details for motorways and trunk roads. Examples are shown inFigures 23.13 to 23.16.

A Concrete Society working party reported in 1974136 on awide range of edge details for all types of roads. The reportcovers function, design considerations, design practice, con-struction, maintenance and relative costs.

23.10 Lighting, signing,communications and safety

23.10.1 Lighting

Lighting can be expected to produce significant savings as aresult of a reduction in night-time accidents, and upgrading oldinstallations can also be expected to reduce accidents. Theprovision of street lighting in urban areas can deter criminalsand there may also be an amenity value. There may be someenvironmental intrusion in rural areas, depending on the loca-tion.

The need for lighting on trunk roads and motorways shouldbe appraised in accordance with TA 49/86.l37 A cost-benefitcalculation and assessments of environmental and road-safetyfactors will generally be required and a framework approachcan be adopted. Street lighting may be required for safetyreasons alone at such locations as follows.

(1) In urban areas subject to a speed restriction of 30 mph.(2) In semi-urban areas subject to a speed restriction of 40 mph.(3) Where the road layout is substandard.(4) Where junctions occur at frequent intervals.(5) At roundabouts.(6) At grade-separated junctions.(7) On fog-prone sections of road.

23.10.1.1 Design principles

The design of all aspects of lighting is covered in BS 5489.l38 Theaim of any road-lighting system is to illuminate all areas andaspects of the road and traffic which are of importance to allusers, including pedestrians, in a manner which is aestheticallypleasing, especially in areas adjacent to areas of historical orvisual interest. The main steps of the design process may besummarized as follows.

(1) The road category and preferred lighting arrangement (i.e.one side, staggered, etc.) should be decided.

(2) The data required for the calculation using BS 5489 andmanufacturers' data should be compiled.

(3) The design spacing should be calculated.(4) The lantern positions on the road plan with respect to fixed

features such as junctions, pedestrian crossings, etc. shouldbe plotted.

(5) The remaining lantern positions on uninterrupted stretchesof road based on the design spacing but 'grading' into thefixed-feature locations should be plotted.

(6) The overall layout should be checked for misleading night-time arrays, general daylight appearance and 'line-of-sight'obstructions.

(7) The column locations should be checked on site for obstruc-tions and visual acceptability. On the basis of the site survey,minor adjustments should be made as necessary.

23.10.1.2 Practical considerations

In addition to the design procedures described above, thefollowing practical requirements relating to the installation ofthe lighting columns and associated equipment should be bornein mind.

Column foundations. The column foundations, whetherplanted or bolted-base, must always include duct routes for theincoming and outgoing cabling which should take account ofthe allowable bending radii of the cables.

The column foundations should also be designed to allow thecolumn door to face away from oncoming traffic wheneverpractical.

Feeder pillars. Feeder pillars should always be located so as toallow safe access for maintenance. The feeder pillar foundationsshould also include ducts for incoming and outgoing cables, asdescribed for columns, above. Spare ducts for future cablesshould be included wherever practical.

Cable routes. Cables should be buried 600 mm below groundlevel and should be laid on 75 mm of sieved sand and coveredwith a further layer of sieved sand to a depth of 75 mm. Cablesshould be run in soft ground wherever possible and the route ofeach cable must be marked with purpose-made cable markersindicating the cable voltage, depth of burial, joint positions andchanges of direction.

Ducts. Where it is not possible to provide cable routes in softground, ducts should be provided, of minimum size 75mmdiameter. Typical locations where ducts are required are roadcrossings, cable routes within concrete structures, cable routesacross vehicular accesses, etc.

Duct routes should take account of the permissible bendingradii of cables and should include cable drawpits at all changesof direction and at intervals of 35 to 50 m, depending on the sizeof the cables involved.

Cast-in conduits. It is necessary in some instances (e.g. wherelighting fittings are fixed on the underside of overbridges) tocast-in conduit routes from a location adjacent to the under-ground cabling to the light fitting(s) via a suitable chamber forlocation of the fused cutout unit normally located in the columnbase.

23.10.1.3 Maintenance and operation

The maintenance and operation of lighting systems is discussedin the British Standards.138 Developmental Standard TD 23/86139

deals with these aspects on trunk roads.

23.10.2 Traffic signs and road markings

A congress held in Vienna on 8 November 1968 resulted in theConvention on Road Signs and Signals. This included recom-mendations for signs and signals and set out standards for them.Further meetings were held at Geneva on 1 May 1971 and on 1March 1973. These resulted in the European Agreement supple-menting the Convention on Road Signs and Signals and theProtocol on Road Markings respectively.

These provisions have been accepted and agreed by theEuropean Conference of Ministers of Transport and the Euro-pean rules concerning road traffic, signs and signals140 co-ordi-nates the provisions of the above.

The UK provisions are laid down in Traffic signs regulationsand general directions 1981^ and subsequent amendments andguidance on use, size, siting and illumination of signs and detailsof road markings are given in the Traffic signs manual40 andCircular roads No. 7/75.141 Further information is contained inDepartment of Transport circulars, standards, advice notes anda comprehensive bibliography,142 which includes publicationsrelating to working drawings for traffic sign design and postsizes, is produced also by the Department.

Permanent traffic signs, excluding signals, can be divided into:

(1) Warning signs, to advise of hazards(2) Regulatory signs, to advise drivers of legal restrictions.(3) Informatory signs giving information about directions,

routes, places and facilities.

Road markings give information to help the drivers to selectlanes and not as a guide base day and night. They may give awarning, a requirement, or a notice of restriction of prohibition.

For night-time visibility, signs may be illuminated externallyor may contain reflective material, although in urban areas theuse of one or other requires special consideration. The use ofreflective material in road markings is a valuable aid to visibilityas is the use of reflective road studs.

Variable message signs are also increasingly used as part oftraffic control at locations where a message is not required to bedisplayed permanently or where alternative messages arerequired at different times. Examples of their use are as part oflane control, speed control, detection and warning to overheightvehicles and warning of adverse weather conditions. They caneither be of the mechanical type (e.g. roller blind, prism or flap)or wholly electric where either an internal light source revealsthe message or of the matrix type where an array of light sourcescreate a range of messages, through the use of fibre optics.

Details of road signs and markings and a comparison of theiruse in many countries of the world were contained in the reportof the Permanent International Association of Road Con-gresses143 to the Geneva congress held in 1973.

23.10.3 Communications

23. JO.3.1 Motorway standard equipment

All motorways in the UK are equipped with a standard Depart-ment of Transport specified communications system whichincludes electric cables, roadside equipment cabinets, signals,mains power supplies and electronic equipment.

Practice and presentation are consistent throughout thecountry. The system provides emergency telephones from themotorist's viewpoint, giving direct connection to the police inthe local motorway control room, together with signals whichindicate either a general need for caution or for specific actions,such as speed restrictions or changes of lane.

All motorways are each equipped with a multipair communi-cations cable along one side only, taking a route as far from thecarriageway as possible and which forms the longitudinal cable,which when connected to others at motorway-to-motorwayjunctions, provides a national cable network used by the motor-way computer centres and control offices to communicate witheach other and with the equipment to be controlled, such assignals, emergency telephones and certain lighting systems.These cables, in their various sizes, cater for all normal require-ments in a motorway communications system.

23.10.3.2 Motorway emergency telephones

These are provided at approximately 1.5km intervals alongcontinuous motorway, normally in pairs, with one for eachcarriageway. This ensures that a motorist can reach a telephoneby walking along the carriageway without attempting to cross it.

Extra pairs of telephones at motorway junctions are sited onthe route but within the junction to accommodate motoristsisolated by the sliproads. In the case of motorway-to-motorwayinterchanges, the link roads and main routes are provided withas many telephones - but not necessarily in pairs - as arerequired to meet the criteria stated above, which preclude thecrossing of a carriageway or a spacing greater than approxi-mately 1.5km.

