Mini roundabouts: good practice guidance

FOREWORD

Gillian MerronParliamentary Under-Secretary

Department for Transport

Mike AllisterImmediate Past President of CSS

The modern concept of a mini-roundabout was introduced in the UK in the early 1970s as ameans to improve capacity and reduce delays at existing junctions where there was limitedscope to introduce other forms of control. Since that time, most local authorities havedeveloped their use to address other issues such as casualty reduction and as a speed-reducing feature within traffic-calmed areas. There are about 5,000 mini-roundabouts aroundthe country and a great deal of experience has been gained in their application.

The purpose of this document is to pull together this wealth of experience so that it can beshared with all those involved in the various aspects of highway management. It is important tonote that this document is not intended as a design standard, but rather to provide guidanceconcerning appropriate locations and situations where mini-roundabouts should be considered.

We would like to thank all those involved in the production of this document for theircommitment and hard work. In particular we wish to thank Faber Maunsell, members of theCSS, the Steering Group and the many authorities and organisations that have providedinformation and examples of good practice.

On behalf of the County Surveyors Society and the Department for Transport, wewholeheartedly commend Mini-Roundabouts – Good Practice Guidance to all with an interestin creating safer roads and the management of traffic within our urban streets.

TABLE OF CONTENTS

1. Introduction and Background ...................................................................................... 1

1.1. General ............................................................................................................... 2

1.2. Purpose of Guidance ......................................................................................... 2

1.3. Background ........................................................................................................ 2

1.4. Relationship with DMRB ..................................................................................... 2

1.5. Disclaimer ........................................................................................................... 2

1.6. Structure of Document ....................................................................................... 2

2. Definition and Use of Mini-Roundabouts ................................................................... 3

2.1. Definition of a Mini-Roundabout ........................................................................ 4

2.2. Use of Mini-Roundabouts .................................................................................. 5

2.3. Improving the Operation of an Existing Junction ............................................... 5

2.4. As an Accident Remedial Measure .................................................................... 6

2.5. As a Traffic Calming Measure ............................................................................ 6

2.6. As an Access to a New Development ................................................................ 6

3. Site Assessment ............................................................................................................. 9

3.1. General ............................................................................................................... 10

3.2. Early Rejection ................................................................................................... 10

3.3. Stage 1 Site Assessment ................................................................................... 10

3.4. Stage 2 Site Assessment ................................................................................... 11

3.5. Visibility ............................................................................................................... 11

3.6. Vehicle Speed .................................................................................................... 12

3.7. Road Character .................................................................................................. 13

3.8. Traffic Volume ..................................................................................................... 15

3.9. Number of Arms ................................................................................................. 15

3.10. Traffic Composition ............................................................................................. 15

3.11. Vulnerable Road Users ...................................................................................... 16

3.12. Consultation ....................................................................................................... 17

3.13. Road Network ..................................................................................................... 17

3.14. Noise and Vibration ............................................................................................ 18

4. Existing Practice ........................................................................................................... 19

4.1. Introduction ......................................................................................................... 20

4.2. Responses to Consultation ................................................................................ 20

4.3. Example Sites .................................................................................................... 23Site: Treffry Lane - B3268 ......................................................................................24Site: A1134 Brooks Road - Brookfield ...................................................................25Site: Poppyfields .....................................................................................................26Site: B5259/B5260 junction ....................................................................................27Site: Colchester Road - Freebournes Road ...........................................................28Site: Fox Lane - West Paddock ..............................................................................29Site: Treswithian Road - Weeth Road ....................................................................30Site: Castle Road - Phillpotts Avenue ....................................................................31Site: The Avenue (north) - The Avenue (south) - St Swithuns Road .....................32Site: Westgate - Sherborne Road ..........................................................................33Site: High Road - Falkers Way (east).....................................................................34Site: A414 Main Road - Well Lane .........................................................................35Site: Kennington Road (north) - Kennington Road (south) - Upper Road .............36Site: The Glebe - Manor Road ...............................................................................37Site: A12 off-skip - Shell Garage Access...............................................................38

4.4. Post Implementation Monitoring ......................................................................... 39

4.5. Maintenance ....................................................................................................... 39

4.6. Driver Behaviour ................................................................................................. 39

4.7. Road User Education ......................................................................................... 40

4.8. Frequently Asked Questions .............................................................................. 40

5. Acknowledgement and References ............................................................................ 45

5.1. Acknowledgements ............................................................................................ 46

5.2. References ......................................................................................................... 46

1. INTRODUCTION AND BACKGROUND

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1.1. GeneralMini-roundabouts have been widelyintroduced on a variety of roads around theUK, from strategic routes (including trunkroads) to residential roads. Practice regardingthe selection and design of mini-roundaboutsvaries between highway authorities, resultingin a degree of confusion regarding the safetyand suitability of mini-roundabouts in somecircumstances. There is also a lack ofawareness of regulations relating to mini-roundabouts.

