design of roundabouts

Geometric Design of

Roundabouts

Summary: This document gives advice and standards for the geometric design ofroundabouts with regard to traffic operation and safety. Amendments andadditions have been made to reflect current good practice in aspects such asentry path curvature assessments, geometry of entries, segregated left turnlanes, over capacity in early years, and speed reducing measures at miniroundabouts.

Printed and Published by theabove Overseeing Departments Crown Copyright 1993 Price:3.00

THE HIGHWAYS AGENCY TD 16/93

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT

THE WELSH OFFICE

Y SWYDDFA GYMREIG

THE DEPARTMENT OF THE ENVIRONMENT FOR

NORTHERN IRELAND

Volume 6 Section 2Part 3 TD 16/93 Registration of Amendments

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYSeptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

REGISTRATION OF AMENDMENTS

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

amendments amendments

Volume 6 Section 2Registration of Amendments Part 3 TD 16/93

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED September 1993

REGISTRATION OF AMENDMENTS

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

amendments amendments

DESIGN MANUAL FOR ROADS AND BRIDGES

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYSeptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

VOLUME 6 ROAD GEOMETRYSECTION 2 JUNCTIONS

PART 3

TD 16/93

GEOMETRIC DESIGN OFROUNDABOUTS

Contents

Chapter

1. Introduction

2. Types of Roundabout

3. Siting of Roundabouts

4. Safety

5. Road Users' Specific Requirements

6. Landscaping

7. Geometric Design Features

8. References

9. Enquiries

Annex 1 Calculation of Roundabout Capacities

Volume 6 Section 2 Chapter 1Part 3 TD 16/93 Introduction1. INTRODUCTION

General

1.1 The treatment of roundabout layout design hasbeen the subject of a recent study which reviewedStandard TD 16/84 and Advice Note TA 42/84 on theGeometric Design of Roundabouts. It maderecommendations on amendments and additions tothose documents based on current good practice.

1.2 Arising out of the study this document nowprovides details of the latest requirements andrecommendations on general design principles andsafety aspects of design.

1.3 This document supersedes Standard TD 16/84and Advice Note TA 42/84.

1.4 Guidance on the most appropriate form ofjunction is given in TA 30 (DMRB 5.1).

Scope

1.5 This document defines three main types ofroundabout and their derivatives for application to newand improved junctions on trunk roads. They haveapplication for use on other roads.

1.6 Requirements are defined in relation to the sizeof roundabouts, effect of approach speed, visibility,entry width, entry deflection and the circulatorycarriageway.

1.7 Recommendations are given on the siting ofroundabouts in urban and rural areas, geometric design,crossfalls and segregated left turning lanes.

1.8 The first major change is the amendment to thedefinition of entry path curvature. This ensures that aradius of curvature greater than 100m cannot now beachieved by driving the straightest path on the approachand through the junction by selecting the outer lane onapproach at certain junctions (see paras 7.25 - 7.32).The second major change is that the visibilityrequirements on the circulatory carriageway aremandatory (see para 7.45).

1.9 Other significant changes include theELECTRONIC COPY - NOTSeptember 1993 PAPER COPIES OF THIS ELECTR

following:-

i. the deletion of the raised profile to SubsidiaryTraffic Deflection Islands;ii. emphasis of the importance of approachcurvature and its effect on safety;

iii. geometric changes to entries;

iv. means of dealing with over capacity in earlyyears of operation;

v. advice on segregated left turn lanes;

vi. advice on traffic speed reducing methods atmini-roundabouts.

Implementation

1.10 This document should be used forthwith on allschemes for the construction, improvement andmaintenance of trunk roads including motorways,currently being prepared provided that, in the opinion ofthe Overseeing Department, this would not result insignificant additional expense or delay progress. DesignOrganisations should confirm its application toparticular schemes with the Overseeing Department.

General Principles

1.11 The principal objective of roundabout design isto secure the safe interchange of traffic betweencrossing traffic streams with minimum delay. This isachieved by a combination of geometric layout featuresthat, ideally, are matched to the volumes of traffic in thetraffic streams, their speed, and to any locationalconstraints that apply.

1.12 There are two broad regimes of roundaboutoperation. The first occurs in urban areas with highpeak flows, often with marked tidal variations andphysical restrictions on the space available. The secondregime occurs in rural areas and is characterised by highapproach speeds, low tidal variation and few physicalconstraints.

1.13 Entry width is an important feature thatdetermines entry capacity and it often needs to be largerin urban situations than in rural cases. On the otherhand the most important determinant of safety is vehicle FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 1/1

deflection imposed at entry because this governs thespeed of vehicles through the junction. It is particularlyimportant whenever approach speeds are high. Entrydeflection is related to the entry path curvature and

Chapter 1 Volume 6 Section 2Introduction Part 3 TD 16/93

limiting this radius of curvature in the vicinity of the integral part of the design process. (Refer to para 7.77entry to 100m maximum ensures that sufficient and DMRB 8.2). deflection will be undergone by entering vehicles tolimit through speeds. 1.16 The provision of road lighting at roundabouts

1.14 The characteristics of roundabout accidents and requirement (DMRB 8.3). Sometimes lightingtheir frequencies in relation to geometric layout design requirements may conflict with environmentaland traffic flows are reported in TRL Report LR 1120 considerations. However, it should be recognised that"Accidents at Four-Arm Roundabouts". The roundabouts are generally safer than other forms of at-relationships derived in this report provide insights into grade junctions and the decision to use a roundaboutthe way various aspects of design interact to influence should not be abandoned solely because of lightingthe types and frequencies of accidents at roundabouts. problems. In sensitive locations it may be possible toThese relationships therefore, constitute the adopt alternative lighting methods and other measuresfundamentals of design for safety. As relationships to make the roundabout more visible. When an existingbetween aspects of design are not always mutually roundabout junction is being modified, the lightingcompatible, minimising the likely incidence of a layout should be checked for suitability with the newparticular type of accident may increase the potential road arrangement and any alteration carried out prior to,for another. Design, therefore is a trade-off between or at the same time as the roadworks. It is important thatoperational efficiency, minimising delays at the approaching drivers see and perceive that they arejunction, and various safety aspects within whateverlocation constraints apply. The latter are often thedominating factor when designing improvements to anexisting junction, particularly in urban areas. The

should normally be regarded as an essential safetyaccident prediction model given in LR 1120 can beused to compare the safety characteristics of alternativedesigns. This model has now been incorporated into theTRL program ARCADY/3 - see TA 44 (DMRB 5.1.1).

1.15 Consideration of the need for, and layout oftraffic signs and road markings should be anELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO1/2Mandatory Sections

1.17 Sections of this document which form partof the standards the Overseeing Departmentexpects in design are highlighted by beingcontained in boxes. These are the sections withwhich the designer must comply. The remainder ofthe document contains advice and enlargementwhich is commended to designers for theirconsideration.

approaching a roundabout and are not misled by theprojection of the lighting layout, particularly at times ofpoor visibility. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993

Volume 6 Section 2 Chapter 2Part 3 TD 16/93 Types of Roundabout

Definitions

2.1 The three main types of roundabout,

2. TYPES OF ROUNDABOUT4m minimum

a

a

a Traffic deflection island

a

b

c

Normal, Mini and Double, and other forms ofroundabouts which are variants of these basic types,ie Ring Junctions, Grade Separated Roundaboutsand Signalised Roundabouts, are defined asfollows:-a Normal Roundabout: a roundabouthaving a one-way circulatory carriageway around akerbed central island 4m or more in diameter andusually with flared approaches to allow multiplevehicle entry. (Fig 2/1).

b Mini Roundabout: a roundabout having aone-way circulatory carriageway around a flush orslightly raised circular marking less than 4m indiameter and with or without flared approaches. (Figs 2/2a and 2/2b).

Normal RoundaboutFigure 2/1ELECTRONIC COPY - NOT FOSeptember 1993 PAPER COPIES OF THIS ELECTRONa

a

Traffic deflection island

b

c

b

c

Hatched trafficdeflection islandKerbed trafficdeflection island

Mini Roundabouts : 4 - Arm Junctionwith flared Approaches

Figure 2/2a

Mini Roundabout : 4 - Arm Junctionwithout Flared Approaches

Figure 2/2bR USE OUTSIDE THE AGENCYIC DOCUMENT ARE UNCONTROLLED 2/1

Chapter 2 Volume 6 Section 2Types of Roundabout Part 3 TD 16/93

c Double Roundabout: an individualjunction with two normal or mini roundabouts

e Ring Junction. A junction arrangementwhere the usual clockwise one-way circulation of

ehicles around a large island is replaced bywo-way circulation with three arm minioundabouts and/or traffic signals at the junction ofeither contiguous (Fig 2/3), or connected by acentral link road or kerbed island (Fig 2/4).

d Grade Separated Roundabout Junction. A roundabout which has at least one entry road viaan inter-connecting slip road from a road at adifferent level, (eg underpasses, flyovers or multiplelevel intersections), (Fig 2/5 and 2/6).

v

tr

e

fw

t

Contigous Double RoundaboutFigure 2/3

Double Roundabout with Short Central Link roadFigure 2/4

No

2.or3 trathis prgea p

M

2.imsashgoThsuro

2.apisldefuesacbywiislwiELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO2/2ach approach arm with the circulatory carriageway.

