design of interurbain intersections

7/31/2019 Design of Interurbain Intersections

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Dcembre 2002

The design of interurbanintersections on major roads

At-grade intersectionsDecember 1998

Produced and disseminated by the

Service d'Etudes Techniques des Routes et AutoroutesCentre de la Scurit et des Techniques Routires46, avenue Aristide Briand - BP 100 - 92225 Bagneux Cedex - FranceTel : (33) 1 46 11 31 31 - Fax (33) 1 46 11 31 69

Translation :Chapters 1 et 2 : Kevin RILEY, Traductions Routes et Transports (Mrs-Erign - 49)Chapter 3 and appendices : Joe BARED, FHWA ( U.S. Washington)Note: The first chapters have been translated into European English, the third and appendices intoAmerican English.

Setraservice

d'tudes

techniques

des routes

et autoroutes

Traduction de guides techniques./ Translation ofTechnical Guides


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FOREWORD

This technical guide deals with the general design and geometry of at-grade intersections onmajor interurban roads. It sets out detailed technical guidelines on this topic which complement the

more general guidelines contained in the publication "Amnagement des Routes Principales".

It replaces two SETRA technical guides: "Les carrefours plans sur routes interurbains" whichdates from March 1980 and " Les Carrefours plans sur routes interurbaines - Carrefours giratoires"which dates from September 1984.

It covers neither intersections in urban areas - these will be dealt with in a document to bepublished by the CERTU ("Guide carrefours urbains") - nor signalized intersections which should notexist outside built-up areas.

"Interurban Intersection Design - At-grade intersections" is intended to be used by all bodiesresponsible for managing the road network. Indeed, most intersections are meeting points between

different networks for which different authorities are responsible.

The guidelines in this document should be considered as the rules of good practice the wholeengineering community.

This document was drafted by

L. DUPONT (SETRA) L. PATTE (SETRA)

P. BOIVIN (SETRA) P. FLACHAT (CETE de Rhne-Alpes) B. GUICHET (CETE de l'Ouest) J.Y. GIRARD (CETE de l'Ouest) G DUPRE (CETE de Normandie Centre)

Work was coordinated by the Direction d'Etudes Conception Routire et Autoroutire(Directorate for Road and Motorway Design Studies) under the supervision of J.M. SANGOUARD.

The drawings and diagrams are the work of G. LEPINE (SETRA) and J.Y. LEBOURG (CETE deNormandie Centre).

This document generated wide-ranging consultation which brought in engineering departments,Directions Dpartementales de l'Equipement (Dpartement Infrastructure Directorates) and expertsfrom the national engineering network. We would like to thank all those who provided comments andinformation.

The term "major roads" is applied in the ARP ("Amnagement des Routes Principales") to roadswhich perform a structural function within the national road network or the Dpartement road networks(the daily traffic on such roads generally exceeds 1500 vehicles).


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CONTENTS

uFOREWORD 5

uCHAPTER 1: GENERAL DESIGN 9

1. BASIC PRINCIPLES OF INTERSECTION DESIGN 10

2. PRELIMINARY STUDIES AND DATA 12

3. CHOICE OF THE TYPE OF INTERSECTION 16

uCHAPTER 2: STANDARD AT-GRADE INTERSECTIONS 25

1. GENERAL PRINCIPLES 27

2. DESIGN OF THE PRIORITY ROAD 37

3. DESIGN OF THE NON-PRIORITY ROAD 57(INTERSECTIONS WITH 3 OR 4 LEGS)

4. ROAD FURNITURE AND SIGNING 63

uCHAPTER 3: ROUNDABOUT INTERSECTIONS 67

1. GENERAL PRINCIPLES 69

2. GEOMETRY OF THE COMPONENTS OF A ROUNABOUT 79

3. SPECIAL ACCOMODATIONS 89

4. ROAD FURNITURE AND SIGNING 94

uGLOSSARY 99

uBIBLIOGRAPHY 107

uAPPENDICES 111

APPENDIX 1: THE SAFETY OF AT-GRADE INTERSECTIONS...113

APPENDIX 2: CAPACITY AND DELAY..117

APPENDIX 3: GEOMETRIC DELAYS AT INTERSECTIONS.121

APPENDIX 4: ESTIMATE OF THE V85 SPEED.123

APPENDIX 5: MEASUREMENT OF THE VISIBILITY CONDITIONS BY THE "STOPWATCHMETHOD"125

APPENDIX 6: DESIGN OF THE CURBS..127


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INTRODUCTION

uSCOPE

This guide deals with the design and construction of at-grade intersections on major roadsoutside urban areas and covers both upgrading of the existing network and the building of new roads.

As at-grade intersections are not compatible with roads that are isolated from their environment,this document does not deal with the geometric design of type L roads (freeways) or type T roads(express roads)1. However, the fundamental principles and the approach to be adopted when choosingthe type of intersection, which are explained in the first chapter "General Design", apply to all types ofintersections on all types of roads.

This guide does cover Type R bypasses2. However, it does not deal with ring roads; as thefunction of most of these is to link different districts they should be considered as urban roads.

Where buildings are either few in number or widely spaced (perhaps constituting a hamlet butnot a built-up area as defined in the Highway Code (art. R1)), the site will generally be classified as non-urban and the technical guidelines for rural areas will be applied.

Cross-town routes, irrespective of the size of the built-up area they pass through, are to beconsidered as urban roads. For these it is necessary to refer to the texts which deal with urban roads, inparticular the CERTU "Guide Carrefours Urbains". In addition, Chapter 7 of the A.R.P. states the generalprinciples that apply to the boundary between rural and urban areas and entrances to built-up areas andcontains some remarks concerning roads that pass through small towns and suburbs at the entrancesto large towns and cities.1

Apart from the very specific exception of the use of a roundabout to provide a "terminal" at the end of a Type T road.

2A bypass is defined in the A.R.P. as a non-urban road passing round a town and which is mainly used by through traffic.


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uCONTENTS OF THE GUIDE

This guide contains technical guidelines for the general design and the geometry of intersectionsand also enumerates the general principles which the designer should take into account.

However, it does not give detailed descriptions of technical features that are the subject ofspecialized guidelines. Although they are mentioned, particularly in the first chapter, preliminaryinvestigations and study techniques are not covered either. For these topics the text refers readers toother documents.

It takes into account the most recent developments and accumulated experience with regard tolevel of service and the interaction between the infrastructure and road safety3. Consequently, designrules and the descriptions of the conditions under which the various types of intersection should beused have been improved or clarified.

This new intersection guide has some major aspects in common with the ARP, andsupplements the recommendations in this. Essentially, these are as follows:

- a definition of technically coherent types of road which are clearly identifiable by drivers;

- operating speeds (V85)are used, in particular to apply visibility recommendations;the "legibility" of the road and facilities are considered;

- considerable importance is given to facilities with a high level of safety;

- acceptance that drivers have the "right to make an error", in particular by designing roadsides devoid ofobstacles and features which are likely to make the consequences of leaving the road more severe.

Consequently, this document modifies the documents it replaces in many important respects (seeForewordabove).

3

The SETRA-CETUR document "Scurit des Routes et des Rues" presents a survey of this knowledge.


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uSTRUCTURE OF THE GUIDE

A coherent design approach consists of a number of stages which can be divided into two mainphases:

in the first, the intersection is adapted to its surroundings. This is covered in Chapter 1, "GeneralDesign". This describes the data to be collected, the preliminary studies to be conducted and therange of facilities which are possible for each type of road. Finally, it states the factors which to beconsidered when deciding whether to construct a standard at-grade intersection or a roundabout;

the second phase is geometric design. This is covered in the second chapter for standard at-gradeintersections and in the third for roundabouts. These chapters both cover general features and thedetailed design of the components which form the intersection.

The principal technical terms used in intersection design are defined in a glossary. Its mainpurpose is to eliminate any terminological problems that readers may encounter when reading thisdocument.

Finally, the appendices contain additional information on safety, the 85th percentile speed andthe design of curbs.

uHOW TO USE THIS GUIDE

Considerations relating in particular to the safety, level of service and cost of facilities haveresulted in guidelines which deal with choosing the type of intersection, general configuration, sizing anddetailed geometry. In practice, a flexible approach should be adopted towards these, taking account oflocal constraints and accident occurrence etc.

In addition, the requirements for existing roads (to which extremely strong constraints mayapply) cannot possibly be as strict as for new roads. For the latter, the principles and guidelinescontained in this document should provide the basis for intersection design.

For existing roads, the design rules which apply for new constructions are to be considered asmedium-term or long-term objectives. Safety, based on a detailed analysis of accidents, should betaken into account when deciding which improvements should be given priority. However, prioritizationshould never interfere with an overall approach to design along a route.

