsoparagraph A4.3).

Content of Site Investigation

A4.14 Site location and use:

• What is the location and nature of the site?

• Are there any features that could be expected torequire road users to be able to stop ormanoeuvre to avoid an accident? For example,junctions, lay-bys, other accesses, crossings,bends or steep gradients.

• What are the site category and the currentInvestigatory Level? Has there been anysubstantial change in the amount or type oftraffic using the road that would influence therequirement for skid resistance and could requirethe Investigatory Level to be changed?

A4.15 Pavement condition data:

• What is the CSC, by how much is it below theInvestigatory Level and over what length? Is theskid resistance uniform along the site or are thereareas of lower skid resistance or large changes inskid resistance? Is the lowest skid resistance inlocations where road users have a specific needto stop or manoeuvre? (The risk of accidentsgenerally increases as the skid resistance falls,but the increase in risk will be greater for siteswhere the road user is likely to need to stopquickly or manoeuvre.)

• Are there any individual 10m lengths that fallsignificantly below the mean for an averaginglength, and is the location of such lengthssignificant, e.g. a short length of low skidresistance within a sharp curve.

• Does the site contain a sharp bend to the left incombination with traffic braking or accelerating,e.g. a sharply curved roundabout approach orexit? In these circumstances the offside wheelpath can become more polished than the nearsidewheel path and the skid resistance in the offsidewheel path can be up to 0.05 units CSC lower

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than that measured in the nearside wheel path.However, this does not mean the skid resistanceis more than 0.05 units CSC below theInvestigatory Level, because the InvestigatoryLevel will have been raised in the vicinity of thecurve to compensate for this effect (Chapter 4).

What is the texture depth and do areas of lowtexture depth (below 0.8mm SMTD) coincide withareas of low skid resistance?

Are there any extreme values of rut depth orlongitudinal profile variance that could affectvehicle handling or drainage of water from thecarriageway?

4.16 Accident history:

A methodology for analysing the accident historyis given in Annex 5.

4.17 Site inspection:

Has a visit to the site been carried out? If so,then what range of weather and traffic conditionshas been observed and over what period? If not,then what other information has been drawnupon?

4.18 Visual assessment:

Is a visual inspection of surface conditionconsistent with the available survey data?

Skid resistance and texture depth are generallymeasured in the nearside wheel track in lane one.Is the rest of the area of the maintained pavementsurface visually consistent with the measuredpath, or are there any localised areas of polishedsurfacing, low texture depth, patching or areasotherwise likely to give rise to uneven skidresistance? If it is likely that the skid resistanceof other lanes could be lower than the lane testedthen additional surveys may need to be carriedout to investigate this. This could occur, eg. If thesurface in other lanes (including the hardshoulder) is different to the lane tested, and theselanes carry a similar volume of heavy traffic tothe lane tested.

If so, is the location such that the lack ofuniformity is likely to increase the risk ofaccidents occurring?

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• Is the area of the maintained pavement surfacefree from debris and other sources ofcontamination? Is water known to drainadequately from the carriageway during heavyrain? Is the pavement free of other defects suchas potholes?

A4.19 Road users:

• What is the volume and type of traffic, includingvulnerable road users? Are observed trafficspeeds appropriate to the nature of the site? Ifthere is significant variation in the speed, type orvolume of traffic during the day, haveobservations been made in an appropriate rangeof traffic conditions? What types of manoeuvresare made and what are the consequences if notcompleted successfully, e.g. head-on or sideimpact at speed are likely to have severeconsequences? Is there any evidence that roadusers consistently fail to negotiate the sitesuccessfully, such as tyre tracks into the verge?

A4.20 Road layout:

• Is the road design still appropriate for the speedand volume of traffic? Is the layout unusual orlikely to be confusing to road users?

• Is the carriageway particularly narrow and is ahard shoulder or 1 metre strip provided? Is theroad layout appropriate for the number and typeof vulnerable road users (pedestrians, cyclists,motorcyclists, equestrians, bus and tram users)?

• Are junction sizes appropriate for all vehiclemovements? Are right turning vehiclesadequately catered for? Are priorities atjunctions clearly defined? Are traffic signalsoperating correctly and are they clearly visible toapproaching motorists?