The telephone instrument is mounted in a standard housing,

which has various reflective labels both on the outside facing theoncoming traffic and on the inside of the housing door. Theselabels show the reference number and letter of the telephone,which the motorist must give to the local motorway controloffice when reporting his situation. Various standard logos arealso part of the telephone housing labels.

Telephone systems are normally, initially, a stand-alone sys-tem installed ready for motorway opening and comprisingseparate groups each of up to sixteen telephones and controlledby a telephone bridging unit near the roadside which sends thegroup's calls over a rented line to the motorway control office.The phase II systems which follow later operate the telephoneunder central computer control, switching them locally on toloaded circuits in the longitudinal communications cable andhence to the motorway control office.

23.10.3.3 Motorway signals

The most basic signal system, called Motorwarn, is that in-stalled temporarily to cover from opening until the permanentsignals are operational. Each signal is separate, consisting of apost set in concrete carrying a pair of amber lanterns, a radioreceiver and a flasher mechanism which causes the lanterns tolight alternately with a period of about 1 s. Power is suppliedfrom a car battery on the ground below and the signal isswitched on or off by radio transmission from an approachingpolice patrol vehicle.

Permanent motorway signals take two forms, depending onwhether it is necessary to apply speed restrictions equally to alllanes of a two-lane or three-lane carriageway, or to applydifferent restrictions to each of any number of lanes. The formersystem is known as 'carriageway signalling' and employs post-mounted matrix-type indicators sited in the central reservation,while the latter is known as 'lane signalling' and mounts similarsignal indicators on a gantry, with one over each lane. Access toa motorway at a junction with an all-purpose road, may becontrolled by post-mounted matrix signals having additionalred flashing lanterns.

All permanent signals discussed above have locally mountedunits, either within the mounting post or on the gantry, knownas 'distributors' and 'controllers' which are integral parts ofsignals. Mains supply is from Electricity Board interface cabi-nets usually set in the boundary fence and feeding signalinstallations over standard pattern mains-supply cable.

Control of signals and transmission over the motorwaylongitudinal cable network until the relevant responder is acti-vated is from motorway control offices via the central computer.The responder is mounted in a cabinet near the roadside, localto the signals under its control, and acts on instructions bearingits electronic address. It also signals to the computer when anyof its emergency telephones are off-hook and switches them tothe motorway control office when instructed.

23.10.3.4 Current developments

All the above information refers to the National MotorwayCommunication System 1 (NMCS 1), but a new system, NMCS2, is being brought into service using distributed computersassociated with motorway control offices. The cabling to signalsand other controlled devices is slightly different but uses similarcable with more cores in the longitudinal cable.

Mono-mode fibre-optic cable is coming into service as astandard cable for closed circuit television, but could easily beused to carry long-distance communications in the future. Newservices such as the automatic traffic surveillance system, fogdetectors and ice detectors are coming into service with NMCS2 and can be easily connected into the existing longitudinal cableat the nearest cabinet.

23.103.5 Traffic signals

The use of traffic signals is dealt with under sections 23.6 and23.12.

23.10.4 Fencing

23.10.4.1 Boundary fencing

Motorway boundary fences are owned and maintained by theDepartment of Transport. It is normal, to simplify supply andmaintenance, to use one type of wooden fence or one type ofstrained wire fence with droppers (Figures 23.35 and 23.36). Thelatter is less obtrusive than the former and would be used, forexample, at the top of cutting slopes to reduce the effect on theskyline.

Other permanent fences generally become the responsibilityof, and are chosen by, the landowner after erection by theDepartment of Transport. They are selected, if possible, fromthe accommodation fences included in the Highway constructiondetails*2 or BS 1722.144 Where appropriate, deer fencing isprovided as shown in Figure 23.37.

Timber for use in the works and preservative treatment fortimber fences should comply with BS 5589145 subject to amend-ments detailed in the Specification for highway works.12

23.10.4.2 Noise barriers

Noise barriers can be made of glass reinforced polyester, glass-fibre cement, plastic-coated steel, aluminium, etc. as well as ofconventional materials such as timber, brick and concrete. Thematerial should be chosen to suit the surroundings. Grassed orplanted earth mounds are aesthetically the most acceptable,particularly in rural areas. Criteria for design should be basedon technical memorandum H 14/76 and Amendment No. I.146

The main principles to be borne in mind are:

(1) The barrier should be sited close to either the noise source orthe position to be protected for optimum effect. It should belong enough to obscure completely the noise source fromview at the observation point.

(2) Normal range of barrier height is between 1 and 3 m. Lessthan 1 m is ineffective and more than 3 m usually unaccept-ably intrusive. Greater heights may be achieved by erectinga barrier on top of an earth mound.

(3) Comparatively light material is usually sufficient because

the limiting consideration is that the noise passing throughthe barrier should be less than that diffracted over or aroundit. The minimum mass required is given by the expression:

M= 3 x antilog ^~^°^ kg/m2

where A (taken as positive) is the potential attenuation indB(A) calculated by the path difference; and M is the massin kg/m2.

23.10.4.3 Safety fences and barriers

A safety fence absorbs some of the energy caused by a vehiclestriking it and redirects the vehicle. A safety barrier providescontainment and vehicle redirection without itself being def-lected or deformed.

The main types of safety fences and barriers used in the UKare: (1) tensioned beam safety fence (corrugated or rectangularhollow section);42 (2) untensioned beam safety fence (corru-gated or open box);42 l47 and (3) British concrete barrier (con-crete profile barrier).147

Use of untensioned corrugated beam fences and concretebarriers is restricted to low speed roads (85 km/h or less) and inthe case of British concrete barriers is limited to urban areas.

Safety fences are provided on the central reservations of allnew motorways. They may also be used on dual-carriagewaytrunk roads and other all-purpose roads where there are hightraffic flows or obstructions such as bridge piers and signgantries. They are also used at the back of verges on embank-ments 6m or more high, and on other embankments wherethere is a road, railway, water or other hazard below, theoutsides of curves of less than 850 m radius on embankmentsover 3 m high, and at obstructions such as bridges, large signsand retaining walls. The requirements for safety fences on newand existing trunk roads are detailed in TD 19/85.148

23.10.5 User facilities

23.10.5.1 Motorway service areas

Department of Transport policy was to provide service areas atintervals of about 40km, with potential infill sites midwaybetween. The land, connections with the motorway, accessroads, landscaping, parking areas, lighting and basic servicesare provided by the Department. The operator, who leases thesite from the Department, pays for the buildings and equipment

All dimensions in millimetresFigure 23.35 Wooden-post and five-rail fence. (After Departmentof Transport (1987) Highway construction details. HMSO, London)

300(minimum)

Concrete grade C 1OPto this level unlessotherwise directed

Main post75 x 150 x 2100 long

square

38 x 87 rails nailed to each post with2 no. 4 x 100 nails

Backfill

Straining post3025 long 100 x 50 x 4steel RHSmaximum spacing 300 m

All dimensions in millimetres

Figure 23.37 High-tensile strained-wire deer fence. (AfterDepartment of Transport (1987) Highway construction details,HMSO, London)

Minimum 450 square or450 diameter augered holeconcrete C 1OP

400 wide x 450 longconcreted OP

.Double strut3025 long 50 x 50 x 4 steelRHS

Intermediate post2875 long 50 x 50 x 3 steelRHSMinimum 200 square or300 diameter augered holeconcrete C 1OP

925 long droppers

925 long droppers

765 long dropper

450 squareor 450 diameteraugered holeTraining post2275 long60 x 60 x 4 RHS

AM dimensions in millimetresFigure 23.36 High-tensile strained-wire fence with droppers.(After Department of Transport (1987) Highway constructiondetails. HMSO, London)

Intermediate post1975 long40 x 40 x 3 RHS

Holes 300 diameter augeredor minimum 200 square

Strut1975 long40 x 40 x 3 RHS

765 long dropper

650 long dropper

mini

mum

min

imum

and is required to maintain the site and buildings in good repair,including the facilities provided initially by the Department, andto provide services for every day of the year, including toilets,food and drink retail outlets, and fuel and repair facilities,although in some cases this latter requirement has been relaxed.