It is generally advisable (for the sake of homogeneity, consistency and performance) to adhereas closely as possible to the standard layouts in this guide. However, merely doing this blindly will not initself guarantee that optimum performance will be achieved. Each intersection is a special case andspecific studies will be required in order to take account of the specific features of the project.


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CHAPTER 1

GENERAL DESIGN

u1. BASIC PRINCIPLES OF INTERSECTION DESIGN 10

1.1. GENERAL DESIGN APPROACH 10

1.2. BASIC PRINCIPLES 11

u2. PRELIMINARY STUDIES AND DATA 12

2.1. DATA 12

2.2. PRELIMINARY SAFETY STUDIES 13

2.3. BASIC TRAFFIC DATA 13

2.4. MEASURING VISIBILITY AT AN EXISTING INTERSECTION 15

2.5. SPEEDS 16

u3. CHOICE OF THE TYPE OF INTERSECTION 16

3.1. PRINCIPLES 16

3.2. TYPES OF ROAD 17

3.3. TYPES OF INTERSECTION 17

3.4. THE RANGE OF DESIGN OPTIONS ACCORDING 18TO THE TYPE OF ROAD

3.5. THE CHOICE BETWEEN TWO TYPES OF INTERSECTION 21(WHICH ARE COMPATIBLE WITH THE TYPE OF ROAD)


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To operate effectively an intersection must:

- be appropriate for the type of road on which it is located, for the site (environment etc.) andthe conditions of use (types of traffic, etc.);- have satisfactory geometric design (both of general features and detail).

This chapter deals with the general design of intersections and provides an overviewof the first of the above points, i.e. the main principles to be followed, and describes theprocess which is to be used to select the type of intersection.

It gives a detailed list of data to be collected and preliminary studies to be performed.This demonstrates the importance of ensuring that an intersection1 is matched to itsenvironment - the type of major road, the nature of the so-called secondary road or roads, thesite, the types of traffic and the exchanges it is intended to accommodate.

1. BASIC PRINCIPLES OF INTERSECTION DESIGN

1.1. GENERAL DESIGN APPROACH

A consistent design approach includes the following stages2

:

identifying the type of road3 involved (or rather the types of roads involved). To makesuch an identification, studies (or perhaps even a planning decision) are required toestablish: (i) its hierarchical position in its network; (ii) the type of road it most resembles(before construction or after upgrading). If this preliminary work at network level has notbeen performed, appropriate studies should be undertaken to determine the type of themajor road (see 3.2.);

establishing a range of design options (types of intersection or interchange). This "rangeof design options" includes all the solutions which are compatible with the type of road in

question (see 3.4).1

Sometimes, rather ambiguously, referred to in French as a local facility.

2This approach is not completely sequential; in particular, data collection can be performed simultaneously

with several phases (data is necessary at the outset and some options or decisions can give rise to

additional data collection in the course of the design process.

3The road typology used in this guide follows that of the ARP. In particular, it has been constructed with

reference to technical rather than administrative factors.


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collecting and analyzing information concerning the site (topography, environment,visibility obstructions, layout of existing road or roads) etc. and its operation (accidents,traffic etc.) (see 2: "Preliminary studies and data");

choosing a type of intersection from among the range of design options;

geometric design as such; this phase can be divided into three parts:

- determining the general configuration (for example, provision of a left-turn lane, positioning ofthe legs of a roundabout, etc.);

- detailed design and dimensioning;

- checks on visibility, capacity and general adequacy (simplicity, compactness etc.), whichcan bring into question planned measures or earlier decisions.

1.2. BASIC PRINCIPLES

Furthermore, all stages of intersection design, whether relating to general design orgeometry, must take account of the following basic principles:

it must be compatible with the type of road and the behaviors this generates;

it must fit in with the rationale of the route (uniformity of facilities, contribution to thecontinuity or division of the major road);

the facility must be legible allowing drivers to recognize the intersection they are enteringeasily, quickly and with certainty;

safety must be optimized for all traffic flows, including very secondary ones;

it must provide free flow for priority traffic;

it must take account of specific groups of drivers (pedestrians, two-wheeled vehicles,

public transport, exceptional vehicles, etc.).


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2. PRELIMINARY STUDIES AND DATA

2.1. DATA

The data below should generally be taken into account during design work for a newintersection or upgrading an existing one. However, systematic data collection should not beconducted in all cases. Furthermore, this is not an exhaustive list.

The most important data are the following:

the position of the intersection on the roads involved, the position of these roads in thenetwork to which they belong, the nature and status of these roads;

the characteristics of the site: topography, road layout, environment (constructions,vegetation, crops, etc.), any visibility obstructions to be taken into account;

use and operation of the intersection: traffic (volumes, nature, movements, etc.),saturation phenomena, specific activities at the site (for example a diner, service station,etc.), location of a bus stop, observable or predictable behaviors (as is the case), and, in

particular, operating speeds on the approaches to the exchange zones;

accidents: number, types and processes (data only available for existing intersections).This data provides the main basis for the safety diagnosis that must systematically beconducted before intersections are modified. In the case of intersections on newinfrastructure, the accident risk can be estimated using "predictive" models, on the basis ofthe traffic and the characteristics of the intersection.

The sections which follow (2.2 to 2.5) provide additional information about thecollection and analysis of some types of basic data (i.e. accidents, traffic, measurement of

visibility conditions, speeds).


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2.2 PRELIMINARY SAFETY STUDIES

In the case of modifications to an intersection which do not specifically target safety,

but which nevertheless aim to improve or at least not to worsen safety, the safety objectiveshould be clearly identified at the same time as the level of service objective (free flow,comfort, etc.).

It is always advisable to conduct a preliminary safety study. This includes: (i)identifying safety measures on the basis of a diagnosis, and (ii) an a prioriassessment of theanticipated effects and any side effects. In the case of adverse side effects, additionalmeasures may be taken (for example the scale of the project can be increased) or themodification can be abandoned.

With regard to this, reference will be made to the methodological guide "Etudespralables des interventions sur l'infrastructure" (SETRA, 1992). Use can also be made ofthe "SECAR" software system for analyzing the safety of non roundabout at-gradeintersections (SETRA; CETE de Normandie - Centre)4 .

2.3 BASIC TRAFFIC DATA

What constitutes essential traffic data depends on whether the project involvesmodifying an existing intersection or designing an intersection on a new road.

2.3.1. INTERSECTIONS ON EXISTING ROADS

In addition to the volume of traffic on the major road, which is generally known, thetraffic volume on the minor road(s) must be measured.

If accidents involving left turning traffic have occurred, or if there seems to be a highproportion of exchange movements, the volume of left turning traffic should be assessed(even if it appears to be small).

However, when traffic volumes are such that capacity problems (long delays for somemovements) are deemed likely, the volumes of all traffic movements must be estimated, forexample when there is heavy traffic on the minor road or very heavy traffic on the major road(even with light traffic on the minor road). When the possibility of periodic congestion cannotbe completely ruled out more thorough knowledge of peak hourly flows5 must be acquired.4

See SETRA Note d'information No. 113 (srie circulation, scurit, exploitation); March 1998.

5The evening peak is generally involved. In some cases the reverse peak can provide useful additional

information; occasionally, a weekly peak hour, or even a seasonal peak can be considered, particularly when

one or other of these is exceeded for at least thirty hours during the year.


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The graphs below show a few criteria for determining the traffic studies that should beconducted. These are given for guidance only6, and are above all intended to warn theengineer that such studies are necessary beforehand.

Fig 1 - A few criteria for the level of traffic studies that should be performed (rural roads with 2 or 3 lanes)

Four leg intersection

0

1000

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0 5000 10000 15000 20000

Traffic on major road (AADT)

he minor

ADT)

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3

Three leg intersection

0

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7000

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0 5000 10000 15000 20000

Traffic on the major road (AADT)

Traffic on the minor road(AADT)

3

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Normally no capacity study conducted

- The traffic levels may cause congestion problems; it is advised to carry out a check on the basis of thedistribution of traffic flows (and perhaps also peak hourly flows).

Periodic congestion is highly likely; a detailed study using directional peak hour data is essential.

2.3.2. INTERSECTIONS ON A NEW ROAD

In addition to the traffic on the major road, traffic on the roads crossed by the new roadmust be measured, and forecasts must be made of the future traffic on these roads once thenew road section has been opened and, in spite of the difficulties involved, of all the turningmovements too. More detailed investigation of peak hourly flows may be conducted,especially when daily traffic volumes mean that the possibility of periodic congestion cannotbe ruled out.6

These graphs are, in particular, based on simple hypotheses about the peak hour, the distribution of turning

movements etc.