1.2 Purpose of GuidanceThis document seeks to help practitionersunderstand what a mini-roundabout is andhow it should be used. It explains thelegislative basis for mini-roundabouts andestablishes current practice based upon realexamples of installation and lessons learned.This document does not explain how a mini-roundabout should be designed; see section1.4 for further information. The intention is toexamine mini-roundabouts in terms of theircurrent use, as a traffic engineering tool. Theroad markings for a mini-roundabout andrelated signs are prescribed in the TrafficSigns Regulations and General Directions2002 (TSRGD). Detailed guidance on thecorrect use of these signs and markings canbe found in Chapters 3 and 5 of the TrafficSigns Manual.The objectives of this document are to:• clarify the definition of a mini-roundabout;• identify what can or cannot be done (i.e.

regulations);• illustrate what could, should or should not

be done (i.e. examples of good and badpractice);

• identify issues to consider when thinkingabout introducing a mini-roundabout; and

• provide a structure to guide the decisionand early design processes.

1.3 BackgroundThis document considers the range of factorsthat may affect the suitability of a site for amini-roundabout. When making a decisionregarding its use, a comparison with otherforms of junction will be undertaken. It isimportant to identify any factors present at ajunction that may suggest a mini-roundaboutis an unsuitable choice as early as possiblein the assessment process. The mini-roundabout can then be discounted andanother junction type investigated. Thedesigner should use judgement andexperience, as well as available guidanceand advice, to decide whether a mini-roundabout is a practicable option.

This document is for use by highwayauthority engineers, or their consultants, andapplies to mini-roundabouts on non-trunkroads.

1.4 Relationship with DMRBGuidance on the design of roundabouts isprovided in TD 16/93. This is to besupplemented with a new TD providingdetailed guidance on mini-roundabouts,which is mandatory for trunk roads butadvisory for applications on local roads. Thedesign guidance contained in the standardwould be applicable to all roads but theguidance on siting and use may differ onlocal roads, which are different in character totrunk roads.

1.5 DisclaimerThis document is intended as guidance. Itdoes not remove or reduce the requirementfor designers to exercise engineeringjudgement when deciding which standards oradvice can be applied, nor does it prohibit theconsideration of departures from standardsor advice in exceptional circumstances.Any justification for departures from theavailable advice and guidance should berecorded and must take into account thegeneral ‘duty of care’ a highway authorityhas, in law, to the road user.Where advice is thought to be safety critical,this is clearly identified. Mini-roundaboutlayouts will usually be subject to a road safetyaudit, in accordance with the highwayauthority’s policy.Although this document contains ranges ofvariables it is not implied that everycombination is acceptable and somecombinations may attract adverse commentsduring a safety audit.This document is intended to representcurrent good practice but is not intended tocover all eventualities or situations that mayarise during the consideration and design ofa particular junction solution.

1.6 Structure of DocumentThe document is structured to reflect thedecision-making process, starting with anunderstanding of what a mini-roundabout isand leading through the site assessmentcriteria to design details.Chapter 2 provides a definition of a mini-roundabout and provides information on howa mini-roundabout can be used. Chapter 3considers site assessment issues. Chapter 4includes a review of existing practice, asummary of the results of the consultationexercise and answers frequently askedquestions.

1. Introduction and Background

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2. DEFINITION AND USE OF MINI-ROUNDABOUTS

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2.1 Definition of a Mini-Roundabout

A mini-roundabout is a type or form ofjunction control at which vehicles circulatearound a white, reflectorised1, central circularroad marking (central island) of between oneand four metres in diameter, as shown inTSRGD diagram 1003.4.