Signalised Roundabout. A roundabouthich has traffic signals installed on one or more of

he approach arms.

rmal Roundabouts

2 The number of entries recommended is either 3 4. Roundabouts perform particularly well witharms, being more efficient than signals, provided theffic demand is well balanced between the arms. If

e number of entries is above 4, driver comprehensionaffected and the roundabout becomes larger with theobability that higher circulatory speeds will benerated. Double roundabouts should be considered asotential solution in these circumstances.

ini Roundabouts

3 Mini-roundabouts can be extremely effective inproving existing urban junctions that experiencefety and side road delay problems. Their layoutould be designed so that drivers are made aware inod time that they are approaching a roundabout. ey should only be used when all the approaches arebject to a 30 mph speed limit or less. Their use onads with higher speed limits is not recommended.

4 Where physical deflection is not possible onproaches, road markings and small traffic deflectionands should be used to induce some vehicleflection. These islands should be kept free of allrniture except the "Keep Left" bollards and othersential signs. If satisfactory deflection can not behieved, the speed of approach traffic can be reduced the use of traffic calming islands as carriagewaydth restrictions on the junction approach. Theseands may also serve as pedestrian refuges. Sufficientdth should be left for cyclists. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993


SeSuperfluouscirculatingcarriageway if no'u' turns present

a

Two Bridge Roundabout at Grade Separated Interchange

Figure 2/5

Grade Separated Interchange with One Bridgeand Two Roundabouts - Dumbell Interchange

Figure 2/6ptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 2/3

Chapter 2 Volume 6 Section 2Types of Roundabout Part 3 TD 16/93

signals. Therefore, provided that excessive approachspeeds are discouraged or prevented, mini-roundaboutscan be used where there are expected to be cyclists.

2.11 Although not based directly on miniroundabout data, ARCADY/3 is as good as any2.5 The circular marking (1 to 4 m diameter) (Fig2/2) should be as large as possible in relation to the siteand be domed up to a maximum height of 125mm at thecentre for a 4 m diameter island. For smaller islands theheight of the dome should be reduced pro-rata. Thisdoming, in conjunction with the presence of someadverse crossfall, will help to make the roundaboutmore conspicuous to drivers. No bollards, signs,lighting columns or other street furniture should beplaced on the dome.

2.6 The dome is usually constructed of bituminousmaterial, concrete or block paving with a hard surround6mm to 15mm proud of the surrounding road surface. Techniques whereby a precast dome is fixed withadhesives to an existing road surface have provedsuccessful.

2.7 The dome should be completely white andreflectorised. A ring of omni-directional reflective roadstuds around the periphery of the dome have been foundeffective. Domes surfaced with materials such asnatural stone sets which do not contrast with thesurrounding road surface are not sufficientlyconspicuous at times of bad visibility.

2.8 At junctions where space is very restricted, therepeated overrunning of the central marking by longvehicles will be unavoidable. In these cases the centralmarkings may be merely a flat circle marked on theroad and its periphery may be delineated by reflectiveroad studs, but in such cases there may be lessobservance of the painted island by light vehicles. Thereis some evidence to suggest that this increases theaccident risk.

2.9 "U" turn manoeuvres at mini-roundabouts areoften unexpected due to the compactness of thejunction. Whilst they should not be prevented, careshould be taken not to create "U" turns unnecessarily. The use of mini-roundabouts at the end of dualcarriageways, or linear traffic management schemeswhere right turns into or out of side roads are prohibitedis not recommended. Most mini-roundabouts involvetight turning manoeuvres which produce severe heavytyre scuffing actions. They should, therefore, beinspected regularly to ensure that road and domemarkings are intact and readily visible.

2.10 Because of the short distance between entries,mini-roundabouts require entering drivers to observeclosely other vehicles in the junction and on theapproaches and react quickly when a gap occurs. Inthese circumstances pedal cyclists can be overlooked. However, the indications are that cyclists are no morevulnerable at mini-roundabouts than at four arm trafficELECTRONIC COPY - NOPAPER COPIES OF THIS ELECT2/4available method for determining mini roundaboutcapacities. However, results should be viewed withsome caution. ARCADY/3 cannot at present be used forsafety and accident assessments for mini roundabouts.

Double Roundabouts

2.12 Cases where double roundabouts can beparticularly useful include:-

a. the improvement of an existing staggeredjunction where it avoids the need to realign one of theapproach roads, and achieves a considerableconstruction cost saving;

b. at unusual or asymmetrical junctions, such as a`scissors' junction, (see Fig 2/4) where the installationof a single island roundabout would require extensiverealignment of the approaches or excessive land take;

c. the joining of two parallel routes separated by afeature such as river, railway line or motorway;

d. at existing crossroads where it separatesopposing right turning movements allowing them topass nearside to nearside as shown in Fig 2/3;

e. at overloaded single roundabouts where, byreducing the circulating flow past critical entries, itincreases capacity;

f. at junctions with more than four entries, adouble roundabout achieves better capacity withacceptable safety characteristics in conjunction with amore efficient use of space, whereas large roundaboutscan generate high circulatory speeds with consequentloss of capacity and safety.

2.13 Where the double roundabout is comprised ofmini-roundabouts they should only be used when all theapproaches are subject to a 30 mph speed limit.T FOR USE OUTSIDE THE AGENCYRONIC DOCUMENT ARE UNCONTROLLED September 1993


Grade Separated Roundabout Junctions Ring Junctions

2.14 The most common forms of roundabout used at 2.18 Some unusual types of roundabouts, forgrade separated junctions are the two bridge type example ring junctions, have been found to work well(Fig 2/5) and the dumbbell type (Fig 2/6). in solving problems at existing junctions. Ring

2.15 Two Bridge Roundabout. There have beenproblems in the past with some layouts of this type dueto their large size, which permit high circulatory speeds.This can cause problems for drivers trying to enter thesystem. Thus when adopting this type of layout everyeffort should be made to achieve compact designs. Ifthis can not be achieved, then on large gyratoriesreducing the width of longer sections can help tocontrol speeds, and can also provide the option for moresuitable entries using a form of "lane gain".

2.16 Dumbbell Roundabout. This type of layout

junctions allow two way traffic on the circulatorysystem and require drivers on the circulatory system togive way. The connection with the entry arms is usuallymade with a small or mini roundabout, or it may besignalled.

2.19 The conversion to ring junctions is an effectivesolution for very large roundabouts which exhibit entryproblems. This type of layout can eliminate congestionproblems without reducing safety.

2.20 A ring junction will not operate successfullyunless the signing is clear, concise and unambiguous,nd careful consideration should be given to this aspectt the design stage.forms a useful intermediate junction between the singlediamond interchange and the two bridge roundabout. Ithas the advantage of compactness and low constructioncosts. The standards for deflection and visibility shouldapply to each of the two roundabouts though in the caseof the connecting link the design speed, and henceforward visibility, is likely to be less than for theexternal arms.

2.17 Capacity assessment should not only considerthe whole junction, but connecting link approachesshould be considered separately to ensure thatinteractive queuing between the roundabouts does notoccur. In situations where there are no "U" turnmovements a full roundabout may not be necessary.However, an unbalanced flow situation can arise, orhigher speeds in the dominant flow, and care should betaken to ensure that this does not result in excessivequeues on exit slip roads.

aa

S

2beodm

tro8ELECTRONIC COPY - NOTSeptember 1993 PAPER COPIES OF THIS ELECTROignalised Roundabout

.21 Where a roundabout does not function wellecause of growth in traffic flow, or is likely toxperience an overloading or an unbalanced flow at oner more entries, or high circulatory speeds, therebyefeating the self-regulating nature of the junction, itay be possible to alleviate the problem by installingaffic signals, (either `continuous' or `part-time'peration) at some or all of the entry points (See DMRB.1) FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 2/5

Volume 6 Section 2 Chapter 3Part 3 TD 16/93 The Siting of Roundabouts

3. THE SITING OF ROUNDABOUTS3ov

w

sr

t

3Dsr

w

Fn

pr

asu

r

Cr

3.1 The decision to provide a roundabout rather Trthan some other form of junction should be based on veoperational, economic and environmental adconsiderations. See TA 23 (DMRB 6.2) and TA 30 pr(DMRB 5.1). Factors to be taken into account at the mdesign stage include, for example, the need to induce do"through traffic" to reduce speed at certain places for prreasons such as:-

a. A significant change in road standard, say from judual to single carriageways or from grade separated Thjunction roads to at-grade junction roads, although prcomplete reliance should not be placed on the neroundabout alone to act as an indicator to drivers;

b. To emphasise the transition from a rural to anurban or suburban environment;

c. Also a roundabout junction can be used in lieuof very sharp changes in route direction which couldnot be achieved by curves, even of substandard radii.

3.2 On single carriageways where overtakingopportunity is limited, the siting of roundabouts canoptimise the length of straight overtaking sections on 3.either side of them. (See TD 9 DMRB 6.1.1). They can rualso be used to provide an overtaking opportunity on Unthe exit side by the provision of a short length of two delanes in the exit direction, either treated as a dualcarriageway or separated by road markings. The lengthof such a section should be determined in the light ofsite conditions.