A committee of inquiry into motorway service areas pro-duced a report in 1978149 which reviewed existing facilities andtheir use and made recommendations for the future.

23.10.5.2 Laybys

On rural all-purpose roads the provision of laybys is recom-mended at the rate of two per 1.6 km, although it is acknow-ledged that the spacing will be dependent on topographicalfeatures and the horizontal and vertical layout of the road.Details of their layout and advice on siting are contained inLayout of roads in rural areas]5Q and the metric supplement.151

23.11 Specifications and materialstesting

23.11.1 Introduction

The Department of Transport Specification for highway works12

covers all aspects of works on highways (except signals), bridgesand other associated structures. There are seven parts, and thefirst six are divided into twenty-six series, each covering aseparate aspect of the work and an introduction. The final partcovers accepted quality assurance schemes, certification-markedquality assurance schemes, approved lists of proprietary systemsand materials, publications referred to in the Specification72 andthe manner in which variations to the specification can be madeby adding, deleting or substituting clauses. Specific criteria maybe inserted in appendices to each section. Clauses may includethe terms 'or as otherwise described in Appendix *****' or 'asdescribed in Appendix *****'. Other terminology used is 'asdescribed in the Contract' or 'as shown on the drawings'. Theadditional information must be added for such clauses tobecome effective. Where clauses include the terms 'unless other-wise agreed by the Engineer' or 'as approved by the Engineer'the engineer is given freedom of action to suit particularcircumstances.

The complementary document, Notes for guidance on thespecification for highway work73 is in six parts which relate to thefirst six parts of the Specification72 Each part is divided in asimilar manner to the Specification.72

The Specification is mandatory for use on Department ofTransport schemes and is used generally for other major road-works in the UK; it usually is incorporated by reference in roadand bridgework contracts.

A specification may be 'end-result' in which only the finishedwork is tested or may be by 'method specification' in which themethod of working is defined. Some blend of the two types ofspecification may be adopted.

23.11.2 Control testing

Control testing may be defined as testing with the aim ofmonitoring compliance with the specification requirements forthe material in question. It is principally carried out by acontractor or supplier and may be predictive in nature. The testmay be the specified test itself or some other test whichcorrelates sufficiently well with the specified test.

23.11.3 Acceptance testing

Acceptance testing is the testing carried out to ascertain whether

compliance has been achieved within the specified test limits.This may involve tests on the components of material prior toplacement, or tests on the material after placement checkingboth the material content and workmanship standard.

23.11.4 Statistical acceptability

In circumstances in which materials are produced as a con-tinuous or semi-continuous process, specifications may require astatistical method of testing aimed at ensuring that early correc-tive measures are taken to prevent the required material para-meter falling outside the specified range.

Examples of the application of the specification are as fol-lows.

23.11.4.1 Earthworks

Cuttings Tests are taken primarily to determine whether theexcavated materials are suitable for forming embankments.Acceptable limits for some of the tests are laid down in thespecification but in others the designer is required to decide thelimits. For cohesive materials, tests that may be required aregrading, plastic limit, undrained shear strength of remouldedmaterial and either moisture content or moisture conditionvalue. A normal permitted moisture content for acceptability isnot less than optimum moisture content (Test 12 of BS 1377)74

and not greater than 1.2 times the plastic limit. The test forgranular materials may be grading, uniformity coefficient, mois-ture content or moisture condition value, and for chalk, thesaturation moisture content. A normal permitted moisturecontent for granular materials for acceptability is 1% fromoptimum to 2% below the optimum as determined by thelaboratory compaction test (Test 12 of BS 1377).74

Embankments. Compaction is normally controlled by amethod specification, although the engineer is permitted tocarry-out field dry-density tests for comparison with similartests on approved work in similar materials to confirm thecompaction applied.

23.11.4.2 Concrete and cement-bound materials

Concrete is tested in its hardened state by cube crushing.152

Nondestructive testing is used mainly for comparative purposesto identify defective areas of completed work.152 Aggregates andcements are tested separately.153"155

23.11.4.3 Bituminous materials

Bituminous materials are tested for their composition156 andcompaction from cores by calculating the percentage refusaldensity as the percentage ratio of the bulk density of the sampleto the final density after compaction to refusal.

23.11.5 Method of measurement

A method of measurement157 is used on Department of Trans-port, and other, major roadworks contracts based on theInstitution of Civil Engineers Conditions of contract.*™ It is alsobased on the Specification for highway works12 and the Highwayconstruction details.*2 The method allows bills of quantities to beprepared in a uniform manner for the benefit of engineers andothers closely associated with highway works. The Library ofstandard item descriptions^ is also provided.

Variations in the conditions of contract, the specification orhighway construction details will require amendments to themethod of measurement to suit the variation. Details of theamendments so made are required to be stated specifically.

23.12 Roads and traffic in urban areas

23.12.1 Introduction

Roads and traffic in urban areas are parts of the complexarrangement and use of the urban fabric. The growth of traffichas caused congestion, particularly in the peak hours and hashad an impact on the environment and the people who live andwork in urban areas with the noise and air pollution associatedwith traffic and increased danger.

Much has been done to combat these problems. Urbanmotorways and other major roads and improvements have beenprovided, but the general tendency is now to make better use ofthe existing fabric by providing traffic-management measures bya variety of means to reduce congestion and delays: (1) traffic-control measures have been introduced; (2) priority has beenprovided to public transport; (3) through traffic has beenremoved from residential areas and shopping centres; (4) pedes-trianized areas and special facilities have been provided forcyclists and pedestrians as well as the disabled; and (5) safetyhas been improved. New roads can still contribute to the generalgood of the area and enable these improvements to take placemore easily and, whilst new roads can have local adverse effects,they can still provide substantial overall benefits if properly sitedand designed.

Roads and traffic in urban areas*2 provides a comprehensivereview, and is intended as a guide to good practice in dealingwith urban area problems.

23.12.2 Hierarchy of roads

It is most useful and desirable to establish a hierarchy of roadswithin an urban area, whereby a policy can be developed for theuse of each road. There is an interaction between the highwaynetwork and land use, and the latter can be controlled inrelation to its position in the road network.

Major through routes, for example, are primarily for trafficuse and the number of accesses to it should be strictly con-trolled. Residential roads, at the other end of the spectrum,should have all through traffic excluded. The various levels ofdistributor roads can be decided and it may be possible to makethe physical characteristics of the road more appropriate to itsplace in the hierarchy. It may be possible to channel more trafficon to main traffic routes thus allowing heavily used shoppingareas to be pedestrianized.

23.12.3 Traffic management

Traffic management is aimed at improving an existing roadnetwork to meet set objectives, without having to resort tosubstantial new construction. Such objectives may includesome, or all, of the following: (1) a reduction in road accidents;(2) environmental improvement; (3) improved access forpeople and goods; and (4) improved traffic flows on majorroutes.

These aims can be achieved by employing various trafficmanagement measures involving combinations of:

(1) Improved road capacity.(2) Giving priority to certain classes of vehicles, e.g. buses,

emergency vehicles and cyclists.(3) Restraining the demand for road space by introducing some

form of restrictive measure.(4) Improved facilities for pedestrians, cyclists and the disabled.