Vehicles entering the junction must give wayto vehicles approaching from the right,circulating the central island.2

The central road marking is either flush orslightly raised as a dome3 (no more than125mm), in order that it can be driven over bylarger vehicles that are physically incapableof manoeuvring around it. The dome is alsoraised to discourage vehicles from drivingover the central island4. Three white arrowsare painted on the carriageway, within thegyratory area, around the central roadmarking, showing the direction of circulation.

Figure 2.1.1: TSRGD diagram 1003.4

Figure 2.1.2: TSRGD diagram 1003.3

1 TSRGD 2002, Regulation 31(1)2 TSRGD 2002, Regulation 25(5)3 TSRGD 2002, Regulation 32(2)(c) – see also Section 3.134 See TSRGD Regulation 16(1) Table item “…a vehicle

proceeding through the junction must keep to the left of thewhite circle at the centre of the marking shown in diagram1003.4, unless the size of the vehicle or the layout of thejunction makes it impracticable to do so.”

A blue mini-roundabout sign (illuminated ifsited within 50 metres of a street lamp withina system of street lighting), as shown indiagram 611.1, precedes the mini-roundabouton each approach. This sign is usuallyaccompanied by the transverse give waymarking shown in diagram 1003.3. However,the mandatory give way markings (diagram1003 and 1023), and give way sign (diagram602), may be used in addition to diagram611.1 where appropriate.5 Where diagrams

1003 and 1023 are used, diagram 602 shouldbe placed above diagram 611.1 as illustratedbelow:

Photo 2.1.1: TSRGD diagram 611.1

Photo 2.1.2: TSRGD diagram 602 andTSRGD diagram 611.1

Warning of the approach to a mini-roundabout can also be provided using theroundabout ahead sign (diagram 510).

When negotiating a mini-roundabout driversmust pass round the central road marking onthe left hand side unless the size of thevehicle or layout makes it impracticable to doso.

Research suggests there are considerablevariations in construction of the roundaboutcentral island. The central island of a mini-roundabout does not conform to diagram1003.4 if:

• it has a diameter less than one metre orgreater than four metres;

• it cannot be driven over;

• it has a surface colouring other thanwhite;

• it is not reflectorised;

• it is constructed of granite setts, blockpaving or other textured material (unlesscoloured white);

• it contains street furniture6;

5 See paragraph 8.17 of Chapter 5 of the Traffic Signs Manual,which explains where GIVE WAY signing should be used.

6 Traffic Signs Manual Chapter 5 Road Markings, para 8.10

A mini-roundabout iseffectively a roadmarking. If the roadmarking is not inaccordance withTSRGD diagram1003.4 it is not amini-roundabout.

2. Definitions and Use of Mini-Roundabouts

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Mini-roundaboutsare generally usedfor one of four mainreasons:• to improve the

operation of anexisting junction;

• as an accidentremedialmeasure;

• as part of a trafficcalming scheme;and

• to provide anaccess to a newdevelopment

2.2 Use of Mini-Roundabouts

Mini-roundabouts were initially developed asa method of improving safety at existingjunctions, but are now increasingly includedas part of new development proposals. Mini-roundabouts may be introduced at junctionsthat experience problems with safety or sideroad delay. They can be used at junctions tobreak up long, straight sections of road or toachieve a sharp deviation of the main routewithout the need for low standard radii.

Mini-roundabouts are often considered as analternative to another junction type due toconstrained highway space or because theyare perceived to be less costly. Earlyexamples were used as an alternative totraffic signals at very constrained sites wherean alternative method of control was needed.

The four main reasons why practitionersconsider mini-roundabouts as a potentialoption are:

• to improve the operation of an existingjunction;

• as an accident remedial measure;

• as part of a traffic calming scheme; or

• to provide an access to a newdevelopment.

2.3 Improving the Operation of anExisting Junction

Mini-roundabouts are used to replace priorityjunctions, traffic signal junctions andconventional roundabouts to improve junctionoperation.

They are usually installed at T-junctions andcrossroad junctions (3 or 4-armed junctions).Mini-roundabouts should not be used atjunctions with five or more arms.

• it has a raised kerb (more than 6mm);

• it has non-prescribed road markings suchas concentric rings;

• it incorporates road studs.