A4.21 Markings, signs and visibility:

• Are all pavement markings, warning anddirection signs appropriate and effective in allconditions (e.g. day, night, fog, rain, on colouredpavement surface)? Have old pavement markingsbeen removed properly? Are there any redundantsigns that could cause confusion? Are signs orother roadside objects on high-speed roadsadequately protected from vehicle impact?

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• Is visibility adequate for drivers to perceive thecorrect path? Do sight lines appear to beadequate at and through junctions and fromminor roads or other accesses? Is the end oflikely vehicle queues visible to motorists? Doeslandscaping, taking into account future growth ofvegetation and the effects of wind and rain,reduce the visibility, including visibility of signs?

A4.22 Additional information:

• Are any other sources of information available,such as reports or visual evidence of damageonly accidents, incidental damage to streetfurniture or reports from the Police? Suchreports are likely to be subjective but are relevantif the reliability of the information is borne out byobservations of the site.

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ACCIDENT DATA

Annex 5Assessment of Accident Data

ANNEX 5: ASSESSMENT OF

A5.1 The most recent available records of personalinjury accidents recorded on the STATS19 databaseover at least a three-year period should be used in theanalysis. The location of accidents should be plotted,identifying accidents that occurred in the wet, or thatinvolved skidding.

A5.2 The following aspects should also be considered:

• Is the location of accidents significant in relationto the observed pattern of skid resistance?

• Is there any other pattern apparent in the locationor type of accidents that would warrant moredetailed analysis or consultation with the personresponsible for accident investigation?

• Have there been any significant changes to thesite or the traffic using it in the analysis period,that could have affected the number of accidents?

A5.3 The number of accidents occurring within aparticular site will be related to the length of the siteand the amount of traffic as well as the length of timeconsidered. This means it is necessary to consider boththe number of accidents and the accident rates, whichcan be calculated given the site length and an estimateof the annual average daily two-way flow.

A5.4 The number of accidents, the average number peryear and the accident rates and severity ratio should betabulated:

Accidents/100km = (Average number ofaccidents per year / site length in km) * 100km

Accidents/108veh-km = Average number ofaccidents per year / (site length in km * vehiclesper day * 365 / 108)

Accident severity ratio = Accidents where acasualty was killed or seriously injured / totalinjury accidents

A5.5 The sites most likely to benefit from treatmentare those where the accident rates observed are greaterthan normal for the type of site. However, if the totalnumber of accidents is rather small, then the sameresource may be more effectively used at other sites.

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A5.6 If a greater than normal proportion of accidentsoccur in wet conditions or involve skidding, thisprovides an indication that increasing the skidresistance may reduce the accident risk. The followingpercentages should be calculated and tabulated:

%Wet = (Number of accidents that occurred inwet conditions / total accidents) * 100

%Skid = (Number of accidents where skiddingwas reported / total accidents) * 100

%Wet skid = (Number of accidents that occurredin wet conditions where skidding was reported /number of accidents that occurred in wetconditions) * 100

A5.7 Here, “wet” is where the road surface wasrecorded as wet/damp or flood in the STATS19 accidentrecord and “skid” is where any of the skidded, jack-knifed and overturned options was recorded in thevehicle record for any of the vehicles involved in theaccident.

A5.8 However, these percentages do not provide agood indication of the influence of the surfacing incases where the total number of accidents is rathersmall, when a small change in the number of wet orskidding accidents results in a substantial change in thepercentages calculated.

A5.9 Control data are necessary to be able to assesswhat the “normal” accident risk is. For trunk roads inEngland, control data for the whole trunk road networkand for the route that the site is a part of can beobtained from the appropriate HA Route Manager.These will provide values for the parameters above,broken down into Motorways, built-up A-roads andnon-built-up A-roads. Non-built-up A-roads are furtherdivided into dual carriageways and single carriageways.The most appropriate category should be selected andboth the national and the route values should becompared with the values for the site underinvestigation, for example, by using Table A5.1.

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Annex 5Assessment of Accident Data

A5.10 The control data will help to indicate whetherthe accident risk at the site is greater than the route orthe national average. This should only be taken as aguide because the route and, particularly, the nationaldata will include different types of site, with differentrisk factors to the site being investigated. If available,accident data from other, similar sites (e.g. with thesame site category as defined in Table 4.1) would alsoprovide a useful comparison. However, there will berelatively few accidents recorded at individual sites,including the site being investigated, and this numberwill be subject to random fluctuation. If there have beenchanges during the analysis period that could haveinfluenced the past accident history, for example, amarked change in traffic, road works or major events,these should be listed with the dates and accidentsaffected and taken into account in the analysis.