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2.3.3. TRAFFIC DATA COLLECTION METHODS

Essentially, the following traffic data collection methods are available for existingroads:

For comprehensive counts:

- counts at permanent counting stations (in particular SIREDO stations) - these techniquesare usually limited to the structural network;

- automatic counts using temporary counters (often pneumatic tubes) which are installed forone or more periods during a week, with the results then extrapolated for an entire year;- manual peak hour counts, extrapolated to give the annual daily average.

With regard to these last two points, where traffic data is incomplete, details of the

methods for estimating the annual average daily traffic (AADT) and additional information aregiven in the technical appendix of the "Circulaire relative aux recensement de la trafficroutire".

Directional traffic counts use the same techniques as comprehensive counts, butthere is no method for extrapolating non-permanent hourly counts to give an average hour oran average peak hour.

2.4. MEASURING VISIBILITY AT AN EXISTING INTERSECTION

The design of a safe project generally involves checking the entry approach visibility(see 2.1.), and the crossing visibility for vehicles on the non-priority road (see Chap.2, 1.2.1).Of the two, the second generally generates the most constraints. It is expressed either interms of a visibility distance or a crossing time. Several methods can be used to ensure thatvisibility conditions is satisfactory.

a)The direct (or so-called "stopwatch") method

This method can only be applied to existing intersections and consists of measuringthe actual intersection crossing time in situ. It is convenient in that it uses the speeds and

visibility distances that are involved in the problem of visibility. Instructions are given inAppendix 5.

b) The indirect method

The method requires knowledge of the operating (V85) speeds of vehicles approachingthe intersection (see 2.5). The required sight distances depend on these (see Chap. 2, 1.2.2.)and can be compared with those actually provided (because of lateral masking, road layout,etc.). In the case of an existing road, visibility distances are measured in situ. Otherwise, theyare estimated a priori, using the plans.


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2.5. SPEEDS

In order to evaluating an intersection's operating conditions and ascertain the sightdistances that are required, measurements of real speeds or forecasts of speeds areessential.

Generally, speeds are measured, in accordance with international practice, using the85th percentile speed (V85), which is the speed below which 85 percent of drivers travel underfree-flow (uncongested) conditions. The V85 speed can be estimated experimentally ortheoretically (see appendix 4).

In the case of an existing road, the 85th percentile speed can be deduced from thedistribution of observed speeds. Measurements are usually made using one of the varioustypes of vehicle detectors or hand-held radar equipment7 . The latter offers operationalflexibility and is able to discriminate easily between free-flow and impeded vehicles, but isdifficult to use over long periods.

3. CHOICE OF THE TYPE OF INTERSECTION

The measurement period or periods must be representative, and a sufficient numberof individual measurements (of free-flow vehicles) must be made. 8

The V85 speed can also be estimated with formulae that take account of the mainfeatures of site geometry (cross-section, horizontal alignment, longitudinal profile, etc.). The

"DIAIVI software (SETRA) can also be used to estimate the operating speed at each point of aproject.

3.1. PRINCIPLES

An intersection must belong to a readily identified type: the operation of an intersectionwhose configuration is too individual is generally poorly understood by drivers and suchintersections frequently cause accidents. Following the instructions set out in Chapters 2 and3 will generally ensure that an intersection belongs to a common, readily identified, type.

The types of intersections installed on a road help to make it clear to drivers what type

of road they are on. When intersections are too varied or inconsistent they are a source ofharmful ambiguity. For example, the construction of interchanges on an ordinary major roadgenerates behaviors downstream which are incompatible with the operating conditions of theroad in question (because of frontage access, at-grade intersections, etc.).7

The document "Mesure des vitesse et ses applications" (SETRA 1997) lists available methods for measuring

vehicle speeds.

8Irrespective of the representativeness of the measurement period (and the accuracy of measurement device),

about 25 measurements are required to estimate the V85 speed to an accuracy of 10 km/h, and 70

measurement to estimate it to within 5 km/h.


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Furthermore, drivers on a certain type of road expect certain types of intersections.For example, drivers on an express road, where there is normally no frontage access andwhere intersections are grade-separated, would be unprepared for frontage access or an at-grade intersection and fail to react appropriately and rapidly if a non-priority vehicle were tocross the road.

Finally, the manner an intersection operates must be compatible with the operatingconditions of the type of road on which it is installed. For example, the difficulty of non-prioritymovements (crossing or left turns) on a standard at-grade intersection is incompatible withthe width and speeds that exist on some roads (for example divided highways).

It is therefore necessary to ascertain that the selected type of intersection iscompatible with the type of road on which it is to be installed. The type of intersection must becompatible with the specific conditions of the site and its operation (traffic, use, closeness toa built-up area, transition between two types of road, safety problems, etc.).

3.2. TYPES OF ROAD

The document "Amnagement des Routes Principales" identifies the following typesof interurban road:

roads which are isolated from their environment, which include

- "freeways" and similar roads (type L): these are divided highways with no at-grade accessor intersections;- "express roads" (type T): these have a single roadway and no at-grade access points or

intersections

other rural major roads:

- divided type R roads ("interurban major roads"): there may be access to roadside plots,which are usually not built on (but traffic is not allowed to cross the central reservation), andat-grade intersections (roundabouts or partial intersections only);

- undivided type R roads (known as "multifunctional roads"): provision of access to roadsideplots, which are usually not built on, presence of at-grade intersections, single roadway;

secondary roads in rural areas: the same characteristics as undivided type R roads, but

with less traffic (rarely more than 1,500 v/d) and smaller width (generally less than 5 m to5.50 m).

3.3. TYPES OF INTERSECTION

As with the types of road, the classification of intersections into homogeneous groupsof designs must favor the clearest possible perception by drivers. Here too, the appearance ofan intersection must suggest a mode of operation which is readily and rapidlyunderstandable. In practice, classifying intersections into major technical families (on thebasis of the shape and type of operation) means this requirement can be met satisfactorily


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From the outset, two large groups of intersections can be identified:

grade-separated intersections or interchanges, in which exchanges are separatedfrom each other and managed away from the major roads (in order to limit secant conflictsto the maximum extent);

at-grade intersections in which all the exchanges between the roads take place on thesame plane. These are of two main types: standard at-grade intersections and roundaboutat-grade intersections.

A "partial intersection" is a standard at-grade intersection where the centralreservation is physically closed. It only permits right turning movements (from the major roadinto a minor road or vice-versa).

3.4. THE RANGE OF DESIGN OPTIONS ACCORDING TO THE TYPE

OF ROADThis results directly from the principles set out in section 3.1. Grade-separated

intersections create a powerful impression of a road which is isolated from its environment;there should be no at-grade intersections on these roads and, apart from a few exceptionalcases, grade-separated intersections should only be used on these roads. In contrast, at-grade intersections are the normal type of design on roads which have a close connectionwith their environment. The choice of the type of at-grade intersection (roundabout orstandard) depends above all on the volume and distribution of traffic and the number ofexchanges which exist or will be created on the roads involved.

3.4.1. TYPE L ROADS

The only possible type of intersection is an interchange (grade-separatedintersection). If an interchange isnot built, the minor road may be grade-separated without anyexchange, transfered to an adjacent intersection or simply closed.

A type L road terminates with either a "provisional end of freeway" design or aconnection with another freeway (interchange) or by a transition to an urban freeway.

3.4.2. TYPE T ROADS

"Type T roads are designed with the objective of providing a high level of service and favoringlong-distance traffic. Therefore, the design of intersections and access points should attemptto limit disruption and speed loss caused by exchange movements between the road and thesecondary roads or the road's environment. It is therefore logical to prohibit frontage accessand grade-separate all points of exchange and crossings. The retention of a small number ofstandard at-grade intersections is incompatible with safety. Roundabout intersections, whichare incompatible with the high level of service objective, can only be considered as aprovisional or permanent "terminal" option at the end of the road, in particular at the entranceto a built-up area (the only other satisfactory arrangement at the end of such a road being a"provisional end of freeway" design with the driver leaving via a deceleration lane and atransition road before joining the ordinary road system)."

(Amnagement des Routes Principales)


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If a section of existing road is converted into a type T road, the exchange and accesspoints must receive appropriate treatment: grade-separation (with or without exchange oftraffic) or removal of the intersection and transfer to a nearby intersection, removal of frontageaccess, provision of access to and creation of a parallel road system (for traffic which is notauthorized to use the type T road).

Otherwise, if the necessary funds are not available, a possible alternative is toupgrade only a clearly marked sub-section, in a completely coherent manner, in accordancewith what has been stated above (see A.R.P. 1.2.b).

Under no circumstances is it acceptable to grade separate the principal access pointswhile deferring the other measures (grade-separation or removal of other intersections,removal of frontage access, provision of access and parallel roads).

Table 1 - Suitable alternatives for type T roads and the general conditions for their use.