3.3 Roundabouts should be sited on level groundpreferably, or in sags rather than at or near the crests ofhills because it is difficult for drivers to appreciate thelayout when approaching on an up gradient. However,there is no evidence that roundabouts on hill tops areintrinsically dangerous if correctly signed and where thevisibility standards have been provided on the approachto the "Give Way" line. Roundabouts should notnormally be sited immediately at the bottom of longdescents where the down grade is significant for LargeGoods Vehicles and loss of control could occur.

3.4 Roundabouts are applicable in urban areas butthey are not generally compatible with Urban

3.ELECTRONIC COPY - NOTSeptember 1993 PAPER COPIES OF THIS ELECTRO.6 Where a proposed roundabout may have anperational effect on an adjacent junction, or viceersa, the interactive effects must be examined andhere appropriate, traffic management measures

uch as prohibited turns (physical or by trafficegulation order) or one way traffic orders may needo be considered.

.8 Roundabout design shall be related to theesign Speed of the approach roads irrespective of

peed limits (but see para 2.3). For completely newoad schemes this shall be estimated in accordanceith the method described in TD 9 (DMRB 6.1.1). or new or improved layout designs on the existingetwork, the design shall be based on the 85ercentile "wet weather" speed on each approachoad. This shall be measured and adjusted inccordance with TA 22 (DMRB 5.1), at a pointufficiently distant from an existing junction as to benaffected by its presence and the speed shall beounded up to the nearest Design Speed above.orresponding positions shall be estimated for new

oundabouts on existing roads.

affic Control (UTC) systems. These systems movehicles through their controlled areas in platoons byjusting traffic signal times to suit the requiredogress. Roundabouts interfere with platoonovement to the extent that subsequent inflows townstreams traffic signals cannot be reliablyedicted, and thus the sequence breaks down.

nctions and accesses are associated with right turns. e banning of such right turns can be accomplished by

oviding a roundabout at a more important junctionarby.

7 Roundabouts are not normally recommended atral dual three lane all-purpose road at-grade junctions.der these conditions it is difficult to achieve adequate

flection.

5 The majority of accidents at major/minor FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 3/1

Chapter 3 Volume 6 Section 2The Siting of Roundabouts Part 3 TD 16/93

3.9 Small roundabouts sometimes use less landthan alternative single lane dualling, and can thus beattractive in sensitive areas.ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED September 19933/2

Volume 6 Section 2 Chapter 4Part 3 TD 16/93 Safety4. SAFETY

4.1 In 1990 there were about 258,000 personal b.injury accidents in Great Britain. Of these, about approa14,100 (5.5%) occurred at roundabouts. The proportion researcof accidents at roundabouts which were fatal was has on0.43%, whereas 1.3% of all other junction accidents and2.8% of link accidents were fatal; this indicates how c.effective roundabouts are in reducing road accident compaseverity at junctions. From Road Accidents in Great is nothBritain 1990, the average accident cost at a roundabout the reccan be calculated as about 50% less than that at all otherjunctions and about 70% less than that on links. 4.5

4.2 A study by Hall and Surl showed that on well poorertrafficked dual carriageways, for similar flows on bothroads a roundabout will generally have fewer accidents a.than a signalised junction. signs, t

4.3 Nevertheless, notwithstanding their good movingrecord, great care must be taken in layout design to left to secure the essential safety aspects. The most common chevroproblem affecting safety is excessive speed, both at chevroentry or within the roundabout. The most significant offsidefactors contributing to high entry and circulating speeds boardsare:- effects

a. Inadequate entry deflection. island

b. A very acute entry angle which encourages fast in a chmerging manoeuvres with circulating traffic. at a ge

c. Poor visibility to the "Give Way" line. are hig

d. Poorly designed or positioned warning and b.advance direction signing. fast du

e. "Reduce Speed Now" signs, where provided, reductibeing incorrectly sited. markin

f. More than four entries leading to a large c.configuration. resistan circula4.4 Additionally safety aspects to be considered in speed odesigning a layout include:- resistan

a. Angle between arms: The accident potential of micro-an entry depends on both the angle clockwise between that theits approach arm and the next approach arm, and the approptraffic flows. A high flow entry should have a large that de

reducin

signs, m

associa

in urba

alterna

TRL R

aggregELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECT

angle to the next entry, and a low flow entry a smallerangle in order to minimise accidents. (Ref TRL ReportLR 1120 Accidents at 4-Arm Roundabouts).Gradient: Whilst it is normal to flattench gradients to about 2% or less at entry,h at a limited number of sites has shown that thisly a small beneficial effect on accident potential.

Visibility to the right at entry: This hasratively little influence upon accident risk; thereing to be gained by increasing visibility aboveommended level.

Measures that have been found to be useful in

safety records include:-

The repositioning or reinforcement of warninghe provision of map type advance direction

the central island chevron sign further to theemphasise the angle of turn, placing anothern sign above the normal position, and placingn signs in the central reserve in line with the lane approach on dual carriageways. Chevron can impinge on circulatory visibility but the can be minimised by positioning the boards (and

kerbline. When approach speeds are low (usually

evron pattern inside the central island perimeterntle slope (paragraph 6.6 and Fig 6/1) as an

h it can provide a useful supplement.

The provision of "Yellow Bar Markings" onal carriageway approaches, (TD 6 DMRB 8.2).

on in accidents can be achieved with yellow bargs.

The provision of appropriate levels of skiddingce on the approaches to roundabouts and on the

tory carriageways. It should be noted that at thef traffic on a circulatory carriageway, skiddingce is derived from the surface texture of the

texture). It is, therefore, very important to ensure aggregates used have skid resisting propertiesriate to the circumstances. It should be notedep surface texture - (the macro-texture) necessary

g accidents at existing roundabouts having

aking the "Give Way" line more conspicuous,

ted turn left sign) 2m back from the central

n areas), a ring of contrasting paving can be laid

tive to chevron boards; where approach speeds

eport LR 1010 demonstrates that a 57%

ates which form the surface of the road - (theT FOR USE OUTSIDE THE AGENCYRONIC DOCUMENT ARE UNCONTROLLED 4/1

for good skid resistance on high speed routes is notrequired for circulatory carriageways. Deep surfacetexture is required however, on the approaches toroundabouts if the 85%ile speed of traffic is greater than

Chapter 4 Volume 6 Section 2Safety Part 3 TD 16/93

55 mph (90 kph). Further information is given inStandard HD 21 (DMRB 7.2.1) and Advice Note HA45 (DMRB 7.2.2).

d. The avoidance of abrupt and excessivesuperelevation in the entry region.

e. The reduction of excessive entry width byhatching or physical means.

f. The provision of "Reduce Speed Now" signsand/or "Count-down" markers.

Two Wheeled Vehicles

4.9 Though roundabouts have an impressive4.6 Care should be taken with the choice of kerbtype for roundabout design. A safety problem can arisewhere certain specialist high profile kerbs are usedaround a central island as they can be a danger tovehicles over running the entry. Observations haveshown that these kerbs can result in loss of control oroverturning of vehicles unless the approach angle issmall and actual vehicle speeds are low. Where highprofile kerbs are to be used on approaches, the kerbscan be hazardous for pedestrians and considerationshould be given to the provision of pedestrianguardrails.

4.7 High circulatory speeds cause associated entryproblems and normally occur at large roundabouts withexcessively long and/or wide circulatory carriageways. But they can also be caused at smaller roundabouts byinadequate deflection at previous entries. The solutionto high circulatory speeds usually has to be fairlydrastic, involving the signalisation of problem entryarms at peak hours. In extreme cases the roundaboutmay have to be converted to a ring junction in whichthe circulatory carriageway is made 2-way and theentries/exits are controlled by individual mini or normalroundabouts, or traffic signals.

4.8 If entry problems are caused by poor visibilityto the right, good results can be achieved by moving the"Give Way" line forward in conjunction with curtailingthe adjacent circulatory carriageway by hatching orextension of the traffic deflection island. ELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTR4/2overall safety record for most vehicle types this doesnot apply equally to two wheeled vehicles. Researchhas shown that at four-arm roundabouts on Class Aroads (TRL Report LR 1120), injury accidentsinvolving two-wheeled vehicles constitute about half ofall those reported. The proportion of accidentsinvolving pedal cyclists is about 15%, although theytypically constitute less than 2% of the traffic flow. Theaccident involvement rates for two-wheeled vehicles,expressed in terms of accidents per road usermovement, are 10-15 times those of cars; with pedalcyclists generally having slightly higher accident ratesthan two-wheeled motor vehicle riders.

4.10 The study at four-arm roundabouts, TRLReport LR 1120, has shown for example that, in 30 and40 mph speed limit areas, there are differences in pedalcycle accident involvement rates for different categoriesof roundabouts. Designers should be aware of thefollowing:-

a. Normal roundabouts with small central islandsand flared entries have accident rates which are abouttwice those of normal roundabouts with large centralislands and unflared entries. This relationship appearsto apply consistently for all types of vehicular roadusers. As previously stated, analysis of accident datasuggests that when all types of accident are considered,entry deflection is the most important factor. (See paras7.25 to 7.36);

b. 70 per cent of pedal cycle accidents at smallernormal roundabouts are of the `entry/circulating' type,for example, motor vehicle entering roundabout collideswith pedal cycle crossing entry;

c. At dual carriageway roundabouts the accidentinvolvement rate for cyclists is about two to three timesgreater than that at dual carriageway traffic signals butfor cars, the opposite is true.