Traffic management measures may be limited to a small, loca-lized area or may encompass an area-wide network. In the lattercase, it should be recognized that measures regarded as provid-

ing solutions for one area can create problems elsewhere. It istherefore vital that all proposals be properly evaluated beforeimplementation, and that adequate data collection and forecast-ing techniques be employed to examine effects over an appropri-ately wide area.

The Association of London Borough Engineers in associationwith the Department of Transport and the Metropolitan Policehave produced a code of practice for traffic management inLondon.160 The guidelines and advice contained within the codemay have useful applications elsewhere.

23.12.4 Methods of assessment

The potential traffic throughput and effectiveness of alternativetraffic management measures can be assessed by trafficsimulation modelling techniques, such as CONTRAM,18

TRAFFICQ,19 SATURN,17 and TRANSYT58 referred to earlierin section 23.4. Such models typically require input data whichincludes link lengths and widths, junction type, traffic flows andestimates of the origins and destinations of the traffic patternsurrounding the study area.

Other computer models such as ARCADY,49 PICADY,49

MIDAS50 and OSCADY54 can be employed for the detailedassessment of individual junction designs within a scheme asoutlined in section 23.6.

New roads may be proposed or new roads in conjunction withtraffic management measures might be suggested as a package,and these proposals can be compared with other possible traffic-management measures.

The assessments should be made with the aid of a frameworkdescribed earlier in this chapter so that all aspects may beconsidered. As with inter-urban roads, public consultation andparticipation is becoming more widespread and the results canbe included; the impact on the environment and people is amajor issue and both benefits and disbenefits will almost invar-iably arise in an urban area. All other appropriate factorsshould also be included. Any specific improvements in trafficand financial benefits in cost-benefit terms arising therefromare, as in inter-urban roads, only two aspects of the comprehen-sive summary which must be produced to allow decision-making. The SACTRA report9 and the government response10

has been mentioned earlier.

23.12.5 Improvements in road capacity

The urban road network consists of a complex interaction ofroad links and the junctions which join them. Measures aimedat improving road capacity can therefore be assessed in terms oflink or junction improvements, either separately or in tandem.

23.12.5.1 Road links

The treatment appropriate to road links between junctionsshould reflect the extent to which they serve the functions ofthrough movement, local distribution and access to frontagepremises.

Link improvements often may be achieved by straightforwardmeasures such as road widening, or by the imposition of waitingand loading restraints161 or turning restrictions. Other forms oflink improvement commonly adopted include tidal flow ar-rangements and the creation of one-way streets.

23.12.5.2 Junctions

The traffic capacity of a road system is commonly constrainedby its junctions. An important consideration when junctionimprovements are being investigated may be whether the exist-ing junction type should be maintained, perhaps with modifica-tions, or whether a different type of junction control would be

more appropriate. Factors influencing the decision will include:(1) the existing and predicted traffic flow and composition;(2) the classification and function of the roads forming thejunction; (3) available highway space; (4) proximity of adjac-ent junctions; (5) road safety record; and (6) other needs, suchas public transport, pedestrians and cyclists. The four maintypes of junction in common use are:

(1) Major/minor.(2) Signalled.(3) Roundabouts and gyratory systems.(4) Grade separation.

Major/minor junctions. Major/minor junctions are the mostcommon type of junction in urban areas, their great advantagebeing that major road traffic is generally not delayed (excepttraffic waiting to turn right into the minor road). Improvementsto heavily trafficked junctions may include the channelization oftraffic to separate and clarify conflicting movements. This maybe achieved by improved road markings and signing or byphysical means, such as traffic islands. Departmental AdviceNotes TA 23/8142 and TA 20/8448 give the main principles forlayout and recommended design standards as stated previously.

Major/minor junctions tend to become unsuitable whenminor road traffic cannot find suitable gaps in the main roadtraffic during times of peak traffic flow. These conditions canresult in excessive queuing and delays in the minor road, oftenleading to accidents or diversionary routing. In these cases, analternative form of junction control will be necessary and trafficsignals often prove beneficial.

Signalled junctions. Traffic signal control is an importantfeature of junction control in urban areas, providing relativelyefficient control within the confines of limited road space.Traffic signals initially were provided to reduce the policemanpower required to control traffic. The use of traffic signalsto control traffic movement can now bring about major reduc-tions in congestion, improve road safety and enable specificstrategies to be introduced which regulate the use of the roadnetwork. Such strategies might be: (1) to reinforce a designatedroute hierarchy; (2) to give priority to public service vehicles;(3) to provide crossing facilities for pedestrians and cyclists;and (4) to maximize traffic flow.

Section 23.6 includes details of computer programs used inconnection with traffic signals and Department of Transportadvice notes on this subject.

New signal controllers using microprocessors are now avail-able for use at isolated junctions. Information on the trafficapproaching from each leg of the junction can be processed atthe signal controller and the signal timings adjusted to provide amore efficient use of the junction.

The numbers of traffic signal systems in urban areas may beextremely high, leading to adverse interaction between neigh-bouring installations. Recent advances in computer technologyhave made it possible to co-ordinate the operation of adjacenttraffic signal sites by the use of area-wide traffic control.12 This inturn has enabled vehicle movements to be controlled over asection of road network, which may result in a reduction invehicle journey times, the number of stops, fuel consumptionand environmental pollution. This is the basis of the majority ofpresent-day urban traffic control schemes. Many different pro-grams are in use: fixed-time programs such as TRANSYT16 58

adjust signal timings based on historical data, but SCOOT162

both receives data on traffic and adjusts signal timings on realtime. Other potential benefits which an urban traffic-controlscheme may provide include: (1) the implementation of diver-sion schemes and variable message systems; (2) creating priori-ties for buses and bus routes; (3) priority for emergency vehicles

responding to incidents; and (4) provision of special signaltiming plans to favour key routes from fire or ambulancestations.

Roundabouts and gyratory systems. Roundabouts provide auseful form of junction control which is generally conducive to'free-flow' conditions when they are operating within theircapacity limitations.49 The different types of roundabouts, theircapacities and designs have been discussed earlier in thischapter. They do not require control equipment and are there-fore not prone to equipment failure. When approach speeds arevery high or some of the approach links are dual carriageways,roundabouts are often the safest way of regulating traffic. Inaddition, they provide good opportunities for vehicles to turnright and allow U-turn manoeuvres, which can be very bene-ficial if restrictions exist elsewhere. However, roundabout solu-tions are unsuitable where linking of traffic flows betweenadjacent junctions is advantageous or where it is beneficial tochange traffic priorities at different times of the day. Theprovision of pedestrian and cyclist facilities may also provedifficult.

In some circumstances (e.g. such as a measure to reduceaccidents or during peak periods) there may be advantages insignalling one or more approach paths to a roundabout.

Gyratory systems - whereby a series of one-way streets arelinked to form a circulatory system - can overcome the limi-tations of several small junctions. However, they can createaccess problems to properties within the central island.

Grade separation. As grade-separated junction facilities arequite expensive and can be visually intrusive they will generallyonly be used at junctions of major importance, where land isavailable and there is little or no adverse environmental impact.

23.12.5.3 Priority management measures

Traffic management measures may be undertaken to providepriority for particular classes of traffic, e.g. public transportvehicles, emergency vehicles, cyclists and pedestrians. Prioritymeasures are normally achieved by allocating special facilitiessuch as pedestrian62 and cycle crossings, bus/emergency vehicleactuated signals163 and bus and cycle lanes. Alternatively, certainclasses of vehicles may be exempted from general traffic restric-tions, e.g. exemption from turning bans or contraflow opera-tions.