Alternatives General conditions for use

Removal of intersection (transfer to a nearbyroundabout or interchange)

Low traffic on minor road, or creation of aparallel road system

Grade separation without exchange Mainly through traffic on minor roadsGrade-separated intersection (interchange) Heavy traffic, considerable exchange

Exception: a roundabout at the end of a Type T road.

3.4.3. TYPE R ROADS

"Type R roads carry long-distance traffic without favoring this over shorter-distancetraffic for which access and ease of exchange movements are important. Frontage access,generally to non built-up land, can be allowed (except when safety criteria are not met, forexample when visibility is inadequate) and there should be a large number of exchangepoints. The most appropriate intersections for these constraints are at-grade, eitherroundabout or non-roundabout (4-way intersection or T-intersection). A grade-separatedintersection can be justified in exceptional circumstances (when a roundabout is saturated,etc.), as can removing or displacing an intersection (in the event of safety problems

associated with its location). "(Amnagement des Routes Principales)

Furthermore, on type R divided highways, at-grade non-roundabout intersectionsshould systematically consist of partial intersections. Roundabouts should always be installedat sites where all the exchanges are brought together.


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Table 2 - Suitable alternatives for type R roads and the general conditions for their use.

Alternatives General conditions for use

Roundabouts Relatively high traffic on minor road, or asafety problem

At-grade intersection (4-way intersection, T-intersection

In the other situations;not acceptable on 4-lane divided highways

Specific case: partial intersection 4-lane divided highways

Exception: grade-separated intersections (with specific justifications - saturation of a roundabout etc.).

Type R bypasses (a special case)

In view of the specific problems affecting these roads as regards the safety ofstandard at-grade intersections (the amount of crossing traffic, frequently unfavorablelocation), the measures to be taken are as follows:

- roundabouts should be constructed at major exchange points (terminal intersections, and

possibly at a central point);

- minor intersections should be modified, either by elimination and transfer to an adjacentintersection, or, if crossing traffic is relatively heavy, by grade separation with no exchange,with costs kept to a minimum;

- no frontage access to ensure the road retains its primary purpose of carrying through-trafficand to avoid urbanization which would result in an ambiguous identity.9

3.4.4. MINOR RURAL ROADS (WHERE NO MAJOR ROADS AREINVOLVED)

The recommended types of intersections are as follows:

- standard at-grade intersections, with nearside priority or a "STOP" or "YIELD" priority rule.10

- roundabout intersections (generally small) for slightly larger intersections9

In the case of a high-traffic road, this is compulsory (see article L152-1 of the "Code de la voirie routire").

10The systematic (or quasi-systematic) giving of priority to a minor road is generally not recommended.


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3.5. THE CHOICE BETWEEN TWO TYPES OF INTERSECTION

(WHICH ARE COMPATIBLE WITH THE TYPE OF ROAD)

3.5.1. GENERAL CONSIDERATIONSThe type of intersection for a given site must not be chosen without reference to the

general design policy for intersections along the route. 11 The consistency of the facilitiesalong a route is important and should be achieved immediately in the case of a new road andtaken into account, at least as a long term aim, in the case of existing roads.

On type R major roads, the choice of a type of intersection (from among the possibleoptions) is usually between the two types of at-grade intersections (standard orroundabout).12 However, the choice of facility only takes this form when the other possiblesolutions (removal of the intersection, grade separation with exchange, for example) havebeen rejected.

The choice is based on local conditions at the site. It may be assisted by multicriteriaanalysis, taking not only safety into account, but also user costs, financial analysis, costbenefit appraisal, etc. 13

In the case of a new construction, the decision will frequently be based on generalknowledge about how a given type of intersection affects the number of accidents, delays etc.At a site with an existing intersection, a safety diagnosis is an essential basis for decision-making. In particular, there is no need to transform an intersection into a roundabout when noaccidents have occurred there.

The safety and journey time benefits depend mainly on the traffic on the major and

minor roads, in particular the nature of through and exchange traffic. This must be known inorder to make an informed choice.

3.5.2. MAIN DECISION CRITERIA

a) Safety

This is a priority criterion. On a major road, a roundabout is safer than a standard at-grade intersection, accidents are generally less frequent and less severe. Informationconcerning the performance of the two types of at-grade intersection is given in Appendix 1.

It should nevertheless be noted that improving a standard at-grade intersection (byadding a left-turn lane or an island on the minor road, for example) can lead to considerablesafety improvements (sometimes at moderate cost). Furthermore, there are normally very

few accidents at grade-separated intersections or partial intersections.11

This general decision may have been made already, for example in the context of the policy for a network with

an established hierarchy.

12The choice between two types of interchange is not deal t with in this document which is concerned with at-

grade intersections.

13See the "Instruction modificatrice provisoire" of 28 July 1995 concerning the methods for evaluating road

investments in rural areas (Direction des Routes).


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b) Cost

The costs of at-grade intersections vary greatly depending on local conditions, theextent to which the existing pavement is reutilized (in the case of reconstruction), the amountof road furniture installed, the construction of feeder roads, etc. Some aspects of design(illumination, landscaping, choice of materials, etc.) can considerably increase the cost of aproject. Furthermore, operating costs (maintenance, electricity consumption if applicable)must also be considered.

However, the cost of improving a standard at-grade intersection is frequently muchlower than the cost of building a roundabout, but it should not be forgotten that limiting thedimensions of a roundabout considerably reduces its cost.

c) Delays

This criterion is also important on roads that carry long distance or medium distancetraffic (which although rarely predominant can be deliberately favored). Local traffic must alsobe considered on roads of secondary importance.

Delays are, basically, of two types, the relative importance of which depends on thetraffic at the site:

- traffic delay (also known as congestion delay). This is due to not having priority andinteractions between vehicles. It can be considered as being the time spent in a queue and atthe front of a queue.

- geometric delay. This is the delay experienced by a vehicle when crossing the facility,when not impeded at all by traffic. This exists because an intersection forces some trafficstreams to slow down. 14

Delays are usually negligible for rural roundabouts. If this is not the case there isprobably a capacity problem which may be detected by the GIRABASE software.15 Figure 2below illustrates the field of application for roundabout intersections with regard to traffic, in

particular their capacity limit.

14

In the case of a roundabout too, drivers need to negotiate the central reservation which makes their trajectory

slightly longer than a straight line, but the corresponding delay is small, at least for the dimensions

recommended in this document (see Chap. 3).

15Produced by the CETE de l'Ouest. In addition, this also estimates delays.


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Fig. 2 - Field of use of roundabout intersections with regard to traffic.

For a 2- or 3-lane type "R" road

0

2 000

4 000

6 000

8 000

10 000

12 000

14 000

AADT on minor

road

AADT on majorroad

For a 4-lane divided type "R" road

0

2 000

4 000

6 000

8 000

10 000

12 000

14 000

16 000

AADT on minor

road

AADT on major

road

The light area represents a reserve capacity of more than 30%. Delays are generallylow.

The dark area represents a reserve capacity16 of between 10% and 30%. Delays canbecome very long in certain cases

Above this, one entry is likely to be saturated.

However, all the users passing through the roundabout are subjected to geometricdelay. The duration of this will vary according to the site. Its average value is 12s for lightvehicles (further information on geometric delay is given in Appendix 3).16

See Appendix 2.


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The delay experienced by non-priority drivers at a standard at-grade intersection isgenerally greater than at a roundabout. It can be estimated using the OCTAVE software17 ,but only become really high when demand on the minor road approaches capacity. Undersuch circumstances, the traffic levels would normally justify the construction of a roundaboutintersection on safety grounds. Figure 1 in section 2.3.1. gives an approximate idea of thecapacity limit of a non-roundabout at-grade intersection.

Geometric delay mainly affects the minor road. It is of the same order of magnitude asfor roundabouts, but tends to be slightly longer (see Appendix 3).

d) Overall consistency between the facilities along the road

In addition, an intersection should always be examined in the context of acomprehensive consideration of an entire section of a route. Without this, genuineoptimization of an investment program is impossible.

It should be understood that an excessive frequency of roundabout intersections along

one route may cause disruption, particularly on the largest roads, by considerably reducingthe level of service provided to through vehicles. In certain specific cases, it can even becounter-productive (by causing traffic to transfer to less safe or less suitable routes, forexample).

3.5.3. ADDITIONAL CRITERIA

Other site characteristics can also weigh in favor of a certain type of facility: thus, thefollowing factors encourage construction of a roundabout:

- the need to mark the boundary between two types of roads (between a divided highway anda single roadway, between a rural road and an urban or suburban road, etc.), to enabledrivers to identify the type of place they are entering and modify their behavior accordingly;

- a situation at the entry of a built-up zone: the "gateway" effect encourages a change indriver behaviors and attention levels which are beneficial to safety in built-up areas; in additiona roundabout provides a functional or aesthetic way of treating public space;

- a large number of legs (>4) which makes it difficult to design an acceptable standard at-grade intersection;

- concerns about access, as a result of which turning movements can be facilitated to thedetriment of journey times on the road;

- more specifically, the need to provide opportunities for turning back, in particular on 3-laneroads (where turning left towards accesses is always a difficult maneuver, particularly fortrucks) or on divided highways.