4.11 Data for pedal cycle accident involvement ratesin 50 to 70 mph speed limits were less reliable, due tolow pedal cycle flows and few pedal cycle accidents,and did not show any significant differences betweentypes of roundabout. The rates observed were similar tothose for smaller normal roundabouts in 30 and 40 mphspeed limits. FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED September 1993

Volume 6 Section 2 Chapter 4Part 3 TD 16/93 Safety

4.12 Comparable data for pedal cycle accidents at c. Sharp turns into exits.mini roundabouts, three-arm roundabouts and singlecarriageway traffic signals are reported in TRL Report d. Excessive crossfall changes on the circulatoryCR 161. carriageway.

Large Goods Vehicles

4.13 The problem of large goods vehicles

e. Excessive adverse crossfall on a nearside laneof the circulatory carriageway.a

a Tight bend

overturning or shedding their loads at roundabouts hasno obvious solution in relation to layout geometry. Whilst there are only about 60 personal injury accidentsa year in this category, there are considerably moredamage-only accidents. Load shedding often involvesgreat congestion, and delay, and is expensive to clear,especially if occurring at major junctions. Experiencesuggests that roundabouts where these problems persistusually exhibit one or more of the following features:-

a. Inadequate entry deflection leading to highentry speeds.

b. Long straight sections of circulatorycarriageway leading into deceptively tight bends. (SeeFig 4/1).

Example of unsatisfactory Tight Bendon Large Roundabout

Figure 4/1

hacenov1578recabpapaabareen

Ac

4.1premoasscapchpredemihedecanmi

4.1admePreELECTRONIC COPY - NOTSeptember 1993 PAPER COPIES OF THIS ELECTRO4.14 An incipient problem for some vehicles may bepresent even if high speeds are not occurring. Research

s shown that an articulated large goods vehicle with atre of gravity height of 2.5m above the ground can

erturn on a 20m radius bend at speeds as low as mph (24 kph). This is reported in TRL Report LR8. Layouts designed in accordance with theommendations in this document should mitigate the

ove problems, although during construction,rticular attention should be paid to ensure thatvement surface tolerances are complied with and thatrupt changes in crossfall are avoided. Normally there advantages in making the exit radii greater than the

try radii.

cident Prediction

5 A further aspect of Road Safety is the use ofdictive accident models as in ARCADY/3. Thesedels are very useful at the feasibility study stage inisting with junction choice. It is not adequate to useacity as the main (or only) criterion for junction

oice. Once a junction type has been chosen thedictive accident model should then be part of the

tail design process to ensure that the final designnimises accident potential. This approach can alsolp to balance accident costs (or savings) againstlay. At present, however, the ARCADY/3 softwarenot deal with safety and accident assessments at

ni roundabouts.

6 The above items are not exhaustive. Furthervice on the investigation of accidents and remedialasures is given in the Accident Investigation andvention Manual. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED 4/3

Volume 6 Section 2 Chapter 5Part 3 TD 16/93 Road Users Specific Requirements5. ROAD USERS SPECI

Cyclists' Facilities

5.1 Roundabouts are a particular hazard for pedalcyclists as has been outlined in paras 4.9 to 4.11. Theoperational performance and safety factors have beenmonitored at a number of experimental schemes aimedat improving cyclist's safety at roundabouts. Thesehave included the use of with flow cycle lanes around 5the circulatory carriageway, conversion of peripheral tfootways to joint cyclist/pedestrian facilities, shared use tof pedestrian subways and signposting alternative cycle troutes away from the roundabout. r

5.2 Evaluation of these has concluded that once a rcyclist has entered a roundabout it is difficult to reduce rrisk, and that the use of shared facilities have limited Tuse depending on the volumes of pedestrians and rcyclists. Nevertheless, bearing in mind the practicalities sand economics, it is important to consider the provision cof facilities which take cyclists out of the circulatory tcarriageway at roundabouts by application of the tfollowing:-

a. Shared use by pedestrians and cyclists of aperipheral cycle track/footway;

b. A signposted alternative cycle route away fromthe roundabout;

c. Full grade separation, eg by a combinedpedestrian/ cyclist subways system;

Failing these, then greater emphasis should be placed bythe designer on the safety aspects of the design of theroundabout layout, rather than high capacity, by carefulattention to the entries and flares.

5.3 If the volume of cyclists is significant but nothigh enough economically to justify segregatedfacilities then consideration should be given tosignalising the roundabout or to an alternative form ofjunction with traffic signals. Further details onassessment methods are available from the TrafficAdvisory Unit.

5.4 Signallised cycle crossings may be appropriatewhere there are no pedestrian requirements butroundabout arms may intersect a cycle track.

AGn

W

E

p

P

5fHttf

aw

b

cw

d

5tpar

trELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECT

5.5 Special consideration should be given to cyclists asegregated left turn lanes. In these cases it might beappropriate to adopt the principles contained in TrafficFIC REQUIREMENTS

.6 Where there is expected to be regular use ofhe approaches by ridden horses, of the order of morehan 20 passages a week, consideration should be giveno the provision of crossing places where theoundabout arms have to be crossed. These should

oundabout to permit suitable visibility to theoundabout by the rider. The principles are set out inA 57 (DMRB 6.3). Segregated routes at the

oundabout are to be preferred. This may involve sometrengthening of verges, but see TA 57. Ridden horsesould share cycle tracks where these are distant fromhe circulating carriageway but should not be expectedo use pedestrian facilities.

dvisory Unit Leaflet 1/88, Provision for Cyclists atrade Separated Junctions. Otherwise, it might beecessary to end the segregated left turn lane at a "Giveay" line on the exit (para 7.64).

questrians' Facilities

referably be crossed at some distance from the actual

edestrians' Facilities

.7 Separate pedestrian routes with crossings awayrom the flared entries to roundabouts are preferable. ere the carriageway widths are less and vehicular

raffic movements are more straightforward. However,his is not always practical, in which case the followingacilities should normally be considered:-

. Unmarked crossing place (ie dropped kerbs),ith a central refuge if possible.

. Zebra crossing, with or without central refuge.

. Some form of controlled crossing with orithout a central refuge which includes for cyclists.

. Subway or footbridge.

.8 The type of facility selected will depend uponhe volumes and movements expected of bothedestrians and traffic and should be designed inccordance with current recommendations andequirements (DMRB 2.2; 6.3; 8.5). The use of differentypes of facility at the same junction is notecommended as this could lead to confusion by

pedestrians and drivers. Crossings should not be placedT FOR USE OUTSIDE THE AGENCYRONIC DOCUMENT ARE UNCONTROLLED 5/1

t across multi-lane entries. They should be located awayfrom the junction where the carriageway is relativelynarrow.

Chapter 5 Volume 6 Section 2Road Users Specific Requirements Part 3 TD 16/93

5.9 If a crossing giving pedestrian priority isprovided close to the entry/exit points of a roundaboutthere will be inevitable consequences for the operationof the roundabout and possibly for safety. Where acrossing must be provided within the junction layout, azebra crossing is preferred; it avoids any ambiguity asto priority that the lights of the pelican can create forthe driver approaching the roundabout "Give Way" line. If a signalised crossing is provided, it should preferablybe of the divided crossing type to avoid excessivedelays at the exit points, because the `blocking back'mechanism causes queues to extend onto the circulatorycarriageway. For information on the effect of zebracrossings on junction flows, see TRL Report SR 724,the calculation methods of which are incorporated inARCADY/3, and for more detailed advice on thelocation of pedestrian crossings, see DMRB 8.5.

5.10 In urban areas, where large numbers ofpedestrians are present, guard rails or other means ofdeterring pedestrians from crossing should be used toprevent indiscriminate crossing of the carriageway. Thedesign of guard railing should not obstruct drivers'visibility requirements. Guard rails, which are designedto maintain drivers' visibility to pedestrians throughthem, and vice-versa, are available, but should be

checked in case blind spots do occur. TA 57 (DMRB6.3) refers.ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED September 19935/2

Volume 6 Section 2 Chapter 6Part 3 TD 16/93 Landscaping

6.1 The design of landscaping within thehighway limits shall be carried out in consultationwith appropriate specialists. The designer shallconsider the maintenance implications and wherethe responsibility for maintenance is passed to athird party, maintenance standards must be agreed. If third parties wish to enhance the standard ofplanting or landscaping at roundabouts, for example:special floral displays, this shall only be with theagreement of the Overseeing Department, and shallnot compromise visibility or safety. Further advice isgiven in the Good Roads Guide (DMRB Vol 10).

6. LANDSCAPING

species of bushes and coppiced trees, without thicktrunks, towards the centre of the island. Specialisedplanting, which might be more appropriate in an urbanarea, generally requires greater maintenance effort if itis to be successful. Any planting must have bulk andsubstance in winter as well as during the summermonths.