Priority measures are often applied where there is a generaldeficiency in the road network in terms of demand related tocapacity. Any such measures should seek to ensure that theoverall effects provide a net gain to the community as a wholeand, where possible, should attempt to minimize the adverseeffects on nonpriority road users.

23.12.6 The provision of information

The efficiency of traffic management schemes can be enhancedgreatly by the careful provision of traffic signing and roadmark-ing systems.

Well-designed, clear and comprehensive signing should pro-vide drivers with information on route choice well in advance ofthe approach to any junction and also warn of any turningprohibitions. Such systems are likely to be enhanced greatly bythe emergence, in the foreseeable future, of electronic routeguidance systems163 such as AUTOGUIDE, a trial of which hasbeen proposed in London, and ALI-SCOUT which has beeninstalled in an area of West Berlin on a trial basis. A trial onroute guidance, the CAC system,143 was undertaken in Tokyosome years ago. In addition, the use of signing can provideinformation on items such as car-parking availability, diversion

routes, guidance to tourist attractions, and direction signing tocyclists and pedestrians. The use of signs and markings has beendealt with earlier in this chapter.

23.12.7 Restrictive measures

Traffic restraint (or demand management) measures may beemployed to control the level of traffic in an area or on aparticular route. For example, in one area restraint may beaimed at through-traffic and in another area at commutermovements.

Restraint measures are designed to encourage those makingtrips to respond to imposed conditions in specific ways. Variousresponses may include: (1) a change in the mode of travel; (2) achange in the time of travel; (3) using a different route; (4) tra-velling to an alternative destination; or (5) not making the tripat all.

Very few restraint schemes have been implemented in the UKother than those involving some form of parking control164

mainly effected through on-street waiting restrictions165 or lorrybans.166 However, more radical measures have been attemptedin other countries such as Singapore, where there are restrictionson entering the central business district in the morning peakhours, and in Hong Kong,163 where a trial on the feasibility ofroad pricing has been undertaken.

23.12.7.1 Facilities for pedestrians and cyclists

Pedestrians and cyclists can be very vulnerable to severe or fatalinjuries on the roads and it is essential that traffic managementmeasures take their safety into account. Typical facilities mayinclude: (1) separate provision of footways or cycle routes;(2) guardrails; (3) pedestrian crossings; (4) subways or foot-bridges; and (5) pedestrianization schemes. In addition, con-sideration needs to be given to the handicapped.167 Where spaceis limited, facilities for pedestrians can sometimes be shared withother users.

23.12.7.2 Residential areas

Previous reference has been made to residential areas. Theresidential precincts in The Netherlands (Woonerf) have beenthe subject of much interest.163 The demonstration projects inThe Netherlands163 adopted three strategies for residential areas:(1) the exclusion of through-traffic by simple means; (2) theexclusion of through-traffic by simple means compiled withsmall-scale measures for reducing speed; and (3) the conversionto a residential precinct. It was felt that, for most areas,preference should be given to a redesign to the second strategywhich gives relatively the best results for the costs involved.

23.12.7.3 Parking

Parking should be considered within the overall policy relatingto the infrastructure and use of the urban area. The demand forparking, particularly in central areas, often exceeds the availablespace. The generous provision of parking space is likely toencourage the use of the private car and the limitation on theamount of parking to be provided may act as some deterrent tousers of private vehicles.

Parking may be provided on-street, or off-street in ground-level or multistorey car parks. On-street parking may be con-trolled by indicated time limits free of charge or some paymentmay be required such as at parking meters. Parking charges,both on- and off-street, usually vary and depend on the locationin relation to the central area.

Residents' parking schemes are often provided where parkingspaces in an area would be otherwise occupied by the cars of

visitors or commuters to the area. Charges may be levied orpermits to park may be provided without charge.

Enforcement is a major part of parking policy. Fines areusually levied when the permitted parking time is exceeded.Vehicles may be removed in certain circumstances, and wheelclamps are now being introduced in certain areas which, inaddition to incurring financial penalties, cause drivers delay andinconvenience before the cars are freed and can be driven away.

23.13 Highway maintenance

23.13.1 Introduction

As outlined in section 23.2, local highway authorities normallyact as agents for the secretaries of state in maintaining trunkroads and motorways, in addition to maintaining their ownhighways.

The object of maintaining a highway is to preserve the fabricin such a condition that it provides safe passage for all trafficthroughout its life. The Report of the committee on highwaymaintenance*6* proposed that maintenance operations bedivided into: (1) structure; (2) aids to movement and safety;and (3) amenity.

It is not easy to ascribe priorities within those groups withoutassessing the type of traffic, its contribution to the communityand the advantages to be gained by incurring the expenditure, oreven the disadvantages, of not doing so.

The report168 proposed national maintenance standards forthe UK; good maintenance extends the life of a road pavementand adds to the convenience and safety of the public using theroad. Good maintenance means:

(1) Day-to-day maintenance to maintain a road in propercondition for the traffic using it. This includes patching,surface dressing, gully emptying, repairs to drainage, kerband footway maintenance, maintenance of bridges andother structures, embankments and verges, repair and main-tenance of traffic lights, carriageway markings, street light-ing and street furniture, snow and ice clearance.

(2) Structural work required to extend the life of the road or toenable a road to carry an increased volume or weight oftraffic.

Tables 23.17 to 23.19 set out the needs in relation to structuralworks, aids to movement and amenity. Regular inspections anda system to determine priorities for short- and long-termattention (i.e. a maintenance rating system) will be required.

Most highway authorities carry out regular detailed inspec-tions of highways at specified frequencies and have recordingsystems for defects which require maintenance action within aspecified period of time.

The Department of Transport has instituted a code of prac-tice169 setting maintenance standards with a routine maintenancemanagement system applying to motorways and trunk roadswhich sets out requirements for a computerized inspectionsystem which is linked to an inventory of the highways andproduces routine maintenance work schedules.

The Department also specifies the statement of service170 anddetailed standards to apply to the winter maintenance activitiesfor motorways and trunk roads.

The local authorities associations in the UK have produced acode of good practice for highway maintenance171 with modelspecifications of maintenance activities as a guide to the prep-aration of highway maintenance policies and standards.

Each highway authority produces particular maintenancepolicies to apply within the area, including those policies andstandards relating to winter maintenance.

Element( ) Item

Carriageway

(I) ConditionShapeIrregularity

(2) Strength

Surface

( I ) Lack of skid resistance only

(2) Patching

(3) Defect through utility services

Drainage

(I) Gully emptying

(2) General performance

(3) Concentrated flow acrosscarriageway

Footways (Urban)

Irregularities related to safe use

Footways (Rural)

Irregularities and surface water related tosafe use

Kerbing

(1) As drainage feature

(2) Edge of footway

Structures

(1) Bridges, culverts, walls

Correction

Reconstruction, resurfacing surfacedressing

Reconstruction, resurfacing

Surface dressing

Surface dressing

(Road markings are entered in Table 23.17)

Width of running water in channels

Install extra channels or gulleys

(See also 'Structures' below)

Relay paving slabs

Resurface flexible material

Minimum maintenance in rural areas

Install to delineate drainage channel andsupport edge of carriageway

Defective kerbing

Kerbs sunk to carriageway level or lower

Necessity for programming of paintingof steelwork

Remark

Depending on degree of fault

Depending on degree of fault

This is corrected within other correctionsoperations as above

To maintain a watertight surface

These should be masked during surfaceoperations

i

Dependent on local conditions - referred toalso under sweeping and cleaning

Standards relate to the flow in channelsreducing the effective width ofcarriageways

Not to be confused with normal flow acrosscambered roads

(1) Suggestions for height of projectionsare given and also rates of inspectionin different areas