Photo 2.1.3: Non-conforming concentric rings

Photo 2.1.4: Street furniture on central islandcreating a small roundabout, not a mini-roundabout

Photo 2.1.5: Street furniture on a domed centralisland in tarmac creating a small roundabout,

not a mini-roundabout

Photo 2.1.6: Non-conforming central marking in settswith white edge marking

Photo 2.3.1: Before view of priority junction

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Refer toMOLASSES andlocal accidentsrecords whenconsidering amini-roundabout

Carefulconsiderationshould be given tointroducing mini-roundabouts aspart of a newdevelopment

A mini-roundabout can improve the operationof a junction by:

• Reducing the dominance ofone traffic flow

As the mini-roundabout works on theprinciple of ‘priority to circulating traffic fromthe right’, a minor traffic flow can be givenpriority over a major traffic flow that wouldotherwise dominate the junction.

• Giving priority to right turners

Again the ‘priority’ principle of operation hasbeen exploited for right-turning traffic, giving itpriority over ahead movements from theopposing direction.

• Facilitating access andreducing delay at side roads

The ‘priority to the right’ rule effectivelyhalves the traffic to which side road flow hasto yield priority, making it easier for side roadtraffic to turn.

• Improving capacity atoverloaded junctions

For a given road space, the mini-roundabouthas a higher capacity than most alternativesand is very flexible in coping with variations inboth volumes and proportions of traffic flowduring the day.

2.4 As an Accident Remedial Measure

Mini-roundabouts are most commonlyintroduced as an accident remedial measure:

• to reduce the number of accidents at ajunction. For 3-arm sites, the meanaccident rate for mini-roundabouts issimilar to that of priority T-junctions andabout 30% less than for signalledjunctions.

• to reduce the severity of accidents at ajunction. The severity of accidents(percentage of fatal and seriousaccidents to all injury accidents) at 3-armmini-roundabout sites is lower than at 3-arm signalled junctions and considerablylower than at 30 mph T-junctions.

The scope for accident reduction will clearlybe dependent on specific junctioncharacteristics, such as traffic flow andgeometry, as well as accident types. Whenconsidering a mini-roundabout as an option,designers should refer to current guidance onaccident numbers such as the MOLASSESdatabase, and locally held records onaccident levels.

2.5 As a Traffic Calming Measure

Mini-roundabouts are also used for trafficcalming:

• As part of a traffic calming scheme.Mini-roundabouts are often considered aspart of area-wide traffic calming schemesin which they are sometimes installed atthe extremities of the scheme or at all orvarious junctions within it.

• Reducing traffic speeds andincreasing driver awareness. The useof a mini-roundabout in isolation as aspeed reducing measure is morecontentious and has met with mixedsuccess. They have also been used toindicate to drivers that they are entering amore residential area. A well designedmini-roundabout can reduce speeds anda poorly designed one may not.

Photo 2.5.1: Mini-roundabout in traffic calmed area

2.6 As an Access to a New Development

Many Local Authorities accept theintroduction of mini-roundabouts as part ofnew development proposals.

Photo 2.6.1: Mini-roundabout as access tonew development

Photo 2.3.2: After view of junction withmini-roundabout

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Designers may use numerical criteria todetermine whether a mini-roundabout issuitable for access to a new development,with some suggesting side road traffic flowsshould be not less than 500 vehicles per day(AADT). Some Local Authorities use differentcriteria. For example, Lancashire, Cheshireand Bedfordshire County Councils prefer touse a ratio, suggesting side road flow shouldbe a minimum of 10-15% of the major roadflow.

A lower flow limit is prescribed becausedifficulties can result from their use at lightlytrafficked side roads, where emergingvehicles or turning movements areunexpected; if side road flows are too lowthen the main road will effectively operateunder free flow conditions.

Consideration should also be given to theusual site constraints and design criteria.

On trunk roads it is unlikely that a mini-roundabout would be an acceptable designsolution for a new junction.

Photo 2.6.2: Mini-roundabout on new estate road

Note: This and other photos illustrate acommon error in the placing of TSRGDdiagram 611.1; this one is upside down.

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3. SITE ASSESSMENT

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Layouts that donot allow cardrivers tonegotiate thecentral islandwithoutoverrunning areunlikely to be gooddesigns.

Assessmentshould beundertaken intwo parts.

3. Site Assessment

3.1 General

Once a practitioner has established that amini-roundabout may be an appropriatechoice, the site needs to be examined toconfirm its suitability.