Site data Control data

Route National Similar sites

Number of accidents in analysis period

Accidents/year

Accidents/100km

Accidents/108vehicle-km

%Wet

%Skid

%Wet skid

Table A5.1 Example Table for Comparison of Site Data and Control Data

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T TYPES OF TEST IN

Annex 6Use of Different Types of Test in Accident Investigation

ANNEX 6: USE OF DIFFERENACCIDENT INVESTIGATION

General

A6.1 Road accidents usually result from a combinationof several factors and these can be difficult to isolateand identify.

A6.2 When investigating accidents, the police areusually attempting to determine the causes of theaccident and to obtain evidence that will assist them inaccident reconstruction and any proceedings againstindividuals that are subsequently deemed necessary. Inthis regard they are particularly interested in thebehaviour of the vehicles involved in the accident. Thespeed of a vehicle, the condition of its brakes,suspension and tyres and the driver’s response willaffect its ability to avoid a collision. The highwayauthority’s objectives will be different, as they will beseeking to determine the extent to which the roadlayout, construction and general surface condition,including its skid resistance, are contributing toaccident risk and whether some form of remedial actionshould be considered.

Friction and Skid Resistance

A6.3 For a vehicle to complete a particular manoeuvresafely there must be adequate friction available betweenthe tyre and the road surface at the particular time. Thelevel of friction required will be influenced by thenature of the manoeuvre and the site layout.

A6.4 The coefficient of friction is the ratio of the forceresisting motion to the vertical force, and strictly refersto a particular tyre on a particular surface at a particulartime. For most vehicles, an average coefficient offriction is normally assumed for all tyres on a vehicleunless there is particular evidence to do otherwise.

A6.5 Skid resistance is a measure of the contributionthat the road provides to tyre/road friction made understandardised conditions. It provides an indication of thegeneral condition of the road but does not relatespecifically to the coefficient of friction that a road userwill experience with particular tyres and the road at aparticular time.

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6.6 The friction available to a manoeuvring vehicleill depend upon the underlying skid resistance of the

oad surface, which should be appropriate for the typef site, but it will also be affected by seasonal factorsnd specific conditions that apply at the material time,uch as the weather e.g. dry, wet or icy, and theresence of other contaminants on the surface such asud or oil.

he Vehicle and the Police Skid Tests

6.7 Before attempting to reconstruct an accident,olice collision investigators usually measure the tyre/oad coefficient of friction in conditions that are aslose as possible to those prevailing at the materialime.

6.8 When a vehicle has been involved in an accidentnd has left skid marks which indicate that one or moref its wheels were locked, an estimate of its likelypeed can be made from the length of the skid marksnd the coefficient of friction, which is directly relatedo the deceleration rate.

6.9 In the police tests, the “locked wheel brakingoefficient” of a vehicle at the accident site is derivedy applying the brakes of a test vehicle quickly, so thathe wheels lock rapidly, and measuring either theistance or the average rate of deceleration that it takeso stop from a known speed. The initial test speed isormally 50 km/h. The vehicle used in the test shoulddeally be the one involved in the accident (or a similarne in terms of model etc) but may often be a policeehicle.

6.10 Stopping distances are indicated using a chalkun that fires at the road when the brakes are firstpplied. Deceleration is measured using specialroprietary equipment that incorporates accelerometersnd is mounted in the test vehicle.

6.11 In dry tests, the average coefficient of friction isssumed to apply at any speed (it is not normally speedependent) and so can be used in calculating vehicles’peeds from stopping distances measured from the skid

marks left at the accident site.

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A6.12 In wet tests the average value recordedrepresents a range of values of friction which willincrease as the skidding vehicle slows down. In thiscontext the results are unsuitable for estimating speedsbut the average value may be used to provide acomparison with wet roads in general to assist ininterpreting other evidence obtained at an accidentscene.

The Road Surface and Skid Resistance Tests

A6.13 A number of factors affect the skid resistance ofthe road surface and these are fully described inAnnex 1. In summary, research has shown that in dryconditions all clean surfaced roads have a high skidresistance but their performance varies greatly in thewet.