(2) Legal implications must be recognized

Rate of inspection given

A normal height of kerb in urban and ruralconditions is given as safety feature forpedestrians

Serves to support edge of carriageway

Rate of inspection to be assessed on localconditions

Table 23.17 Structural elements of highway maintenance

Element( ) Item

Safety of road user is paramount

(2) Embankments and cuttings; toinclude ditches where appropriate

Table 23.18 Aids to movement and safety

Element( ) Item

Road Markings(I) Advisory markings

(2) Mandatory and statutory markings

(3) Reflecting studs

Traffic signs and bollards(I) Illuminated signs and bollards

(2) Nonilluminated signs

(3) Traffic signals

Pedestrian Crossings( I ) Beacons(2) Road markings

Road Lighting(I ) Lanterns

Correction

Deterioration of concrete or other fabric

Underwater inspection where necessary

Regular inspection for incipient slips orfailures

Advice to adjoining landowners aboutstructural or drainage defects

Correction

Remake

Remake

Loose or ineffective studs should bereplaced. A wholesale change might bemade at the end of effective life

Regular inspection for(i) light failure(ii) drainage(iii) cleaning(iv) supports and frames

Regular inspection for(i) drainage(ii) cleaning(iii) supports

Regular inspection for(i) light failure(ii) general maintenance and cleaning(iii) phasing(iv) alignment(v) mechanism(vi) painting

As for traffic lights(i) Slippery surfaces should be corrected(ii) Obscure markings should be made

good

Regular inspection for(i) illumination(ii) cleaningConditions to be reported duringinspection

Remark

Legal implications are important

Remark

General recognition should be possible

These markings must lie within legal limits

An inspection before onset of winterconditions is advisable and equally aninspection should be made after wintermaintenance operations are complete

Particular attention to be paid to mandatorysigns, e.g. 'Stop' signs, etc.

See above

(1) All highway personnel should reportfaults wherever discovered

(2) Contract maintenance and guarantees tobe operated. On-call arrangements tobe made

Legal implications are important

The safety of the road user - driver andpedestrian - is paramount in this connection

Table 23.17 - continued

Element( ) Item

(2) Columns

Guard Rails and safety fencesPedestrians and vehicular

Winter maintenance(1) Precautionary salting

(2) Snow and ice clearance

(3) Snow fencing

(4) Salt storage

Correction

Conditions to be reported duringinspection

To be included in regular inspections.Where risk to public is involved speedyaction is necessary.Inspection to cover:(i) condition(ii) painting(iii) cleaning

On receipt of frost warning roads shouldbe treatedThe rate of salt-spread should be 14g/m2

Salt in accordance with BS 3247: 1970,Part 1

Use of specialist equipment

Remark

Note legal implications

Treatment should be applied within a limitedperiod after a warning:(a) Rural main roads and motorways - 2 h(b) Other important roads and accesses to

emergency services - 2 h(c) Urban main roads - 1 hCrews of salting equipment should be onstand-by duty. Neat salt should be used.This should be mixed with grit in specialcircumstances only

Major routes as in (a) above should neverbecome impassable to traffic.

This is related to traffic flows which havethe effect along with salt of keepingsnow from accumulating. Snow ploughswith blades effectively remove slush frommain routes; crews should be on standbyduty.

Roads as in (c) above should not beimpassable for longer than 1 h.

Public transport is a major factor in theclearance of snow.

Roads in other priorities should generally becleared in 4 to 6 h unless conditions areexceptional.

Pedestrian ways may require special treatmentin town centres.

Footpath clearance should be confined tobusy areas, steep hills, etc.

Local knowledge is necessary to establishthe siting and timing of snow-fenceerection. Care must be taken to clear thiswith landowners

Care must be taken to site salt heaps toavoid damage to local crops,watercourses

Table 23.18 - continued

Element( ) Item

Grass cutting

(I) Prevention of obstruction of sightlines

(2) Maintenance on certain roads ofreasonable pedestrian access

(3) Control of noxious weeds

Hedge Trimming

Prevention of obstruction of visibility atbends and at traffic signs

Trees

Correction

Grass cutting

Use of chemical sprays

Tractor-mounted equipment might beemployed

This repeats very much the information under hedge trimming. Trees may call forspecialist advice which is often available in the Parks Department of a LocalAuthority

Sweeping and Cleaning

(1) Objects and material shed byvehicles

Heavy and dangerous items must beremoved by hand

This is material which can cause brokenwindscreens and mechanical damage

Remark

Standards vary for urban or rural situations

Rural(a) Major roads - 1 .8 m of the verge

should be kept below 15 cm.Elsewhere one or two cuts should beemployed to keep grass to 30 cm long

(b) Minor roads - one cut per year willnormally suffice

(c) Spraying of grassed areas can be usedto control noxious weeds.Consideration must be given towidth of road, nature of traffic andnot least, the culture of the grassgrowth

Urban

(a) Major roads - grass should be keptdown to 8 cm

(b) Minor roads - minimum maintenanceconsistent with the environment.

These should be used with caution whereaccess for cutting is not available and forgrowth control generally.Examples are: around sign supports;central reserves with safety barriers; urbanwalkways

This is not normally a function of thehighway authority. They have power torequire land owners to reduce hedgeheights and to control trees.

Legal implications should be noted carefully

Legal implications and ownership areimportant factors

The rates of inspection and activity on thisaccount vary with the weight of traffic,between daily inspection on motorwaysand weekly visits on less busy roads.

All highway staff should be aware of thenecessity to remove dangerous itemswhenever they see them

Table 23.19 Amenity items

The structural needs of the road including resistance toskidding, surface irregularity, changes in traffic and usage of theroad and a 'maintenance' rating168 for, say, each 500 m length ofroad, requires to be derived. Computer programs have beendeveloped to handle maintenance ratings for an extensive roadnetwork.

23.13.2 Assessment of structural needs

Two methods of assessing structural needs are the CHART172173

and MARCH174 systems. Maintenance assessment surveys arerequired by the Department of Transport in the determinationof structural maintenance needs of motorways and trunk roads.The CHART and deflection surveys assist in the identificationof priorities for structural carriageway maintenance.

The CHART is a visual and computerized inspection methodof recording specific defects which, with the aid of a computermodel, evaluates the condition of the road and recommendstypes of treatment with priorities.

Deflection measurements of flexible pavements are primarilytaken by deflectograph, which is now used extensively onmotorway and trunk road surveys. It is supplemented as neces-sary by deflection beam measurements.175177

The deflection beam is usually used on short lengths of roadof 1 km or less. The deflectograph provides a more rapidmethod of measurement than the beam and is better-suited forroutine surveys of long lengths of road.

Skid resistance is checked in the UK using the sideways forcecoefficient routine investigation machine (SCRIM) developedby the TRRL. This provides a printout of sideways forcecoefficient (SFC) at 10m intervals along a road and can testabout 1500km per year.178 I8°

23.13.3 Aids to movement and safety

Items falling under this heading are the routine requirements forsigning, lighting and similar items and the winter maintenancerequirements which have to be met as they arise.

23.13.3.1 Amenity items

Items included under the general heading of the amenity func-tions of a road also have a safety element in that grass-cutting,tree-lopping, etc. ensure that sight-lines at bends and thevisibility of traffic signs are maintained.

23.13.3.2 Maintenance organization

The organization on the ground ranges from the outdatedindividual or lengthman method to the more cost-effectivemobile gang/team. The latter method utilizes mechanical equip-ment with specialist teams for surface dressing, patching ofcarriageways, road markings, lighting, signing, bridge mainten-ance, motorway maintenance, etc.