Engineers need to be aware of thecomplexity of assessing the suitability of asite for the installation of a mini-roundabout.Many variables contribute to its suitability andpotential success. Factors need to bequantified and their significance determined,including whether the initial design of themini-roundabout can be modified to mitigateany potential problems.

3.2 Early Rejection

Mini-roundabouts are unlikely to be anappropriate junction treatment at thefollowing locations:

• on a dual carriageway;

• at a junction with five or more arms; and

• where the 85th percentile speed exceeds35 mph (see section 3.6 for furtherinformation); and

• where there is no scope to reduceapproach speeds.

The procedure for assessing site suitabilityshould be undertaken in two stages.

3.3 Stage 1 Site Assessment

As part of the assessment it is recommendedthat a record is kept of all relevant factors,including details of site surveys, in order thata fully informed decision can be made, and ifnecessary a comparison between otherjunction options. Site visits by the designer indaylight and during the hours of darkness arerecommended. A sample site assessmentform is provided at the back of thisdocument.

The first stage of assessment will includeseveral key decisions:

IS THERE ENOUGH SPACE AVAILABLEFOR THE CONSTRUCTION OF A MINI-ROUNDABOUT?

The width of the carriageway and extent ofland designated as public highway willdetermine whether there is enough spaceavailable for the construction of a mini-roundabout. Available space at the junctionmay be sufficient to enable a conventionalroundabout to be constructed instead. Wheninvestigating if space is sufficient,consideration will need to be given to theavailability of private land.

The inscribed circle diameter (ICD) of a mini-roundabout is the diameter of the largestcircle that can be inscribed within the junctionkerbs. A suggested maximum ICD for a mini-roundabout is 28 metres. Above thisdimension a conventional roundabout shouldbe used. Designers should also consider aminimum ICD, taking account of therequirement for drivers to drive around, andnot over, the central island. Figure 3.3 belowshows the desirable minimum ICD basedupon a medium sized car.

Layouts that do not allow car drivers tonegotiate the central island withoutoverrunning are unlikely to be good designs.

Figure 3.3: Sample Desirable Minimum ICD

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A mini-roundaboutshould not beconsidered a simplelining and signingexercise.

Visibility will place aconstraint on thedesign and measuresrequired as part of thelayout.

Photo 3.3.1: Constrained site where all vehicles fromside road have to overrun central island

WILL THE INSTALLATION OF A MINI-ROUNDABOUT ADVERSELY AFFECT THESAFETY PERFORMANCE OF THEJUNCTION, WHETHER OR NOT THE MINI-ROUNDABOUT IS BEING INTRODUCED ASAN ACCIDENT REMEDIAL MEASURE?

It is essential that the accident record for anexisting junction be investigated in order topredict the effect that a mini-roundaboutwould have on safety at a particular site.

The improved safety performance of a newmini-roundabout junction is also dependenton the improvements to the general roadenvironment, such as the renewal of linesand signs, a new surface or improvedlighting, as well as the change of junctioncontrol. However, sustaining this level ofbenefit will be dependent on regular junctionmaintenance.

IS A MINI-ROUNDABOUT LIKELY TO BEAFFORDABLE AND ECONOMICALLYVIABLE?

Mini-roundabouts are often consideredbecause they are perceived to be relativelyinexpensive compared to other junction typesand it is important that any junctionimprovement provides an economic solutionin addition to improving operational and/orsafety benefits.

When considering a mini-roundabout as asafety measure, economic justification isassessed, i.e. the first year rate of return(FYRR) should be calculated.

Whilst not providing a ‘perfect’ solution, amini-roundabout may provide sufficientimprovement over the existing junctionperformance to justify installation on a valuefor money basis.

However, it is also important that the wholelife cost of the junction is taken into account.Mini-roundabouts incur ongoing maintenancecosts and these should not be overlooked.For example, a domed central island subjectto high turning movements by HGVs may bescuffed regularly and will need to be re-painted to maintain conspicuity.

The successful design of a mini-roundaboutmay require:

• carriageway realignment;

• build-outs;

• street lighting (provision andmodification);

• new crossing facilities;

• modifications to drainage;

• carriageway resurfacing; and

• traffic islands.

It is essential that these costs are notoverlooked. A mini-roundabout should not beconsidered a simple lining and signingexercise.