6.4 In rural areas planting should be restricted toindigenous species and be related to the surroundinglandscape. In an open moorland, for example, any6.2 Apart from the amenity benefits, the landscapetreatment of roundabouts can have practical advantagesfrom a traffic engineering point of view. By earthmodelling, perhaps in conjunction with planting, thepresence of the roundabout can be made more obviousto approaching traffic. The screening of traffic on theopposite side of the roundabout to the point of entrycan, without restricting necessary visibility, avoiddistraction and confusion caused by traffic movementsof no concern to a driver. Planting can provide apositive background to chevron signs and directionsigns on the central island while visually uniting thevarious vertical features and reducing any appearance ofclutter.

6.3 By careful planning, the areas required forvisibility envelopes can be planted with species havinga low mature height, with higher and denserELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECTRplanted as the demands of visibility permit with dueallowance for the situation that will develop withmatured growth.

6.5 Generally the planting of roundabout centralisland less than 10m in diameter is inappropriate as theneed to provide driver visibility leaves only a smallcentral area available. Such a restricted area of plantingis out of scale with the roundabout as a whole, andbecomes an incongruous "blob".

6.6 Recent experiments with a ring of black andwhite paving laid in a chevron pattern inside the centralisland perimeter at a gentle slope have proved

islands and they can be effective from a safety point ofview (Fig 6/1). The chevron markings are a road signcurrently available by special authorisation, although

planting of other than local species would appearincongruous and landscape treatment would normallybe restricted to ground modelling. Conversely inwoodland areas roundabouts should be as densely

successful in improving the conspicuity of central

their inclusion among the Traffic Signs Regulations isbeing explored.T FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 6/1

Chapter 6 Volume 6 Section 2Landscaping Part 3 TD 16/93

6X

Section X-X

Contrasting Paving Chevron Markings for Normal Roundabouts

Figure 6/1ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED September 1993/2

Volume 6 Section 2 Chapter 7Part 3 TD 16/93 Geometric Design Features

FEATURESD1 2

Ae

r

v

Point of maximumentry deflection atright hand end of give-way line

A Entry widthApproach half widthEntry RadiusInscribed circleDiameter

evrD

Definitions

7.1 Entry Width: e, is measured from the pointA along the normal to the nearside kerb, see Fig 7/1.

7.2 The Approach Half Width: v, is measuredat a point in the approach upstream from any entryflare, from the median line (or offside edge ofcarriageway on dual carriageways) to the nearsidekerb, along a normal, see Fig 7/1.

7.3 The Average Effective Flare Length: lN, isfound as shown in Fig 7/2. The line GF'D is theprojection of the nearside kerb from the approachtowards the "give way" line, parallel to the medianHA and at a distance of v from it. BA is the linealong which e is measured (and is therefore normalto GBJ), and thus D is at a

7. GEOMETRIC DESIGN

Geometric Design Features

Figure 7/1ELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECTdistance of [e-v] from B. The line CF' is parallel toBG (the nearside kerb) and at a distance of [e-v]/2from it. Usually the line CF' is therefore curved andits length is measured along the curve to obtain lN.

7.4 The Sharpness of Flare: S, is defined bythe relationship:

S=1.6[e-v]/lN

and is a measure of the rate at which extra width isdeveloped in the entry flare. Large values of Scorrespond to short severe flares and small values tolong gradual flares.

7.5 The Inscribed Circle Diameter: D, is thediameter of the largest circle that can be inscribedwithin the junction outline, see Fig 7/1. In caseswhere the outline is asymmetric, the local value inthe region of the entry is taken. The extreme casearises for a double roundabout at a "scissors"crossroads. Fig 7/3 illustrates the determination of Din such cases.

Average Effective Flare Length

Figure 7/2T FOR USE OUTSIDE THE AGENCYRONIC DOCUMENT ARE UNCONTROLLED 7/1

NK

A'

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93D1D2

M LD1D2

applies to entries K and Lapplies to entries M and N

A

F B

D

Entry AngleE

7.6 The Entry Angle: N, serves as a geometricproxy for the conflict angle between entering andcirculating streams. For roundabouts, having morethan a distance of about 30m between the offsides ofan entry and the next exit the construction isillustrated in Figs 7/4 and 7/5.

Fig 7/4 refers to roundabouts where the circulatorycarriageway between an entry and the next exit isapproximately straight. AD is parallel to the straightcirculatory carriageway where A is as in Fig 7/1 andD is the point nearest to A on the median island (ormarking) of the following entry.

Fig 7/5 shows the equivalent construction forroundabouts with curved circulatory carriageways(or those where the line AD in Fig 7/4 is clearly notparallel to the circulatory

t

Iw

ttllbi

Fia

Geometric Design Features

Figure 7/3

Entry Angle

Figure 7/4ELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO7/2D'C F

BE

Entry Angle

C L H

0K

F

G

J

B

Entry angle defined as (90 - /2)0

carriageway) ANDN replaces AD as the line parallelo the circulatory carriageway.

n both cases the line BC is a tangent to the line EF,hich is midway between the nearside kerbline and

he median line or the edge of any median island onhe offside, where this line intersects the "give way"ine. N is measured as the acute angle between theines BC and AD in Fig 7/4, and as the acute angleetween BC and the tangent to ANDN at the point ofntersection between BC and ANDN shown in Fig 7/5.

or all other roundabouts the construction is shownn Fig 7/6. The line BC is the same as in Figs 7/4nd 7/5. The line GH is the tangent to the line JK,

Entry Angle

Figure 7/5

Entry Angle

Figure 7/6 FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993


which is in the following exit midway between the

nearside kerb and the median line or the edge of anymedian island on the offside, where this lineintersects the outer edge of the circulatorycarriageway. BC and GH intersect at L. N is thendefined by:

N = 90-[angle GLB]/2 ie HLB/2

when the righthand side of the equation is positive.

When the righthand side of the equation is zero ornegative, N=0. Angle GLB is measured on the"outside" of the roundabout, that is, on the sidefacing away from the central island.

7.7 The Entry Radius: r is measured as theminimum radius of curvature of the nearsidekerbline at entry, see Fig 7/1. For some designs thearc of minimum radius may extend into thefollowing exit, but this is not important providedthat a half or more of the arc length is within theentry region.

7.8 Desirable Minimum Stopping SightDistance: as defined in TD 9 (DMRB 6.1.1)

7.9 Entry Path Curvature: This is a measureof the amount of entry deflection to the left imposedon vehicles at the entry to a roundabout, see paras7.25 to 7.32.

7.10 Traffic Deflection Island: a raised area(usually kerbed) on the carriageway, which islocated and shaped so as to direct and also separatetraffic movements onto and from a roundabout.

7.11 Ghost Islands used for Subsidiary TrafficDeflection: a shaped area, flush with the roadsurface, delineated by road markings, and within theentry width of the approach to a roundabout, solocated to deflect and direct traffic movements intothe circulatory carriageway.

7.15not l

Entries

7.12 The design of roundabout entries is a complexprocedure, there are several variables which need to beaddressed to ensure a design which is safe and hasadequate capacity.

The calculation of capacity is outlined in Annex 1.

require

Entry Flare LEntry Entry Appro

Entry

7.14 width generaand norelatiosignifiapart fcapaciwith lois not circumlanes babnormthe evespace

Lane wa miniwide llarge gentry,

7.16 accounpossibto use wherethe entrecom

7.17 On rurhigh threservan arc island are lowbe achensureELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECTR Lane widths at the "Give Way" line shall beess than 3.0m.

7.13 The designer has flexibility in the applicationof the parameters to meet best the particular site

ments and constraints. The variables are:-

Widthength

AngleRadiusach carriageway half width

Width

It is good practice to add at least one extra laneto the lanes on the entry approach, but as al rule not more than two lanes should be added entry should be more than four lanes wide. Thenship between entry width and capacity is quitecant. Entry width is the largest single factor,rom approach carriageway half width, affectingty. There may be some cases, usually associatedw predicted flows, where increased entry width

operationally necessary but in thesestances it is still recommended that two entrye provided; this will give added flexibility atal flow periods in the future, a passing facility innt of breakdown, and will ease the problem of

provision for long vehicles turning.

idths should be tapered back in the entry flare tomum width of 2m. It is generally better to useane widths, because they are more suitable foroods vehicles. For example, at a 10m wide3 x 3.33m lanes are better than 4 x 2.5m lanes.

The development of entry lanes should taket of the anticipated turning proportions and

le lane bias, since drivers often have a tendencythe nearside lane. The use of lane bifurcation a lane widens into two should maximise use ofry width. The use of short offside lanes is notmended.