23.13.3.3 Winter maintenance

Setting-up a winter maintenance system involves determiningthe standards to be adopted and then examining the meteoro-logical data for the district so as to establish the period duringwhich the winter maintenance organization will be expected tofunction. The organization with its resources and plant willundertake de-icing, gritting, snow clearance and similar work.

23.13.3.4 Signing for highway maintenance

Signing for maintenance work on UK highways is covered in theTraffic signs manual.™ Information in the Traffic signs manualfor major roads is amended and supplemented by DepartmentalStandard TD 14/83.m This standard includes the TRW series ofdrawings which detail roadwork-signing requirements onmotorways and dual carriageways including contraflow trafficcontrol.

For minor roads (unclassified roads which form about half ofthe total road mileage in the UK), Department of TransportAdvice Note TA 47/85l82 deals with the control of traffic atroadworks on single-carriageway roads and is complemented by

Table 23.19 - continued

Element( ) Item

(2) Vegetation and detritus

Correction

This is material which can block drainagesystems which can obscure roadmarkings and cause dirty windscreens

Remark

Legal implications are important

Again rates of inspection and activity aredictated by traffic but where traffic isheaviest, cleaning and scavenging isdifficult. In rural areas carriagewaysweeping is not necessary more often thanat 2-monthly intervals.

In town centres daily attendance is required,reducing to weekly attention in residentialareas. This should include footpaths.

The rate of gully emptying is dependent onthe build-up of such material. In dryweather it may be necessary to top upgulleys with water to permit drainagetraps to operate. This can fit in withroad-washing operations

Department of Transport Standard TD 21/85183 concerningportable traffic signals at roadworks.

All signs must comply with the Traffic signs regulations andgeneral directions.53

Staffordshire County Council, with the assistance of elevenother county councils, have produced a useful 54-page bookletentitled Safety at roadworks.***

23.13.3.5 Staff for maintenance

The type of staff required for maintenance includes those:(1) for inspection, estimating and programming work; (2) forsuperintending day-to-day operations; and (3) for carrying outthe physical work, either by direct labour or contract.

The officer controlling highways in a local authority willencompass the whole range of operations: (1) provision of newhighways and bridges and their physical effects on the countrythrough drainage arrangements, earthworks, etc.; (2) the main-tenance of the roads, and its management in terms of control oftraffic; and (3) surveillance of operations of public utilities andof security generally. He will probably require a second-in-command over the whole range of interests if the total length ofhighways is in excess of about 3000 km. Figure 23.38 illustratesa typical organization.

The Report of the Committee on Highway Maintenance168

indicated that about 800 km of road would form a useful areaunit; this could be increased significantly if comprehensivesupport services were readily available.

The area surveyor will be responsible for all maintenance inthe area and will have to deal with some of the design problemsmet from time to time. It will be necessary to interpret surveydata provided as a basis for work programming and to deal withthe financial implications.

The area superintendent will need to organize labour andundertake ground surveys for work-programming purposes.The number of foremen must depend on the nature of the areaand of the men under their control, e.g. rural conditions varyfrom urban conditions. Many of the inspections will be carried

out by the foremen or inspectors and sometimes by members oftheir teams. The 'general services' available for maintenancework could include, for example: (1) a signs store, trafficsignals, etc.; (2) a plant depot; (3) a winter maintenance depotand salt stores; and (4) workshops. An 'emergency' team willcomprise a pool of skilled labour which can be used for specialoperations.

23.13.3.6 Alternative staffing structure

The Local Government Planning and Land Act 1980 requireslocal authority direct labour organizations to compete withcontractors in the private sector in tendering for highway workscontracts of maintenance and construction. Some local authori-ties have in consequence set up separate contracting organiza-tions similar to the public works contractor within the localauthority. The structure of such a highways department isdivided into client and contracting organizations. On the clientside, area surveyors/engineers with technical and administrativesupport staff work 'on the ground' and are responsible to themaintenance officer at headquarters. Contract works managerswith direct-labour resources and staff, responsible to the worksofficer at headquarters, operate in the field to the client'srequirements.

The contractors in these cases also provide the highwaysemergency services, including winter maintenance for the auth-ority. The Department of Transport provides facilities for theiragents.

23.14 Low-cost roads in developingcountries

23.14.1 Introduction

This section comprises only an outline guide to the basicprinciples of road design and construction in developing coun-tries. The nature of roads in these countries varies widely

Chief Officer

Deputy (when required)

AssistantNew Highways

Not developed inthis section

AreaSurveyor

AreaSurveyor

AreaSurveyor

AreaSurveyor

GeneralServices

AssistantHighway Maintenance

AssistantSurveyor

AreaSuperintendent

AreaForeman

AreaForeman

AreaSurveyor

EmergencyTeam

Figure 23.38 Typical operational structure for highwaymaintenance staff

depending upon the particular geographic, economic and politi-cal situation of the country concerned. Some countries followBritish design and construction practices, some follow practicesin the US and others those relating to yet other countries. Mostcountries will have developed road standards of their own, oftenbased on a blend of one or more of the above but taking intoaccount local conditions and requirements, availability of localmaterials and other local aspects, and it is essential that thedesigner should refer to the particular standards relating to thecountry in which he is operating. A publication produced forUNESCO185 covers all aspects of design construction and main-tenance of these roads.

Roads in developing countries have a number of commoncharacteristics which influence the approach to design andgenerally require a philosophy different from that in the de-veloped world. These characteristics include:

(1) Long lengths of road under consideration at one time.(2) Low traffic volumes.(3) Limited finance.(4) Traffic composition (ox carts, bicycles, overloaded lorries,

etc.).(5) Tropical climate and geology (intense seasonal rainfall,

lateritic soils, etc.).(6) Poorly trained workforce and limited availability of con-

struction materials.

The design and method of construction of a new road will varysignificantly depending on the above characteristics and also onconsiderations of cost-benefit analysis, funding, and strategicand political factors. A solution should be selected (whateverthe circumstances of a particular scheme) which is appropriateto the type and volume of traffic, geology, climate and availabi-lity of labour and machinery.

The construction of new roads in a developing country is avital element in opening up the countryside and boostingindustrial and agricultural development. Finance frequently isprovided, at least in part, by an external funding agency. Theeconomic return initially may be small in relation to the largecapital expenditure and therefore it is generally of prime import-ance to keep the construction cost to the minimum. This can beachieved in a number of ways:

(1) Phased development: it may be appropriate to construct agravel road in the first instance with the aim of adding asurfacing as traffic volumes and finance become available.

(2) Appropriate construction techniques: this may involvelabour-intensive techniques in countries with a large labourforce, but mechanization can often be a more appropriatesolution.

(3) Appropriate design standards: these in respect of highwaygeometry, drainage and construction.

23.14.2 Route development

Routes for roads will often follow existing tracks but where anew alignment is to be provided the following principles willgenerally apply:

( I ) A route should be chosen to avoid structural and drainageproblems.

(2) A route should be chosen to avoid excessive earthworks.(3) The alignment should be kept simple, as simple curves and

straights are easier to set out in the field. Land boundariesand property acquisition will not generally be a limitingfactor in developing countries.

(4) Where possible the vertical alignment should be slightlyabove ground level to aid drainage. Where this is not

possible drainage considerations are of paramount import-ance.

(5) The route should, where possible, be aligned near sources oflocally available construction materials.

Suitable mapping often does not exist or is unavailable and itmay be necessary to rely on aerial photography and walking theroute to fix a line.

The road geometry will vary depending on the volume andtype of traffic and the terrain. The cross-section, in general,should incorporate a verge at each side. This enables vehicles topull off the road and also protects the edge of the pavementconstruction from erosion and damage. Figure 23.39 showstypical cross section standards adopted in tropical countries andalso typical standards for low-volume roads.