A6.14 On wet surfaces ‘slipperiness’ is caused by thelubricating effect of the water film which becomesmore effective as vehicle speeds increase, so that skidresistance generally falls as vehicle travel faster. Therisk of an accident involving skidding increasessignificantly when roads are wet.

A6.15 On some newly laid asphalt surfaces, thebitumen layer coating the aggregate at the surface canaffect both wet and dry skid resistance. More details aregiven in HD 36 (DMRB 7.5.1).

A6.16 The wet skid resistance of a road surface isroutinely measured on trunk roads using SCRIM.SCRIM has a test wheel fitted with a smooth tyre whichis mounted mid-machine in line with the nearside wheeltrack and angled at 20 degrees to the direction of travel.A controlled jet of water wets the road surfaceimmediately in front of the test wheel. As the vehiclemoves forward the test wheel, whilst freely rotating inits own plane, scuffs in a forward direction on the wetroad surface generating a sideway-force. The ratio ofthis force to the vertical force between the test wheeland the road gives a measure of the skid resistance thatcan then be recorded continuously along the road.

A6.17 Because skid resistance varies with speed,SCRIM tests are made at standardised operating speeds,normally about 50km/h or 80km/h depending on thetype of road. All test results are corrected to anequivalent standard speed of 50km/h before comparingwith Investigatory Levels.

A6.18 Because the test wheel is set at an angle to thedirection of travel, the effective slip speed of the test

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measurement is therefore one of low-speed skidstance: at a 50km/h standard test speed, the sliped is approximately 17km/h.

.19 Extensive research has established relationshipsween wet skid resistance as measured by SCRIM accident risk for different types of site. Table 4.1s investigatory skid resistance bands for differentgories of site. Roads are routinely monitored with

RIM to detect any sections where skid resistancey have fallen to a level where skidding accident riskthat type of site may increase; such sites areestigated and if necessary remedial works are put ind.

.20 Some UK Highway Authorities may use other devices to measure wet skid resistance in particularumstances. As with SCRIM tests, these tests arended to investigate the general condition of the road

face and not specific friction conditions. The devicesrate on different principles to SCRIM, such as usingine locked-wheel friction for spot checks or an in- fixed-slip technique in which the test wheel ised to rotate at a slower speed than the vehicle speed so skids over the surface. The different principles slip speeds mean that different values are obtained they cannot necessarily be compared with SCRIMfficient values.

.21 Another device used to assess skid resistance ast of an accident investigation is the pendulum tester, EN 13036-4:2003. Like the other tests, results from pendulum tester do not relate directly to the actualfficient of friction experienced by a particular tyre particular situation. This device measures skidstance on a small length of road (approximatelymm) and therefore positioning and frequency ofpling are critical. It was originally designed toulate a patterned tyre skidding at 50km/h but withnges to tyre compounds, construction and treadterns, this can no longer be safely assumed. SCRIMasures skidding resistance using a smooth tyre andrefore the surface texture will influence thetionship between SCRIM measurements anddulum Test Values (PTVs). The portable tester is therefore recommended for use on fine texturedfaces and results on coarse textured surfaces can beleading because of operational difficulties.

evance of the Tests

.22 When considering these two general types of, it is important to understand that:

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• Police skid tests are carried out in differingconditions and are used at accident sites to assistin accident reconstruction. They are frequentlymade in dry conditions. The measurements arenot suitable for assessing whether a roadsurface is substandard or in need of remedialtreatment. However, if a dry skid test indicates alower than expected dry road skid resistance, thisshould be drawn to the attention of the highwayauthority so that the cause can be investigated;

• the SCRIM test is a standardised wet road testand is used for routine monitoring of the skidresistance of the nearside wheel track of the roadsurface to assess if maintenance is required. It isnot specifically carried out after an accident. Theresults of SCRIM tests may be used to assesswhether the road surface might have been acontributory factor where it is known that avehicle skidded on a wet road, but even if thiswere considered likely, it would not necessarilyimply that the road condition was sub-standard;

• results given by the SCRIM test and the policetests can appear similar. SCRIM Coefficients,which represent friction, have a typical range of0.30 to 0.60. The Police skid test produces valuesthat may look numerically similar but becausethey are from quite different measurements theyare not related.

A6.23 Both groups of test are valid only for theparticular purposes for which they were derived.

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