However, these criteria do not remove the need to consider the principle selectioncriteria (see 3.5.2.). On their own they are not sufficient to justify the construction of aroundabout. In particular, a roundabout is unnecessary when traffic demand on the minorroad is insignificant.17

The OCTAVE software (SETRA, 1998) deals with the capacity of unsignalized intersections.


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CHAPTER 2

STANDARD AT-GRADE INTERSECTIONS

u1. GENERAL CONSIDERATIONS

1.1 DESIGN PRINCIPLES

1.2. RULES OF PLACEMENT

u 2. DESIGN OF THE PRIORITY ROAD

2.1. HORIZONTAL ALIGNMENT AND LONGITUDINAL PROFILE

2.2 THE NUMBER OF TRAFFIC LANES

2.3. FACILITIES TO ASSIST LEFT-TURNING MOVEMENTS

2.4. CROSS-SECTION WHERE THERE IS A CENTRAL FACILITY ON THEPRIORITY ROAD

2.5. SPLITTER ISLANDS (OR CENTRAL RESERVATION)

2.6. FACILITIES FOR RIGHT-TURNING MOVEMENTS FROM THEMAJOR ROAD

2.7. ACCELERATION LANES ON A MAJOR ROAD

2.8. NARROW ROADS

u3. DESIGN OF THE NON-PRIORITY ROAD

(INTERSECTIONS WITH 3 OR 4 LEGS)

3.1 LAYOUT OF THE SECONDARY ROAD

3.2. SPLITTER ISLANDS ON THE NON-PRIORITY ROAD

3.3. ENTRY AND EXIT LANES

3.4. EXTREMELY MINOR NON-PRIORITY ROADS

3.5. PARTIAL INTERSECTIONS ON DIVIDED HIGHWAYS

u4. ROAD FURNITURE AND SIGNING

4.1. SIGNING

4.2. SAFETY BARRIERS (RESTRAINING DEVICES

4.3. ILLUMINATION


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This chapter deals with the construction and geometric design of non-roundabout at-grade intersections, known as standard at-grade intersections.

Standard at-grade intersections provide a lower average level of safety than othertypes of intersection (roundabout, grade-separated). The priority in their design must thereforebe to maximize safety - capacity problems are comparatively rare in rural areas.

This chapter lays down guidelines to enable this objective to be achieved (while at thesame time adapting the facility to the type of traffic) and also states the rules and parametersfor constructing and sizing intersection components.

Fig 1 - Principal components and parameters of a standard at-grade intersection.

Entry radius (Re)Exit radius (R s)

Entry lane

Minor leg

(non priority)

Nez dlot

Width of minor leg (l )

Exit lane

Island ti s

Major leg

( priority)

Left-turn lane

Island nose

S litte risland

Through lane

On major type R roads, standard at-grade intersections normally involve theintersection of two (or more) roads belonging to networks with different positions in the roadhierarchy.1 This means that the intersection consists of one "major" road which must have priority andone or more "minor roads" which can also be described as "non-priority" roads.1 A roundabout is normally constructed at at-grade intersections between two major roads which belong to networks with

the same position in the hierarchy.


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1. GENERAL CONSIDERATIONS

The conceptual and geometric design phases of standard at-grade intersections musttake into account the basic principles enumerated in Chapter 1. This essentially involvescompatibility with the type of road, integration with the rationale of the route, legibility of thefacility, optimization of safety, provision of a high rate of flow for priority flows and takingaccount of specific types of road users.

In addition to these fundamental principles which apply to intersections of all types, theconstruction and design of a standard at-grade intersection requires the following specificprecautions.

With regard to the route:- the number of conflict points (i.e. the number of intersections on the major road) must belimited;

- there must be an adequate distance between two successive intersections. If not,intersections must be grouped together to form a single facility (which means the objective inthe preceding paragraph is also attained);

- special attention must be given to changes in the type of intersection or the priority rule alonga route.

Upstream of the facility:

- the geometry or the environment should be modified to encourage speeds which areappropriate for the type of facility and the priority rule;

- surroundings which assist good legibility.

In the approach to the intersection:

- satisfactory entry approach visibility of conflict points;

- visible and legible advance signing which informs drivers in the clearest possible terms ofthe type of facility they are about to encounter and the priority rules which apply there(directional and priority signing).

Within the intersection:

- satisfactory crossing visibility;

- the use of certain types of intersection which are compact, straightforward, tested andrapidly identifiable and whose operation is well understood by drivers: T-intersections or 4-way intersections (in addition to roundabouts);

- homogeneous geometrical features along a route; with facilities which comply as closely aspossible with the standard layouts described in the sections below;


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- features which assist good legibility (it is generally only necessary to comply with the rule ofsimplicity to achieve this);

- the simplest possible signing, which is consistent with the layout and placed where it is

clearly visible (particularly in the case of directional signing).

Fig. 2- Principal recommended and not recommended layouts for non-roundabout at-grade intersections.

1. Recommended

2. Not recommended (examples)


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Several types of intersection must be rejected because they do not comply with theseprinciples and frequently cause accidents:

- "bulb" type intersections that are too large and rather complex, in which non priority trafficflows too freely;

- Y-intersections which often have similar shortcomings and which suffer from a degree ofambiguity; these should be replaced by either a T-intersection or a roundabout depending onwhich is more appropriate.

- intersections in which there is a "left turning transition lane coming from the right", theoperation of which is ambiguous (left turning movements are unusual at at-gradeintersections);

- many other atypical types of intersection, frequently large, where the large number of islandsand transition lanes confuses drivers.

In addition, intersections with a nearside priority rule must not be allowed on type Rroads, because there is a risk they will not be understood by drivers on the major road, whohave generally had priority for long distances upstream of the intersection. Such intersectionsare prohibited on roads which are classed as trunk roads.

Signalized intersections, which may surprise drivers and have a poor safety record,should also be ruled out in rural areas. It is generally advantageous to replace them byroundabout intersections, even in suburban areas or on cross-town routes.

To apply these principles of layout and design we need to distinguish between:

new roads: the principles set out above provide the basis for the design of planned

intersections;

existing roads: priority should be given to facilities which improve safety, and to a lesserdegree capacity. Any plan to reconstruct an intersection should be preceded by accidentand traffic analysis (see Chap 1). The design rules given in the remainder of this Chaptershould be considered as general guidance for improvements to existing roads.

1.2. RULES OF PLACEMENT

The four sections below set out the design considerations that apply to standard at-grade intersections. They cover visibility, legibility, the distance between intersections and the

limitation of the number of intersections

1.2.1. VISIBILITY

a) Visibility requirement at an intersection

For safety reasons, drivers waiting on the minor road or at an access point must haveenough time to see whether there is a vehicle on the major road, decide to perform a crossingmaneuver and start and complete it 2 before the arrival of a priority vehicle that was maskedto begin with.

2 This includes maneuvers to cross the intersection and merge into the traffic on the major road.


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Drivers turning left into the minor road must be provided with a similar length of timewith regard to opposing traffic on the major road.

The time required to cross the priority road, known as the "crossing time", naturallydepends on its width.

Table 1: Crossing time3according to the width of the crossed road and the priority rule (to be taken into

account when calculating the visibility distance).

Cross-section of major road

2-lanes2 lanes

+

left-turn lane

4-lane divided :merging from the

right

at partial

intersections

STOPrecommended time 8 s 9 s 8 s

absolute minimum 6 s 7 s 6 s

YIELDrecommended time 10 s 11 s 9 s

absolute minimum 8 s 9 s 7 s

Left-turn lanerecommended time 8 s

into minor road absolute minimum 6 s

N.B. These times should be increased by 1 s when access is via an upward grade of more than 2%,which should, furthermore, be avoided (see 3.1.3.).

It must also be ensured that approaching vehicles have adequate visibility of the nosesof splitter islands on the major road and the secondary legs. This generally constitutes less ofa design constraint than the crossing time condition, and is set out in 2.1. for the major road.

b) Provision of visibility

This relates to the intersection crossing time and involves clearing a sight triangle foreach conflict between two traffic streams: there must be no visual obstruction within thistriangle.

The triangle is located 1 m above a plane which passes through the centerline of both

roads. Its corners are located as follows: (i) the conflict point between the two traffic streamsin question, (ii) an assumed observation point on the non-priority road beyond which a drivermust be able to see a vehicle traveling on the major road, and (iii) an observed point on themajor road. These elements vary depending on the priority rule (see Fig. 4 and 5).3 The recommended times provide a greater margin of safety and are better suited to slow-starting vehicles (trucks, two-

wheel vehicles).