The alignment of entry lanes is also critical. al roundabouts where design speeds are relativelye kerbline of the deflection island (or central

e in the case of a dual carriageway) should be onwhich, when projected forward, meets the centraltangentially. In urban areas, where design speedser, this is less important, but nevertheless should

ieved where possible. Care should be taken to that the resultant entry angle is not too low norT FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 7/3

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93

gth lN (Fig 7/2).

try Angle

2 The effect of entry angle on entry capacity isgative; as the angle increases capacity decreasesghtly. However, care should be taken in the choice oftry angle since high and low angles may result inreased accident potential. The angle should, ifssible, lie between 20 and 60 degrees, though somehway authorities prefer 30 to 40 degrees. Low entry

gles force drivers into merging positions where theyst either look over their shoulders to their right or

empt a true merge using their mirrors (with theendant problems of disregarding the "Give Way" line7.20 Flares on the approach to roundabouts shallbe such that:-

a. the maximum entry width shall not exceed10.5m for single and 15.0m for dual carriagewayapproach roads;

b. the average effective flare length shall notexceed 100m, but it should be noted that beyond 30or 40m any expected extra capacity is derived fromextrapolation beyond the bounds of experimentaldata and should therefore be treated with caution;

that entry path curvature is not too great. len

7.18 For capacity assessment, the entry width shouldbe taken as the width which drivers are likely to use. Where the offside kerbline forms a vehicle path whichis tangential to the central islands the entry width andeffective entry width are the same.

7.19 It is usual to consider design flows 15 yearsafter opening for highway schemes. This can result inroundabout entries with too many lanes for earlier yearflows and lead to operational problems. A design yearlayout will determine overall geometry and landrequirements for the roundabout but for the early yearsit may be necessary for the designer to consider aninterim stage. This approach can result in reduced entrywidths and entry lanes. Consideration can also be givento an interim reduction of the circulatory carriagewaywidth either by an increase in diameter of the centralisland or by extending islands forward into thecirculatory carriageway.

Flare Design At Entry

7.21 The capacity of an entry can be improved byincreasing the average effective flare length, though thisis of limited effect. A minimum length of about 5m isdesirable in urban areas, whilst a length of 25m isconsidered adequate in rural areas. Flare lengthsgreater than 25m may assist in geometric layout buthave little effect in increasing capacity. Flare lengthsshould not be greater than 100m as beyond this thedesign becomes one of link widening. Where thedesign speed is high, entry widening should bedeveloped gradually avoiding any sharp angles. Inurban areas the use of long flare lengths is often notpossible due to land constraints and capacity may haveto be achieved using wider entries and shorter flares.(As a rough guide, the total length of the entry widening(BG) should be about twice the average effective flare

En

7.2neslienincpohiganmu

attattanprobraaccvaex

En

7.2abcapgorouin 10inaporadELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO7/4d generation of high entry speeds). High entry anglesduce excessive entry deflection and can lead to sharpking at entries accompanied by "nose to tail"idents, especially in rural areas. The best entry angle

lue is about 30 degrees. Figs 7/7 and 7/8 show twotreme cases.

try Radius

3 Entry capacity increases with entry radius up toout 20m, higher radii result in very little increase inacity. The minimum entry radius should be 6m, a

od practical design is about 20m. Where andabout is designed to cater for large goods vehicles

particular, the entry radius should not be less thanm. Large entry radii will almost certainly result indequate entry deflection, for example it will not bessible to achieve the deflection standard if the entryius is 100m or more. FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993


pse

eth

tlyEntry angle

Example of Too High an Entry Angle andalso Excessive Entry Deflection

Figure 7/8

Entry Kerbing

7.24 As entries are almost always kerbed, hardstrishould be terminated when entry widening begins. Thsimplest procedure is to place the kerbs at the back ofthe hardstrip and then terminate the hardstrip edge linby profiling it back towards the kerbs in a short smoocurve or taper. (See Figs 7/9 and 7/10). This is notappropriate where there is regular use by cyclists whomay wish to continue to the edge of the circulatorycarriageway by using the hard strip. Where theroundabout is provided with cycle facilities (such as aperipheral cycle track), the hardstrip should link directo these facilities.Entry angle is defined as (90 - /2)0

Example of Too LoSubstanda

FELECTRONIC COPYSeptember 1993 PAPER COPIES OF THIS EL0

w an Entry Angle and alsord Entry Deflection

igure 7/7 - NOT FOR USE OUTSIDE THE AGENCYECTRONIC DOCUMENT ARE UNCONTROLLED 7/5


71m

1m

d

d

b

bc

a

a

a

a

a

a

a

a

cb

1m

d

b

b

b

1m

1m

1m

dd

d

KerbsEdge LinesEdge Lineprofiled backtowards the kerbEdge of Carriageway

abc

d

Method of Terminating Edge Strips on Single CarriagewayApproach to a Roundabout

Figure 7/9

Method of Terminating Edge Strips on Dual CarriagewayApproach to a Roundabout



Entry Deflection

7.25 Entry Path Curvature is one of the mostimportant determinants of safety at roundabouts. Itis a measure of the amount of entry deflection to theleft imposed on vehicles at entry to the roundabout.

e. That the vehicle proceeds towards the "GiveWay" line. Then either:-

it proceeds towards the central island of theroundabout passing through a point not less than 1m7.26 For design purposes only at both new andimproved `normal' type roundabouts, the vehicleentry path shall be such that when scribed inaccordance with the following construction, thetightest radius of the entry path curvature shall notexceed 100 metres. The method of constructing andmeasuring the entry path curvature is describedbelow, and shown in Figs 7/11 to 7/15. Fig 7/13shows an approach with negative curvature, Fig 7/14shows an approach with positive approachcurvature, and Fig 7/15 a roundabout at a "Y"junction.

7.27 Assume:-

a. The entering vehicle is 2m wide and will betaking the `straight ahead' movement at a 4 armroundabout and across the head of the Tee at a 3 armroundabout.

b. That there is no other traffic on theapproach and on the circulatory carriageway.

c. That the driver will negotiate the siteconstraints with minimum deflections and that lanemarkings by the give way line will be ignored.

d. The initial approach position for entry pathcurvature measured from a point not less than 50mfrom the Give Way line, is within the range:-

1m from the nearside kerb1m from the centre line of a single carriageway or1m from the offside kerb of a dual carriageway

This will ensure that all approach alignments areexamined and that no vehicle path can exceed therecommended maximum radius of curvature.ELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECTRfrom the nearside channel or kerb, the position ofwhich relative to the starting point (d above)depends on the amount of approach flare to the left(Fig 7/11 and Fig 7/13);or where a subsidiary traffic deflection ghost islandexists, it is assumed to pass whichever side of theisland involves the least deflection (Fig 7/12).

f. The vehicle is then assumed to continue ona smooth path with its centre-line never passingcloser than 1m from the central island (it may bemore in some configurations)

7.28 Draw, to a scale not less than 1/500 using aflexible curve or equivalent, the centre line of themost realistic path that a vehicle would take in itscomplete passage through the junction on a smoothalignment without sharp transitions. More than oneindependent assessment of the vehicle paths shall becarried out.

7.29 This tightest radius shall be measured bymeans of suitable templates. See para 7.32.

7.30 The exact path drawn will be a matter ofpersonal judgement and the results should be examinedfor compliance and consistency with the appropriateparagraphs in this section.

7.31 One convenient method of construction of therequired path is to imagine the advance of all thechannel or kerb lines and centre line in the case ofsingle carriageways (together with central islands anddeflection islands) into the carriageway by 1m. Thevehicle path will be the line of least resistance, whosecentre line will normally, but not always, be tangentialto these construction lines; in the entry, at the centralisland and in the exit. Any reverse of curvature in thevehicle path around the central island must be drawn sothat there is no sharp deviation between that curve andthe entry curve. Particular care in checking entry pathcurvature is required when considering small centralisland designs.T FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 7/7

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93Y

Xc

a

1m min

bb

1m minimum

Determination of Entry Path CurvatureFigure 7/11

b

b

1m

a

1mc

c

X

X

Y

1m

Y

1m1m min

Determination of Entry Path Curvature(when subsidiary traffic deflection islands exist)

Figure 7/12The radius should be measured over a distance of 20-25m; it is the minimum which occurs along theapproach entry path in the vicinity of the Give Wayline but not more than 50m in advance of it.

a Commencement point either 1m from nearsidekerb or 1m from offside kerb for D2L; 1m fromnearside kerb or 1m from centreline S2L, at apoint not less than 50m from the Give Way line.

b

Vehicle entry path curvature

1m

cELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYPAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED September 19937/8


Se1mb

Xc

1m minY

a

1m

Determination of Entry Path Curvature(on a curved approach arm with negative approach curvature)

Figure 7/13

1m min

1m minY Xc

a

1mb

Determination of Entry Path Curvature(on a curved approach arm with positive approach curvature)

Figure 7/14

The radius should be measured over a distance of 20-25m; it is the minimum which occurs along theapproach entry path in the vicinity of the Give Wayline but not more than 50m in advance of it.

a Commencement point 1m from the offside kerbfor D2L or 1m from centreline S2L, not less than50m from the Give Way line.

b

Vehicle entry path curvature.cELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 7/9

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93The radius should be measured over a distance of20-25m; it is the minimum which occurs along theapproach entry path in the vicinity of the Give Wayline but not more than 50m in advance of it.

Commencement point 1m from offside kerb for D2or 1m from centreline for S2L; not less than 50mfrom the Give Way line.

a

Vehicle entry path curvaturec

b

a

Centreline offset15-20m (Note

'easy exits)a

To Measure the Entry Path Curvature

7.32 The entry path curvature is measured on thecurved length of path in the vicinity of the "GiveWay" line (but not more than 50m in advance of it)between points X and Y (See Figs 7/11 to 7/15)about 20m to 25m but not less than 20m in length,over which the tightest radius occurs.