23.14.3 Drainage

Water is the main enemy of road construction particularly intropical countries where rainfall is frequently very intense. Asalready stated, drainage of the road formation is of paramountimportance and is best achieved by setting the road slightlyhigher than the surrounding land and providing adequate side-drains as shown in Figure 23.40. Only in urban areas is positivedrainage appropriate.

Drainage should be simple. Side-drains will generally dis-charge directly into streams or on to surrounding fields. Side-drains of appropriate cross-section are easily maintained by theuse of a grader. Only on steep gradients need the ditch be linedwith stone pitching to prevent erosion.

Cross-drainage should be kept to a minimum but whererequired is best achieved by the use of simple concrete pipes orcorrugated steel structures. Complex reinforced concrete struc-tures are not appropriate in developing countries except forlong-span bridges.

A work by Watkins and Fiddes186 deals with rainfall runoffand highway drainage.

23.14.4 Pavement construction

Many different types of pavement construction are usedthroughout the world and the type chosen will depend on theforecast volumes of traffic, the finance available and otherfactors specific to the country in question.

The pavement in its simplest form will consist of an earthtrack, the existing soil being graded to a smooth runningsurface. Many tropical soils will perform well provided they arewell shaped and adequate drainage is provided.

A modified surface can be used if local soils are unsuitable orif traffic volumes demand a stronger road. This can be achievedeither by stabilizing the existing soil in situ to improve itsproperties or importation of another suitable soil from nearby.Imported soils of suitable characteristics for road surfacing aretypically natural gravels or crushed stones with a significant claycontent.

Existing soil can be stabilized by many techniques. Forexample, cement may be added in small quantities (4 to 5%) tostrengthen the soil, or lime can be added, which has the effect ofreducing the plasticity index. The quantities to be used must bedetermined by trials.

Single-size and wind-blown soils are notoriously difficult todeal with but have been successfully stabilized using bitumen;this is expensive and other organic products such molasses maybe appropriate in certain countries where such agricultural by-products are plentiful.

Gravel roads under heavy traffic flow require sealing toprovide improved riding quality and protect the surface fromthe ingress of rainwater and axle load damage. The simplest

Figure 23.40 The importance of good drainage, (a) Goodpractice; (b) bad practice. (After Watkins and Fiddes (1984)Highway and urban hydrology in the tropics. Pentech Press,London).

Good practice

Bad practice

Earth from roadwayand ditch

Water from ditch andsurrounding countrydraining on to road.Road deteriorates rapidly

Earth bladed fromside drainsformation kept drained.Road life prolonged

Figure 23.39 Typical cross-section of a low-cost road

F I at terra in

Rolling or hillyterrain

Mountainousterrain

Average daily trafficPaved width ^L1 (m)Shoulder width L2 (m)Design speed (km/h)Maximum grade %Shoulder width L2 (m)Design speed (km/h)Maximum grade %Shoulder width L2 (m)Design speed (km/h)Maximum grade %

>15007.33

10043

10061

807

Paved class500-1500

6.72-310052

80-10061

60-808

50-5006.12

100628071608

Grave/ class50-500 < 50

6.1 5.52 1

80-100 60-806 81 1

60-80 50-608 10

50-60 30-5010 12

Side drain ShoulderFlat terrain

Existinggroundlevel

0.6 m minimumSurface seal

BaseSub-base

Formation

0,6 m minimum

Stone-pitchedside drain

Mountainous terrainSide drainShoulderCarriageway

form is a bituminous seal.188 This can be upgraded to a doublesurface seal or a thin asphaltic concrete surfacing if the eco-nomic benefit can be shown. In some parts of the world, othermaterials such as waterbound macadams and penetration maca-dams, are employed. These can be appropriate if labour-intensive techniques are to be used.

Pavement design is undertaken by an empirical method aswith other roads using the Californian bearing ratio of thesubgrade in combination with the predicted number of standardaxles using the road within the design life. Figure 23.41 shows atypical design chart from Road Note 31 . l87 Care should be takenin design, since not only is the percentage of heavy vehicles likelyto be very high in a developing country but overloading iscommon practice. Careful traffic forecasting, including fieldmeasurement of actual axle loads, is essential to a realisticdesign.

If it is desirable to provide, at the time of construction, apavement capable of carrying more than 500 000 standard axles,the designer may choose either a 150mm base with a 50mmbituminous surfacing or a 200 mm base with a double surfacedressing. For both of these alternatives, the recommended sub-base thickness is indicated by the broken line. Alternatively, abase 150mm thick with a double surface dressing may be laidinitially and the thickness increased when 500000 standardaxles have been carried. The extra thickness may consist of50 mm of bituminous surfacing or at least 75 mm of crushedstone with a double surface dressing. The largest aggregate sizein the crushed stone must not exceed 19 mm and the old surfacemust be prepared by scarifying to a depth of 50 mm. For this

stage construction procedure, the recommended thickness ofsub-base is indicated by the solid line. Table 23.20 shows howrapidly the damaging power increases with increasing axle load.

23.14.5 Structures

Complex steel or reinforced concrete structures are generallyinappropriate for developing countries since their constructionwill often involve imported materials and resultant high cost.Also the necessary expertise in construction techniques andquality control may not be readily available. Expertise inmaintenance may also not be readily available and all structuresshould be designed with the ease of maintenance in mind.

The use of pontoon bridges, drifts or Irish bridges on low-trafficked routes may be appropriate. If a steel or reinforcedconcrete structure is unavoidable the designer should bear inmind the limitations of the contractor undertaking the work andthe design should be kept simple.

Again, many countries will have their own locally developedbridging techniques which suit local conditions; hand-dug cais-son foundations, for instance, are used widely in Asia.

23.14.6 Maintenance185 I89 I9°Where low-cost construction is used the road pavement isparticularly susceptible to rapid deterioration and good main-tenance is vital to prolong the life of the roads. Many mainten-ance activities must be carried out at frequent intervals.

Many loan agencies now put great emphasis on maintenance,

Cumulative number of standard axles in one direction (x 106)Figure 23.41 Pavement design chart for flexible pavements.(After Transport and Road Research Laboratory (1977) A guide tothe structural design of bitumen-surfaced roads in tropical andsub-tropical countries. HMSO, London)

Surfacing dressing

150 mm of base

Either50 mm of bituminous

surfacing and150 mm of base

or200 mm of base

with surfacedressing

Minimum thickness of sub-base of 100 mm to be used withsubgrades of CBR 8 to 24%. Material used in this zone tohave CBR of not less than 25%

Thick

ness

of s

ub-b

ase

and/

or s

electe

d fil

l (m

m)

since many low-cost roads built with financial aid have deterior-ated prematurely because of the lack of maintenance. Manyprojects now include the setting-up and running of an ongoingmaintenance management system as an integral part of theproject.

Since the cost to the road user rises significantly as thestandard of the pavement surface deteriorates, the economicadvantage of maintenance is very marked. This is particularlytrue of low-cost roads with light construction where lack ofmaintenance can lead to the rapid development of corrugations,ruts and potholes.

Acknowledgements

The author acknowledges the patient help and assistance of hiscolleagues in the Croydon and Winchester offices of Mott, Hay& Anderson in the preparation of this chapter, and in particularH. Williams, J. Prince and Mrs P. Corston.

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Table 23.20 Factor for converting numbers of axles to theeqivalent number of standard 8200 kg (18 000 Ib) axles

Axle load(kg)

910181027203630454054406350726081609970998010890117901270013610145201543016320172301814019070199802088021790

(Ib)

20004000600080001000012000140001600018000200002200024000260002800030000320003400036000380004000042000440004600048000

Equivalence factor

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