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The assumed observation point is 2 m from the right edge of the non-priority road, setback 4 m from the stop-line4 at a STOP-controlled intersection and set back 15 to 20 m fromthe YIELD line5 where this rule applies. In the case of left turns into the minor road the positionof the assumed observation point is to be determined on a case by case basis depending onthe configuration of the intersection.

The observed point is at a height of 1 m above the centerline of the priority road,6

and at a distance from the conflict point which corresponds to the distance traveled by priorityvehicles during the crossing time (f); this distance is known as the crossing sight distance(D). The 85th percentile speed (V85) is used to calculate D(see Appendix 4).

7

The distance Dis given by: D= V85 x t, where the value of tis given below (Table 1)and varies according the width of the road to be crossed and the priority rule.

Fig. 3 - Crossing sight distance (D) on the basis of the 85th percentile speed on themajor road and thecrossing time (t).

0

50

100

150

200

250

300

350

400

50 60 70 80 90 100 110 120

V85 on major road ( km/h)

Sight distance ( m) .

t =11s

t = 6 s

t = 7 s

t = 8 s

t = 9 s

t =10s

Site triangles will be provided (4 for a 4-way intersections, 2 for a T-intersections, 1 fora partial intersections on a divided highway) on the basis of the elements described aboveand the priority rules which apply. The visibility for left turning maneuvers from the major roadwill be checked later (this generally imposes fewer constraints than left-turning movementsfrom the secondary road).

4 This is the point where 95 % of drivers actually stop.

5 This is the position where road users gain information when enterring an intersection where the YIELD priority rule applies.

6 If the major road is a two-way road on which overtaking is allowed the lane in question is the left-hand lane (for traffic

coming from the right) for sight triangles to the right of the observation point. In all other cases it is the right-hand lane.

7 The V85 speed used to calculate the crossing sight distance is computed using the speeds of all vehicles, including those

exceeding the posted limit.


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Fig. 4 - Sight triangles for drivers on the minor road, depending on the priority rule.

YIELD priority rule (AB3a sign)

2m

2

72

Observation point with yield

priority sign

15 to

20 m

2m

D (278m for example, when t=10 s and V85=100 km/h)

AB3a

Observed point to theright ( overtaking allowed

on the major road )

Observed point to the left

STOP priority rule

2

724m

AB4

Observation point with

stop sign

AB4

2m

2m

D (222m for example when t=8 s and V 85=100 km/h )

Observed oint to the left

Observed point to the right

( ovetaking allowed on the

major road

Fig 5 - Visibility for the left-turn maneuver into the minor road

D (222m for exam le, when t=8s and V85=100 km/h


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These crossing sight distances will be used in geometric design and the managementof roadsides.

c) Precautions as regards horizontal alignment and longitudinal profile

On a new road, intersections and access points must not be located on curvedalignment.8 It is, however, acceptable to install a T-intersection or an access point on theoutside of a bend whose radius is such that no more than the normal cross-fall is required, oncondition that sight distances are adequate. Right turning movements from the major roadmust not be too tangential.

Installing an intersection where there is a salient angle is inadvisable. On a new roadthis option is to be rejected if the longitudinal profile makes it impossible to comply with thesight conditions stated above.

For an existing road, any measures required for poorly-located intersections or

access points can be determined on the basis of a check on visibility and/or accident analysis(see e, below).

d) Precautions concerning roadside management

In the vicinity of an intersection, anything located near the road (signs or roadfurniture9, slopes, trees, crops or other vegetation, buildings, engineering structures, walls,parked vehicles, etc.) can potentially mask visibility. Thus, the visibility conditions set outabove demand a zone that is free of lateral masking with sufficient guarantees that it willremain so. Exceptionally, localized masking may be tolerated, on condition that it does notinterfere with vision.

In order for traffic signs to be outside the sight triangles, they should be set backroughly 200m from a YIELD line and at least 50 m from a STOP line. 10

e) The case of an existing facility where sight distance requirements are notmet

On an existing road, when it is not possible to remove the masking that impairsvisibility at an intersection, other measures must be considered. There are a number of waysin which the visibility requirements stated above can be attained; we shall mention thefollowing:

- realigning the minor roads - for example, this can transform a 4-way intersection into two T-

intersections, known as a staggered intersection (see 3.1.2.) - in some cases with a smallradius in a salient angle, it can be advantageous to move the centerline of the minor (non-priority) road to the central point of the curve;

- transferring exchanges to an adjacent intersection;

- in exceptional cases, modifying the layout of the major road (horizontal alignment,longitudinal profile);8 Apart from the adverse effects on visibility, it is more difficult to judge speeds on a bend and information gathering is more

difficult when the non-priority branch joins the major road on the inside of a curve.

9 Safety barriers can also mask visibility, particularly when there is a summit curve on the major road.

10 If it is assumed that vehicles are 0.70 m away from the right shoulder (itself 2 m wide) and the 85th percentile speed is 100km/h.


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- in the case of partial intersections installed on 4-lane divided highways, installing orextending a acceleration lane;

If, ultimately, it proves impossible to meet sight distance requirements, it is necessary

to be extremely demanding with regard to the legibility of the facility (see 1.2.2). This mayentail installing warning devices, in particular, to encourage drivers to obey the posted speedlimits on the major road. To remain effective such measures must, of course, be exceptional.Furthermore, remedial measures of this type can never be completely satisfactory.

1.2.2. LEGIBILITY

Drivers arriving at an intersection must understand easily and rapidly how it operates ,the behavior that is expected of them (for example slowing down and yielding) and what theother drivers are doing or will do.

The following conditions are necessary to ensure satisfactory legibility:

- compatibility of sight distances with approach speeds;- facilities or features that highlight the presence of the intersection (in particular splitter

islands)- uniformity of geometric features along a route;- facilities which comply as far as possible with the "standard layouts"- the simplest possible signing, which is consistent and placed where it is clearly visible

Landscaping (or more generally a modification of roadsides) assists driver attention

and may facilitate interpretation of the road. For example, it can make minor roads visible atan early stage (a transverse line of trees, see Fig. 6, etc.), emphasize loss of priority (ascreen of vegetation that blocks vision at T-intersections, see Fig. 7 etc.).

However, such measures are difficult to implement and must be approached on acase-by-case basis (systematization must be completely avoided). Without specialprecautions they might reduce primary safety (for example by masking visibility of the minorroad, giving the impression that the non-priority road is uninterrupted) or secondary safety (itis essential to take account of zones of limited severity). 11

Finally, to ensure that the initial legibility provided when the facility is constructed hassome durability, the following should be considered from the design phase: foreseeablechanges in the use of surrounding land, the needs of residents, maintenance constraints, etc.This means involving local partners (Commune, residents, etc.) in any project.

11 See ARP, Chap 2.


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Fig. 6 - Sighting of the crossed minor road as a result of a transverse line.

Fig. 7 Screen of vegetation blocking vision of the minor leg of a T-intersection.


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1.2.3. THE DISTANCE BETWEEN INTERSECTIONS

Excessively close spacing of intersections frequently has an adverse effect on thefollowing factors (i) visibility, (ii) legibility - it becomes difficult for drivers to match their

behavior to the situation and anticipate events on the road, the overabundance of signalslengthens the time taken to perceive and understand messages -, (iii) the placement of trafficsigns (regulatory, prohibitory, priority and directional), and (iv) the conditions for overtaking. Ingeneral, such changes reduce safety.

The impact on the above factors depends on the type of intersection, operatingspeeds (V85) on the route, etc. There is therefore no universally valid formal rule for theminimum distance between two successive intersections. However, a minimum distance of250 m can generally be considered to be satisfactory12 , but specific site characteristics canmake this very inadequate.

The presence of a center lane can also lead to a minimum distance beingrecommended between two successive intersections, in order to allow sufficient opportunityfor safe overtaking. In the case of an existing road, however, this recommendation should notmean that the possibility of a central facility is rejected if it is necessary for the safety of theintersection.

Table 2 - Minimum recommended distance between successive intersections, and residual length for

overtaking13, depending on operating speeds.

V85 (km/h) 60-70 80-90 100-110

Minimum recommended

distance (m)

600 900 1200

Length for overtaking (m) 300 40 600

1.2.4. LIMITING THE NUMBER OF INTERSECTIONS

Limiting the number of intersections is particularly beneficial :

- for safety, as it limits the number of potential conflict zones, and grouping existingintersections together makes it possible to retain only those with the required approach andcrossing sight distances;

- for traffic flow and the comfort of drivers on the major road, as the number of zones whereflow is disturbed by exchanges between roads is limited and there is more opportunity toovertake slow vehicles.12 "Staggered" intersections should be considered as a single facility rather than two T-intersections close together.