Determination of Entry Path Curvature for a Roundabout at a Y Junction

Figure 7/15

Achieving Entry Deflection

7.33 A good method for creating entry deflection onnew schemes where there are no other constraints is tostagger the arms, as shown in Fig 7/16. This will helpwith the overall design, reduce the size of roundabouts,minimise land acquisition and assist with theconstruction of "easy" exits.

Entry Deflection by Staggering Approach Roads

Figure 7/161m minELECTRONIC COPY - NOT FPAPER COPIES OF THIS ELECTRO7/101m min

b

1m minc X

Y

aOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993

Volume 6 Section 2 Chapter 7Example Showing how Deflection Island Design can Increa

Figure 7ELECTRONIC COPY - September 1993 PAPER COPIES OF THIS ELECse Entry Deflection at an Existing Roundabout

/17Part 3 TD 16/93 Geometric Design Features

7.34 It is not good practice to generate entry necessitate small normal roundabouts which cannotdeflection by sharply deviating the approach roads to provide sufficient entry deflection to the left by meansthe right close to the roundabout and then to the left at of the central island alone. In these cases deflectionentry. Approach curves should be fairly gentle but should be generated by means of enlarged traffic islandsthere are cases when horizontal radii below the or by subsidiary traffic deflection ghost islands in theminimum for the general design speed of the approach entry, (Figs 7/17 and 7/18). link may be used, provided always that they areproceeded by the "Roundabout Ahead" warning sign as 7.36 Subsidiary Deflection Ghost Islands are areasdefined in the Traffic Signs Regulations and General defined by road markings, flush with the road surface.Directions. However, tight radii will require large They should not be kerbed or raised. The conspicuity ofamounts of verge widening to provide adequate forward subsidiary traffic deflection islands should be improvedvisibility and add to the verge maintenance by surfacing the area in white reflectorised material. Torequirements. There is evidence to suggest that a gentle highlight the perimeter during the hours of darkness,right hand bend leading to a left hand deflection at entry reflecting road studs should be affixed to theis more safe than a gentle left hand bend. carriageway surrounding the islands along the

7.35 In urban areas, the restrictions on space the design of subsidiary islands to avoid any confusionavailable coupled with the turning width requirements with the primary traffic deflection island.of large goods vehicles may

alignment of the warning line. Care must be taken inNOT FOR USE OUTSIDE THE AGENCYTRONIC DOCUMENT ARE UNCONTROLLED 7/11

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93a Ghost island for subsidiary traffic deflection

Visibility

7.37 The forward visibility at the approach to aroundabout shall be as laid down in TD 9 (DMRB6.1.1) measured to the "Give Way" line as shown inFig 7/19.

7.38 The following guidelines represent goodpractice concerning the provision of visibility and,when subject to relaxation, there is a need foradditional signing to alert drivers of all vehicles topotential hazards.

Eye and Object heights

7.39 Visibilities, with the exception of visibilityto the right at entry and across the

central island, shall be assessed in accordance withTD 9 (DRMB 6.1.1). Visibility to the right andacross the central island shall beobtainable from a driver's eye height of 1.05m to anobject height of 1.05m, and the envelope ofvisibility shall extend to 2.0m above the roadsurface.

7.40 Where traffic and direction signs are to beerected on a central reservation, verge, or deflectionisland within the envelope of visibility, including tothe right, the mounting height shall not be less than2.0m above the carriageway surface and theenvelope checked on sites with changes of gradient.

Entry Deflection Achieved by Subsidiary Traffic Deflection Islands

Figure 7/18a

aELECTRONIC COPY - NOTPAPER COPIES OF THIS ELECTRO7/12a

a FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993

Volume 6 Section 2 Chapter 7Part 3 TD 16/93 Geometric Design Features7.3ma

5.5m

7.3m Dual Carriageway

5.5m

a10m

10m Single Carriageway

7.3m

3.65m

a

7.3m Single Carriageway

Vehicle position centre of nearside laneDesirable Minimum Stopping Sight Distance (SSD) for Approach Road Design Speeda

Measurement of Stopping Sight DistanceFigure 7/19ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYSeptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 7/13


Visibility to the Right

approach, and the visibility along the approach to no

Forward Visibility at Entrymore than the stopping sight distances for the designspeed of the approach, by the selective use oflandscaping.

Inscribed Circle Diameter (m)

< 40

40-60

60 - 100

> 100

TableELECTRONIC COPY - NPAPER COPIES OF THIS ELEC7/14vegetation or surface treatment. In these situations,limited penetration into the visibility envelope byvegetative growth of a dispersed nature would notbe unacceptable.

Visibility Distance (m)("a" in Fig 7/20a)

Whole junction

40

50

70

7/17.41 Drivers of all vehicles approaching the"Give Way" line shall be able to see the full width ofthe circulatory carriageway to their right, from the"Give Way" line for a distance appropriate to thesight stopping distance for the circulatory traffic(measured along the centre line of the circulatorycarriageway) as indicated in Table 7/1, and shown inFig 7/20a. This also applies to roundabouts that haveparapet walls on either side of the ciculatorycarriageway.7.42 This visibility shall be checked from thecentre of the offside lane at a distance of 15m backfrom the "Give Way" line, as shown in Fig 7/20b.Checks shall be made that crossfall design orconstruction and sign location do not restrictvisibility.

7.43 It should be noted that excessive visibility atentry or visibility between adjacent entries can resultin approach and entry speeds greater than desirablefor the junction geometry. Consideration shall begiven to limiting in particular the visibility ofadjacent entries to that from 15m back on the

7.44 Drivers of all vehicles approaching the"Give Way" line shall be able to see the full width ofthe circulatory carriageway ahead of them for adistance (measured along the centre line of thecirculatory carriageway) appropriate to the size ofthe roundabout (as indicated in Table 7/1). Thevisibility shall be checked from the centre of thenearside lane at a distance of 15m back from the"Give Way" line as shown in Fig 7/21.

Circulatory Visibility

7.45 Drivers of all vehicles circulating on aroundabout shall be able to see the full width of theciculatory carriageway ahead of them for a distanceappropriate to the size of roundabout (as indicated inTable 7/1). This visiblity shall be checked from apoint 2m in from the central island as shown in Fig7/22. It is often useful to improve the conspicuity ofcentral islands by the use of landscaping but thiscould obstruct circulatory visibility. The circulatoryvisibility envelope will encroach onto the height ofOT FOR USE OUTSIDE THE AGENCYTRONIC DOCUMENT ARE UNCONTROLLED September 1993

Volume 6 Section 2 Chapter 7Part 3 TD 16/93 Geometric Design FeaturesArea of circulatory carriageway overwhich visibility shall be obtainedfrom viewpoint

c

a

b

c15mc

b

Area of circulatory carriageway overwhich visibility shall be obtained from viewpoint

Sight stopping distance for circulatingtrafficLimit of visibility splayHalf lane width

a

bc

b

Visibility to the Right Required at Entry ( from Give Way line )Figure 7/20a

Visibility to the Right Required at Entry (15m back from Give Way line)

Figure 7/20bELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCYSeptember 1993 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 7/15

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93b

b

15m

c

a

Area of circulatory carriageway overwhich visibility shall be obtainedfrom viewpoint

Distance related to circulatory speedLimit of visibility splayHalf lane width

abc

Area of circulatory carriageway overwhich visibility shall be obtainedfrom viewpoint

2ma

b

Distance related to circulatory speedLimit of visibility splay

ab

Forward Visibilty Required at Entry

Figure 7/21

Circulatory Visibility



Pedestrian Crossing Visibility

7.46 Drivers of all vehicles approaching a

drivers and is not obscured by a vertical curveformed in the road surface. This can be achieved bythe provision of a short length, say 10m, of level

roach road immediately prior to the "give way".pedestrian crossing across an entry shall have aminimum distance of visibility to it of DesirableStopping Sight Distance for the design speed of thelink. See TD 9 (DMRB 6.1.1). At the "Give Way"line, drivers of all vehicles shall be able to see thefull width of a pedestrian crossing across the nextexit if the crossing is within 50m of the roundabout. (See Fig 7/23). In urban areas, adjacent roadsidedevelopment may however prevent this visibilitysplay being fully established.

Visual Intrusions

7.47 Signs, street furniture and planting shall notbe placed within the visibility envelopes so as toobstruct visibility, but infringements by isolated slimprojections such as lamp columns, sign supports orbridge columns can be ignored provided they areless than 550mm wide. The only exception to thiswill be positioning of bollards on deflection islandsand staggered chevron boards on central islands(para 4.5a). Where possible, footways should belocated outside visibility envelopes but where this isnot possible care shall be taken to minimise theeffects of pedestrians on visibility requirements.