The cost of a mini-roundabout can varygreatly depending on the level of workinvolved. Local authority consultationsuggests the range of costs for 3 or 4-armsingle mini-roundabouts are (at 2003 outturnprices):

3-arm £10,000 - £30,000

4-arm £15,000 - £50,000

3.4 Stage 2 Site Assessment

The second stage of assessment requiresengineering judgement in order to ascertainwhether a mini-roundabout is an appropriatejunction improvement option by evaluating thefollowing factors:

• visibility;

• vehicle speed;

• road character;

• traffic volume;

• number of arms;

• traffic composition;

• vulnerable road users;

• road network; and

• noise and vibration.

Following the Stage 2 Assessment thedecision to introduce a mini-roundaboutwould be confirmed and issues to considerduring the design process are identified. Theconclusion may be that a mini-roundabout isnot the best option.

3.5 Visibility

3.5.1 Visibility of the mini-roundabout

For a mini-roundabout to operate asintended, it is essential that the junction typecan be recognised and that drivers haveadequate forward visibility of the junction.

Parking on the approaches to mini-roundabouts is a particular problem andconsideration should be given to applyingparking restrictions on the approach arms.

Consider the whole lifecosts not justconstruction costs andaccident savings.

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Both the speedlimit and theapproach speedsshould be takeninto account.

Local practice regarding signing on theapproach and visibility of the give waymarkings and signs varies.

3.5.2 Visibility of conflicting approaches

Some practitioners have commented that‘excessive’ visibility to the right has been aproblem, with drivers deciding whether toyield or not on the junction approach and notat the give way line.

However, this is seldom the case at mini-roundabouts with adequate entry angles.

3.6 Vehicle SpeedMini-roundabouts are not a suitable junctionoption at locations where vehicles willapproach the junction at high speed.

The location and design of the mini-roundabout should ensure that vehicles haveslowed down to an appropriate speed prior toreaching the junction, can stop whennecessary and should then maintain anappropriate speed around the circulatorycarriageway.

The design of a mini-roundabout should alsodiscourage drivers from accelerating throughthe roundabout and on exit. If, prior toentering the mini-roundabout, a driver canalready see that they will be able to negotiatethe junction quickly (and due to the small sizeof mini-roundabout junctions this is oftenpossible) they may be encouraged tomaintain a higher speed. Vehiclesaccelerating on exit may endangerpedestrians and/or cyclists and equestriansat nearby crossing facilities, whethercontrolled or uncontrolled.

Chapter 5 of the Traffic Signs Manualadvises that mini-roundabouts should only beused on roads with a speed limit of 30 mphor less.7

Some local authority practitioners believe thespeed limits on the approach roads are ofless relevance than the actual approachspeed of vehicles. Experience has shownthat mini-roundabouts can work safely in 40mph limit areas if the vehicle approachspeeds are reduced prior to entry to say 20-25 mph.

In addition to noting existing speed limits andany proposals for changes, it isrecommended that the approach speed of allarms is obtained as part of the assessmentprocess. The photos show examples of mini-roundabouts outside 30 mph limits that haveproven to operate safely in accident terms.

Observations of visibility and vehicleapproach speeds have indicated that wherevisibility of side road traffic was more than 30metres from a point 2.4 metres back from theoffside give way marking then the speedreducing effect of the mini-roundabout wassignificantly reduced.

Photo 3.5.2: Site with restricted approach visibility

Having entered the junction, drivers willrequire adequate visibility on exit, particularlyif there is a pedestrian crossing immediatelydownstream.

Photo 3.5.3: Site with good visibilityon all approaches

Photo 3.5.1: Site with ‘excessive’ visibility on raisedjunction to encourage reduction in approach speeds

7 Chapter 5 Traffic Signs Manual 2003: “Mini-roundabouts shouldonly be used when all approaches are subject to a speed limit of30mph or less. Their use on roads with a higher limit is notrecommended…”, para 8.11

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3.7 Road Character

The individual characteristics of the roadjunction at which a mini-roundabout is beingconsidered will determine the site’s suitability.

The following factors relating to roadcharacter will therefore be discussed:

• gradients;

• highway status;

• number of carriageways or lanes;

• pedestrian and cycle facilities;

• public transport infrastructure;

• street lighting; and

• urban or rural nature of road.

GRADIENTS

Ideally, mini-roundabouts should be locatedon level ground or in sags but not at the topof hills. Installations at the bottom of longdescents or on steep gradients should beavoided.