13 This is a theoretical distance which may be reduced by other factors (for example the layout).


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2. DESIGN OF THE PRIORITY ROAD

2.1. HORIZONTAL ALIGNMENT AND LONGITUDINAL PROFILE

The general rules for the installation of intersections are given in section 1.2. of thisChapter.

It is particularly important for drivers traveling on the priority road to have an adequatesight distance with respect to the noses of raised traffic islands (the so-called "entry approachsight distance"). This sight distance must be at least equal to the stopping distance14 for avehicle traveling at the V85 speed of the approaching traffic on the major road.

15 For newroads this condition will usually be met at all points along the route. For improvements tointersections on existing roads, the table below shows the stopping distances used for designpurposes.

Table 3 - stopping distances (d) on the basis of the V85speed.

V85 (km/h) 50 60 70 80 90 100

don a straight section (m) 50 65 85 105 130 160don a bend (m)16 55 72 95 121 151 187

2.2 THE NUMBER OF TRAFFIC LANES

Generally, the wider the major road, the more dangerous the intersection. This meansthat at the intersection the number of through lanes should be limited in both directions.

2.2.1. INTERSECTIONS ON 2-LANE ROADS

At a standard at-grade intersection, only one through lane should be retained in eachdirection. Widening to 3 or 4 lanes at an intersection must therefore not occur: suchconfigurations are dangerous and result in little improvement in traffic flow on the major road.

Fig. 8 - Designs for intersections on 2-lane roads.

14 The stopping distance dis made up of the braking distance (distanced covered during braking which reduces the speed

from V85 to 0 under specified wet pavement conditions) and the distance covered during the reaction time (taken as 2 s at

speeds of 100 km/h and less). In order to calculate the stopping distance d, readers are referred to 4.2.b. in the A.R.P.

15 It is also possible for only speeds below the speed limit (typically 90 km/h) to be used for calculating the 85th percentile

speed.

16 The stopping distance on a bend should be used for radii R< 5 x V85 (where V85 is in m/s and R is the radius of the bend).


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2.2.2. INTERSECTIONS ON 3-LANE (AND EXCEPTIONALLY 4-LANE) ROADS

It is strongly recommended that the 3-lane (and even more the four-lane) configurationbe abandoned at intersections. On such road links traffic should be reduced to a single lane ineach direction upstream of the intersection.

Fig.9 - Designs for intersections on 3- or 4-lane roads

2.2.3 INTERSECTIONS ON 4-LANE DIVIDED HIGHWAYS

On type R 4-lane divided highways, standard at-grade intersections will systematicallytake the form of partial intersections. Roundabouts will be systematically installed at thelocations where all interchange movements are transferred (with reduction to a single lane ineach direction upstream of these roundabouts).17

Fig 10 - Design principle for type R 4-lane divided highways.

2.3. FACILITIES TO ASSIST LEFT-TURNING MOVEMENTS (FROM

THE MAJOR ROAD INTO THE MINOR ROAD)

The guidance given below on the choice of facilities for left turning movements (lateralwidening, central facility, construction of a roundabout), are essentially the outcome of safetyconsiderations, but also take account of driving comfort and the cost of facilities. The trafficlevels which are stated should not be taken rigidly but considered in relation to localconstraints and any specific traffic peaks.

Furthermore, on an existing road, detailed accident analysis will reveal the relativeproportion of accidents involving left turns and criss-crossing movements and evaluatewhether it is appropriate to install a left-turning facility and what form this should take.

17 Opportunities for turning back must be frequent, about 5 km apart to avoid lengthening jounrneys excessively (this distance

should, however, depend on real needs and whether or not access roads have been constructed).


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2.3.1. ON A 2-LANE ROAD

The scale of design measures at a standard at-grade intersection on a 2-lane road willdepend on the type of intersection (T-intersection or 4-way intersection)18 and the traffic

levels.

a) A shoulder at T-intersections (or access points) on a 2-lane road

For T-intersections with low left-turning demand (less than 100 v/d) or for residentaccess points, constructing a shoulder opposite the minor road (or access point), reducesthe risk of accidents caused by the left-turning movement (by providing for an avoidingmaneuver to the right of the turning vehicle). In the absence of a continuous (and sufficientlywide) shoulder along the major road, local treatment as shown in the diagram below (Fig. 11)is satisfactory.

Fig. 11 - Facility to assist left-turning movements at a low-traffic T-intersection.

10 m15 to 35 m10 m 10 m 1,50 to 2,00 m

5 m

The aim should be to achieve a width of at least 5 m between the centerline of theroad and the edge of the widened section. This generally involves widening the road bybetween 1.50 m and 2.00 m.

A total length of less than 40 m should be avoided (a short facility of this type is likelyto encourage hesitation and impede the avoidance maneuver). The total length may beincreased to 65 m when there are trucks turning left. Nothing should be done which mightdeter drivers from using the shoulder for an avoidance maneuver: the surface must be ofgood quality, without special markings (normal edge marking highlights the boundary between

the roadway and the shoulder, etc.).

However, use of the shoulder as a traffic lane should not be over encouraged: theshoulder should not be treated as a lane (for example with marking on its outer edge, or along entry taper), and any signing which attempts to specify a particular use of the shouldershould be avoided.

N.B. Parked vehicles do not generally cause a problem, as parking demand tends to be very low in ruralareas. Where problems arise a prohibitory sign may be installed (off the usable part of the shoulder).Very occasional parking does not reduce the usefulness of this facility.18 Staggered intersections are covered in 3.1.2.


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b) creation of a left-turn lane

Generally, the creation of deceleration lanes for left-turning movements is justified onthe grounds of likely safety improvements, increased intersection capacity beyond certain

traffic levels and, lastly, because such movements are difficult when no facilities are provided(in particular where traffic is heavy). Irrespective of their length, these special lanes reducethe hazards associated with left-turning movements by removing stationary left turningvehicles from the through lanes. It must, however, be stressed that they perform very muchless well when there is only a ghost (as opposed to a raised) island.

For a T-intersection on a 2-lane road, when left-turning demand reaches asignificant level (over 100 v/d) it is advisable to construct a left-turn lane.

For 4-way intersections, the construction of this type of lane reduces the accidentrisk for left turns but increases the risk of a collision in the second part of the maneuverbetween a vehicle from the minor road crossing the major road and a priority vehicle. A facility

of this type is therefore only recommended when left-turning demand is quite high (more than200 v/d), and when the traffic crossing the intersection is not too heavy. The geometriccharacteristics of left-turn lanes are given in 2.5.

2.3.2. ON A 3-LANE ROAD

a) On a new 3-lane road

On safety grounds, the center lane must be systematically be closed at all at-gradeintersections and resident access points into which left turns are possible. The central space

should be converted into a (perhaps short) left-turn lane or a central refuge in the case of anaccess point. This means that the number of access points must be restricted in order tomaintain a sufficient number of zones where the 3-lanes can be used to increase overtakingcapacity.

b) On an existing 3-lane road

It is strongly recommended that the center lane be discontinued at all at-gradeintersections. A diagnostic study should be performed for access points (analysis ofaccidents, left-turning demand towards the access) in order to determine whether centralprotection is necessary. When there is a very large number of access points (for example in

suburban areas) resident access may take priority over ease of overtaking and the centerlane be used almost exclusively to provide left-turning facilities. Alternatively, dual use may bemaintained by converting the 3-lane road into a wide 2-lane road (with shoulders for avoidingleft-turning vehicles).


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2.3.3. CRITERIA FOR ACHIEVING THE BEST COMPROMISE

Table 4 - General rules for facilities for left-turn movements from the major road.

1. For a T-intersection or a resident access point

Traffic onResident access

T-intersection or important access point (left turning traffic)

major

roadless than 100 v/d 100 to 400 v/d more than 300 -

400 v/d

2-lane roads

< 8000 v/d no change

no change or

surfacing shoulder left-turn laneleft-turn lane

> 8000 v/dor

surfacing of shoulder

same or

left-turn

lane

or

roundabout

3-lane roads

< 8000 v/d

left-turn lane

or

left-turn lane

or left-turn lane left-turn lane

> 8000 v/d

removal of access point

(and provision of new

access road dans le cas

dun crneau de

dpassement)

removal of intersection with

transfer to an adjacent

intersection

left-turn lane

or roundabout

or

roundabout

2. For a 4-way intersection

Traffic on

major

left turning traffic

road less than 200 v/d 200 to 400 v/d more than 400 v/d

2-lane roads

< 8000 v/d no changeleft-turn left-turn lane

> 8000 v/dsame or left-turn lane lane or

roundabout

3 lane roads

< 8000 v/dleft-turn lane

or left-turn lane left-turn lane

> 8000 v/d removal of intersection with

transfer to an adjacentintersection

left-turn lane

or roundabout

or

roundabout


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2.4. CROSS-SECTION WHERE THERE IS ACENTRAL FACILITY

ON THE PRIORITY ROAD

Traffic lanes should generally have the same width within an at-grade intersection as

in the links between intersections. However, if the roadway's typical cross-section is between5 and 6 m wide, the traffic lanes at the intersection will be 3 m wide. Furthermore, at anintersection there is no justification for having through lanes wider than 3.50 m.