Visibility at Grade Separated Junctions

7.48 Where roundabouts are above the mainthrough route, it is most important to providevisibility at the slip road entries. Layouts shall bechecked at the initial design stage to ensure thatentry visibilities will not be obstructed by safetyfences, barriers, bridge parapets or other features.Hatching out on the outside of roundabouts can beused to advantage to improve the situation wherevisibility for traffic entering from slip roads islimited. This can serve to achieve safer operation atdedicated left turn lanes from entry slip roads. If aroundabout is part of a flyover junction, the flyoverabutments shall be set back to provide therecommended visibilities at the slip road entries. Otherwise severely restricted visibilities of thisnature at entries usually generate delays and causeaccidents. It is important that the "give way" line isclearly visible to approaching

appline

7.5shaDia

7.5shatheif scon

Circ

7.49be cirefer

7.52reverlinkibetwmeththerethe ncurv

7.53propcarricase carriadjacIt wocarriand areduinterELECTRONIC COPY - NOSeptember 1993 PAPER COPIES OF THIS ELECTR0 The width of the circulatory carriagewayll not exceed 15m.The largest Inscribed Circlemeter (ICD) for a mini roundabout shall be 28m.

1 The width of the circulatory carriagewayll be constant and lie between 1.0 and 1.2 times maximum entry width. However, see para 7.58mall Inscribed Circle Diameters (ICDs) are beingtemplated.

ulatory Carriageway

The circulatory carriageway should, if possible,rcular in plan, avoiding deceptively tight bends asred to in para 4.13b and shown in Fig 4/1.

It is normal practice to avoid short lengths ofse curve between entry and adjacent exits byng these curves or joining them with straightseen the entry radius and the exit radius. Oneod is to increase the exit radius. However, where is a considerable distance between the entry andext exit, as at three entry roundabouts, reverseature may result. (See Fig 7/20).

There may be situations where the turningortions are such that one section of circulatoryageway will have a relatively low flow. In thisthere may be an over provision in circulatingageway width and an area of carriageway, usuallyent to an entry deflection island, becomes unused. uld be possible to reduce the circulatory

ageway width by extending the deflection islanddvancing the give way line. This method of

cing circulatory width may also be adopted as anim measure in the early years of a scheme.T FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 7/17

Chapter 7 Volume 6 Section 2Geometric Design Features Part 3 TD 16/93c

ba

Minimum area over which unobstructedvisibility is required from viewpointwhen crossing is within 50m of exitLimit of visibility splayHalf lane width

a

bc

Figure 7/23 : Visibility Required at Entry to Pedestrian Crossing at next Exit

7.54 For larger roundabouts this reduction incirculatory width can be achieved by the use of hatchmarkings and is often associated with taking out of usethe offside entry lane. If such measures are to beconsidered as an interim geometric design feature forearly years traffic flows, consideration should be givento the use of contrasting hard surfacing for these areas.

7.55 For smaller roundabouts it is more appropriateto consider interim circulatory carriageway reduction byincreasing the size of the central island. If this is to beintroduced from the outset a preferable measure wouldbe the use of contrasting hard surfacing but hatchmarkings could also be considered.

Inscribed Circle Diameter (ICD)

7.56 The following advice is based on the sweptturning paths generated by a 15.5m long articulatedvehicle with a single axle at the rear of the trailer. Thisis referred to below as the "Design Vehicle". Theturning width required by this type of vehicle is greaterthan that for all other vehicles within the normalmaximum dimensions permitted in the current VehicleConstruction and Use Regulations, or likely to bepermitted in the near future. The requirements for othervehicles (including an 11m long rigid vehicle, 12m longcoach, 18.35m drawbar-trailer combination, and a16.5m articulated vehicle) are less onerous.

8.010.012.0


7.57 The smallest ICD for a normal roundabout that overrun by the rear wheels of articulated vehicles and14.016.018.0

89

10

Turning Widths Required for

FigurELECTRONIC COPY - NOTSeptember 1993 PAPER COPIES OF THIS ELECTR5.06.07.0

.0

.0

.0

13.914.415.015.616.317.0

29.830.832.033.234.636.0

Smaller Normal Roundabouts

e 7/24Main central islandLow profile subsidiary central islandwhere providedRemaining circulatory carriagewaywidth 1.0-1.2 x maximum entry width.Design vehicle1m clearance minimumInscribed Circle Diameter

ab

c

def

In these cases no entry deflectionislands should protude withinthe ICD.

4.06.0

3.04.0

13.013.4

28.028.8

Central Island Diameter (m) R1(m) R2(m) Minimum ICD (m)

b

c f

R1

e

a

R2 d

will accommodate the "Design Vehicle" is 28m. If this trailers. Such islands should have the same profile ascannot be accommodated, a mini roundabout should be the circulatory carriageway with a maximum upstand ofused. It should be noted that it may be difficult, if not 50mm.impossible, to meet the entry deflection requirementwith normal roundabouts which have ICDs up to 40m. 7.58 The turning space requirements for the "DesignIn this case consideration could be given to the Vehicle" at normal roundabouts from 28m to 36m ICDinstallation of a low profile central island which would are shown in Fig 7/24. For ICDs above these values,provide adequate deflection for standard vehicles but and/or where low profile central islands are to beallow installed the circulatory carriageway width should be

checked against TA 20 (DMRB 6.2). But usually therule in para 7.51 will provide more than adequate width. FOR USE OUTSIDE THE AGENCYONIC DOCUMENT ARE UNCONTROLLED 7/19


Exits

7.59 The spacing of an exit and the precedingentry shall not be less than that which results fromthe combination of the minimum entry radius (6m)and the minimum exit radius (20m), thoughdesirable radii of 20m, and 40m respectively shouldbe used where possible. If an existing roundabout is

to be modified to include an additional entry, caremust be taken to ensure that this does not affectsafety at the preceding entry and following exit. Itmay be necessary to redesign the whole junction ifadequate spacing between adjacent entry/exit cannotbe achieved.

7.60 The principle of "easy exits" shall beapplied. A nearside kerb radius of about 40m at themouth of the exit is desirable but for larger ruralroundabouts this may be increased to suit the overalljunction geometry. In any case, this radius shall notbe below 20m or greater than 100m.

7.61 At the beginning of an exit, its width, measurednormally to the exit radius, should, where possible,allow for an extra traffic lane over and above that of thelink downstream. For example, if the downstream linkis S2 or WS2 the width at the exit should be 7.0m or7.5m, and if the link is D2AP, the width should be 10mto 11m. This extra width should be reduced on thenearside in such a way as to avoid exiting vehiclesencroaching onto the entering carriageway at the end ofthe traffic deflection island. Normally, this would be ata taper of 1:15 to 1:20, though where the exit is on anup gradient, the local widening may be extended toreduce intermittent congestion from slow moving largervehicles and to provide an overtaking opportunity forfaster vehicles. Similarly, if the exit road is on a lefthand curve it may be necessary to extend the taperlength and the length of the traffic deflection island. Within single carriageway exits, a minimum width of6m, measured normally to the nearside kerb, should beprovided adjacent to traffic deflection islands to allowtraffic to pass a broken down vehicle. Fig 7/25 shows atypical single carriageway exit embodying some of theabove principles. On exits the edge line shouldcontinue along the line of the kerbing once this isterminated. (See Figs 7/9 and 7/10)

C

7r

pgw

aw

tw

es6StewELECTRONIC COPY - NOT PAPER COPIES OF THIS ELECTRO7/206m min

20-50m

7.0-7.5ma

a Exit radius 40-100m

Typical Single Carriageway ExitFigure 7 / 25

rossfall and Longitudinal Gradient

.62 Steep gradients should be avoided atoundabout approaches or flattened to a maximum of 2er cent before entry. Crossfall and longitudinalradient combine to provide the necessary slope thatill drain surface water from the carriageway. Thus,

lthough the following paragraphs are for simplicityritten in terms of crossfall, the value and direction of

he greatest slope must always be taken into accounthen considering drainage. It may well be that large or

longated roundabouts may have to be designed usingteeper gradients within them in line with TD 9 (DMRB.1.1) in order to meet environmental concerns. uperelevation is arranged to assist vehicles when

ravelling round a curve. Its values, when used, arequal to or greater than those necessary for surfaceater drainage.FOR USE OUTSIDE THE AGENCYNIC DOCUMENT ARE UNCONTROLLED September 1993


nly apply where vehicle speeds are relatively low.

or Exits

.67 Superelevation, related to the horizontallignment, should be provided where necessary to assistehicles to accelerate safely away from the roundabout. owever, as with entries, crossfalls adjacent to theundabout should be those required for surface water

rainage. If the exit leads into a right hand curve,perelevation should not be introduced too quickly and such a value that vehicles tend to encroach into an

djacent (dual or opposing single carriageway) lane.

dverse Crossfall

.68 Adverse Crossfall is crossfall that acts againste desired movement of a vehicle when turning. It canad to driver discomfort and even safety hazards andould, if possible, be eliminated from the paths of theain traffic movements at normal roundabouts. Miniundabouts and smaller normal roundabouts in urban7.63 Superelevation is not required on the ocirculatory carriageways of roundabouts whereascrossfall is required, to drain surface water, but on theapproaches and exits superelevation can assist drivers tonegotiate the associated curves.

7.64 Normal crossfall for drainage on roundaboutsshould be 2 per cent (1 in 50). Crossfall should notexceed 2.5 per cent (1 in 40). To avoid ponding,longitudinal edge profiles should be graded at not lessthan 0.67 percent (1 in 150), with 0.5 per cent (1 in 200)considered the mi

design of roundabouts

Documents

electronic document

geometric design features8

reviewedstandard td

document nowprovides

highways agency td

entry width

entry deflection

agencypaper copies