Drivers may have difficulty assessing thelayout of a junction that they are approachingon an up gradient and there is a risk thatlarge goods vehicles may lose control ifapproaching a junction on a down gradient.

Photo 3.7.1: Mini-roundabout on hill descent withspeed reducing measures on approach

The slope of the mini-roundabout shouldfollow the slope of the junction. Someadverse crossfall will be acceptable, providedapproach speed can be controlled.

HIGHWAY STATUS

Particular care should be taken ifconstructing a mini-roundabout where one ormore of the side roads do not form part ofthe public highway. This may become morecommon as mini-roundabouts areincreasingly used as accesses to newdevelopment, e.g. as accesses tosupermarket car parks, industrial estates etc.

A mini-roundabout is reliant on driversadhering to traffic signs and road markingsinstalled on the approaches. Visibility is aparticular concern when this is over land notin the control of the highway authority andsubsequent development may prejudicesafety.Photo 3.6.3: Mini-roundabout on raised junction

Photo 3.6.2: Mini-roundabout on national speed limitroad – at this site the approach roads are narrowcountry lanes where the speeds are constrained.

Photo 3.6.1: Example of mini-roundabout within40 mph speed limit with constrained approach

At some junctions, the approach speed ofvehicles may be low due to the physicalcharacteristics of the road or existing trafficcalming features. At other junctions where amini-roundabout is to be introduced, speed-reducing measures may need to be includedas part of the junction improvements.

At sites where the current speed limitexceeds 30 mph, consideration should begiven to changing the speed limit and/or othermeasures to reduce approach speeds.Simply reducing the speed limit may notaffect approach speeds.

Mini-roundabouts are sometimes intended toact as speed control measures. It is importantto ensure that the design, including the layoutof islands, build-outs and approaches,enables the mini-roundabout to serve as aspeed reducing feature without compromisingthe safety or operation of the junction.

Road characteristicswill have an effect onthe suitability of amini-roundabout.

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Additional streetlighting should beconsidered whenintroducing amini-roundabout.

Good practice would suggest that a sufficientlength of the approach road in question isadopted to retain control for signing andmaintenance purposes.

NUMBER OF CARRIAGEWAYS OR LANES

It is not considered good practice to introducemini-roundabouts on dual carriageway roads,although, at junctions with single-lanedualling, they may be acceptable.

Care must be taken when designing mini-roundabout junctions with multiple laneapproaches as lane discipline may be poorand vehicular paths through the roundaboutcan vary. Two-lane approaches canencourage drivers to overrun the centralisland and can impair visibility. Sufficientdeflection is difficult to provide and moreattention may need to be given to reducingapproach speeds.

The number of lanes on the approach to amini-roundabout should not exceed thenumber of exit lanes.

Photo 3.7.2: Multi-lane approach

PEDESTRIAN AND CYCLE FACILITIES

The installation of a mini-roundabout may beconsidered at a junction that has existingpedestrian and cycle facilities such as:

• pedestrian refuges;

• Zebra, Pelican, Puffin or Toucancrossings;

• dropped kerbs and tactile paving;

• off-road cycle tracks;

• cycle lanes.

It is important to ensure the mini-roundaboutdoes not compromise the use of existingfacilities by pedestrians and cyclists. In somecases, existing facilities may need to bealtered or relocated or new pedestrian andcycle measures introduced as part of thejunction improvement. This may result insubstantial additional cost.

Crossings located within 20 metres of mini-roundabouts have been shown to operateeffectively. This may be due to relatively lowspeeds through the junction. Wherecrossings are further away, approach and

exit speed may be unaffected by the mini-roundabout.

Photo 3.7.3: Mini-roundabout incorporatingsegregated cycle facilities and zebra crossing

PUBLIC TRANSPORT INFRASTRUCTURE

Mini-roundabouts can cause difficulties forbuses and it is unlikely that bus lanes canoperate safely through a mini-roundaboutbecause of left-turners. See TSM Chapter 5para 17.10 for guidance on terminating buslanes on the approach to a roundabout. Thepresence of a bus stop or bus bay should notresult in problems, provided refuges andislands are designed with bus movements inmind.