The central island should be separated from the continuous line (3 u wide)19 that runsadjacent to it by an unpainted area with a width of at least 2 u: the shoulder on the left willtherefore have a minimum width of 5 u, but a considerably greater width is preferable (0.50 mfor a road with a width of 6 m or more wide, for example).

For a roadway whose typical cross-section is 7 m wide, this would give, for eachtraffic direction, a minimum paved width between the edge of the central island and the line atthe outside edge of the pavement of 3.80 m, which would break down as follows: 3.50 m of

traffic lane + continuous line of 3 u+ space of 2 u(minimum), where u= 6 cm.

The right shoulder (which carries the edge marking) has the same width as it does inthe road's typical cross-section (with, however, a minimum of 1 m). When there is a highlevel of light two-wheel traffic, it is preferable to surface a 1.25 m width of shoulder. 20

Fig.12 - Cross-section at a standard at-grade intersection as a function of the width of thetypicalcross-section.

1. Width of between 5 and 6 m (typical cross-section)

3,00 m

right shoulder

1,00 m0,30 0,40 m

"LC"(3 u)

"T3"(3 u)

carriageway (1 through lane)left shoulder

2. Width6 m (typical cross-section)

"LC"

(3 u)

"T3"

(3 u)

3,00 to 3,50 1,00 m0,30 0,50 m

right shouldercarriageway ( 1 through lane)left shoulder

3. four-lane divided highway"LC"

(3 u)

"T1"

(2 u)

"T3"

(3 u)

median

7,00 m

carria eway (2 throu h lanes

1 00 m

ri ht shoulderleft shoulder

A minimum 1.25 m width of the right shoulder must be paved is if there is a large amount of two-wheeltraffic.

- The lane and the right shoulder should be at least as wide as in the typical cross-section of the road .The left shoulder and the median strip are the same width as in the typical cross-section of the road

19

uis the width unit (see 4.1.6).

20 These features are to be considered is minima and do not remove the need to comply with the rules set out in the ARP.


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2.5. SPLITTER ISLANDS (OR CENTRAL RESERVATION)

2.5.1. THE FUNCTIONS OF SPLITTER ISLANDS ON THE MAJORROAD

The main function of splitter islands on the major road is to protect vehicles turning leftfrom rear-end collisions by physically separating the left-turn lane. Correctly designed splitterislands (in particular with advance signing of the island nose by means of stripes as laid downin the regulations) improves overall perception of the intersection for priority traffic (warningeffect generated by introducing a clearly identifiable image into the "roadscape".

Raised splitter islands on the major road are essential in the following situations:

- on all types of roads when there is a left-turn lane, except in the specific case of narrowroads (see 2.8);

- on 3-lane roads when there is a considerable amount of crossing traffic (


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Fig. 13 - Geometric design criteria for theapproach to splitter islands on major roads with reference to the

typical cross-section of the road.

1. Roadway width < 6m (typical cross-section)

10 50 m L/2=3 m

=

2 10 m

r=200 mr=100 m

3 m

3 m

1/15me

L/6=13 m

1 60 m

2. Roadway width 6m (typical cross-section)

22 50 m L/2=58 50 m

2 30 m

1/15me

r=100 m r=200 m r=300 m

J5

3 m 3 50 m

5 m

L/6=19 50 m

2 00 m

Table 5 - Values of L (length of advance signing; see "Instruction Ministrielle sur la signalisation

routire; Livre 1 - Partie 7.".

Width of roadway L (m) L/2 (m)> 7 m 156 78

5 to 7 m 117 58.


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2.5.4. LENGTH OF SPLITTER ISLANDS

The length of the left-turn lane (without taper) does not affect safety, and can therefore

be limited to the length required to store left-turning vehicles (generally quite short), andoccasionally reduced to just a central refuge. Other factors may, however, impose greaterlengths (for example the guaranteed level of comfort along a route, or the choice of a largecapacity reserve).

The total length (for one traffic direction) of a splitter island (including its advancemarking) will be determined by the length of the displacement, the tapered lane and the left-turn lane. It can vary between 90 m in the case of intersections with low volumes of left-turning traffic to more than 170 m in the case of the largest intersections. The length of theadvance marking of the splitter island nose will always be equal to L/2 when the lanes aredisplaced symmetrically with regard to the centerline of the road and equal to L in the case of

unilateral displacement.

Table 6 - Length (in m) of the components of splitter islands, in the principal scenarios (with

symmetrical displacement, depending on the width of the typical cross-section of the road and

the composition of left turning traffic.

Advancemarking

Displacement Straightsection

Taper Storage a b-

Roadway < 6 m (negligible left-turning truck traffic)

39 to 58,5 10,5 to 16 10 15 15 0.25 to 1.10 3.00 to 3.85Roadway < 6 m (considerable left-turning truck traffic)

39 to 58,5 10,5 to 16 10 15 25 0.25 to 1.10 3.25 to 4.10

Roadway 6 m (negligible left-turning truck traffic)

58,5 16.5 to 22.5 > 10 20-30 20-50 0.25 to 2.00 3.25 to 5.00

Roadway 6 m (considerable left-turning truck traffic)

58,5 16.5 to 22.5 > 10 20 - 30 40 - 60 0.25 to 1.75 3.50 to 5.00

Advance marking L/2

39 to 58,50 m

Displacement 1/15

10,5 to 22,5 m

Straight

section.

>10 m

Taper

15 to 30 m

Storage

15 to 60 m

Total lengh : 90 to 170 m

1,60 to

2,00 m

2,75 to 3,25

a =

0 25 to 2b =

3 to 5

a is the width of the part of the island between the left-turn lane and the through lane for opposingtraffic- b is the total width of the central island, i.e. the sum of a and the width of the left-turn lane.


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The nomograph below (Fig. 14) shows the number of vehicles that should beconsidered when designing the storage zone. The storage length is obtained by simplyconsidering the average space that vehicles occupy. The following formula, for example,could be used : Ls = (7 + 10p).Ns (where Ls is in meters, p is the proportion of trucks in the thestream in question, and Ns is the number of vehicles obtained from the nomograph.

23

Fig 14 - The storage capacity to be provided (number of vehicles) on the left-turn lane (values given by

the OCTAVE software which deals with the capacity of unsignalized intersections, see Appendix 2).

0

50

100

150

200

250

300

350

400

0 100 200 300 400 500 600 700 800 900 1000 1100 1200

O osin traffic on ma or road (v/h

Left turnin traffic (v/h .

2

121086

5

4

3

2.5.5. LONGITUDINAL POSITION OF ISLAND TIPS

The splitter islands on the major road must (i) assist vehicle guidance (to obtain an

optimum path), in particular in the case of left-turning movements (from the minor road intothe major road or vice-versa) and (ii) not be located in the path of straight ahead or left-turningvehicles on the minor road. In practice, the position of the island heads is given by thetangency point of the splitter island turning circles when exiting the minor road and from theleft edge of the relevant traffic lanes on the major road (see 3.2 and Fig. 15).

Fig 15 - the central part of the splitter island in the case of a 5 m central reservation on a road with a 7 m

wide typical cross-section.

4 m

0,50*

0,50*

3 m

2 m 3,50 m

5,00 m

3,50 m

* 0,30 mini

LC (3u)

T'3 (3u)

T2 (5u)4 m

Storage : 20 m to 50 mTaper : 20 m to 30 m

LC (3u)T2 (5u)

Rie

R is

23 This formula can be modified to take account of specific sizes of turning vehicles.


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2.5.6. DETAILED DESIGN OF SPLITTER ISLANDS

It is always preferable for raised islands to have mountable curbs (see Appendix 6"Design of the curbs"). This is important because the island markings become less visible inwet conditions, and, also, some drivers do not obey continuous lines).

Islands must be free from aggressive obstacles (road lighting columns, large signpoles, etc.). All singing (priority, regulatory, or directional signs, J5 markers) which is normallylocated on splitter islands, must be installed with a minimum distance of 0.70 m between theedge of the sign and the edge of the nearest lane.

Furthermore, as far as possible, nothing which masks visibility should be installed onan island.

Raised islands must be constructed from materials with a different surface from

design of interurbain intersections

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