Careful design is needed where mini-roundabouts are sited near railway levelcrossings. Designers need to be aware of thedangers of traffic queuing back from the mini-roundabout across the level crossing, ortraffic from the level crossing blocking themini-roundabout. The former situation isparticularly dangerous. If a mini-roundaboutis being considered at a junction near a levelcrossing, consultation with the railwayauthority is essential.

Photo 3.7.4: Mini-roundabout near level crossing

STREET LIGHTING

It is important that mini-roundabout junctionsare visible to approaching drivers. If the mini-roundabout is to be installed in an unlit areaconsideration needs to be given to ensuringthe mini-roundabout is conspicuous at night.This may mean improvements to signing orproviding street lighting. Where a system ofstreet lighting is provided then it should

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comply with the recommendations in BS5489, and advice sought from a lightingengineer.

Flush central islands are considered to bemore difficult for street lighting to illuminatethan domed islands, as they have no profile.Successfully lighting mini-roundabout centralislands in wet conditions is particularlydifficult. A domed central island is moreconspicuous.

The illumination of the ‘give way’ traffic signs(602 or 611.1) is a requirement where streetlighting exists. For advance direction signs,the requirements are that either they are lit orthey are reflectorised which also applies to‘New Roundabout Ahead’ signs (7014),where used.

It should be noted that the TSRGD do notpermit the use of reflective road studs toincrease the conspicuity of the central island.

URBAN OR RURAL NATURE OF ROAD

The urban or rural character of a road is notgenerally considered to predetermine theappropriateness of a mini-roundabout as ajunction solution, but consideration needs tobe given to the visual impact on the ruralenvironment.

Photo 3.7.5: Mini-roundabout in rural type location

Mini-roundabouts can be used on both ruralvillage and urban roads.

Mini-roundabouts have been increasinglyused in rural areas, particularly at busyintersections in villages or as part of ruraltraffic calming schemes. Some mini-roundabouts have been installed on ruralroads away from settlements. There aremany potential problems with the use of mini-roundabouts in such locations and theirinstallation is discouraged (see 3.10).

3.8 Traffic Volume

Additional capacity could, under certaincircumstances, be provided by theintroduction of a mini-roundabout but therewill be limits. Practitioners should refer to acapacity assessment programme such asARCADY to assess capacity implications ingreater detail.

It is suggested that 4-arm mini-roundaboutsshould not be introduced where total entryflows are below 500 veh/hr, or minor road

Photo 3.10.1: HGV scuffing of central island

Alternative junctiontypes should beconsidered for 4 or5 arm junctions.

flows are less than 15% of the major roadflow. Mini-roundabouts are particularly suitedto handling high proportions of right-turningtraffic.

3.9 Number of Arms

Originally, mini-roundabouts were onlyconsidered for junctions with three-arms. In1975, the recommendations changed andmini-roundabouts on trunk roads wereallowed at both 3 and 4-arm junctions. Since1984, these recommendations have notspecified the number of arms.

Mini-roundabouts are known to be widelyintroduced at both 3 and 4-arm junctions.

Adequate deflection may be difficult toachieve with more than three arms. The useof mini-roundabouts to accommodate one-way slip roads or very minor accesses mayprove more advantageous than thealternatives.

However, the installation of mini-roundaboutsat junctions with more than four arms is notrecommended, even on local roads. 4 and 5-arm mini-roundabouts have a variable safetyrecord and may not perform as well asalternative junction types.

Where a junction has five or more arms adouble mini-roundabout may be used,although a signal-controlled junction may bemore appropriate.

3.10 Traffic Composition

It is not considered advisable for mini-roundabouts to be sited at junctions that areused by a high proportion of heavy goodsvehicles, agricultural vehicles or buses andcoaches.

Although the design of mini-roundabouts isintended to allow long (or wide) vehicles totraverse the central road marking, thecontinual overrunning by vehicles will causetyre scuffing and the rapid deterioration ofthe mini-roundabout road markings. Withoutregular inspection and maintenance, thecentral road marking will lose conspicuity anddrivers will not be able to determine thecircular path around the roundabout.

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Also, the overrunning of a domed centralisland may cause driver (and bus passenger)discomfort, noise and vibrations.

In addition to long vehicles, considerationshould also be given to the appropriatenessof siting mini-roundabouts on roads with highnumbers of pedestrians, cyclists,motorcyclists, equestrians or other vulnerableroad users.

It is recommended that the emergencyservices are consulted about proposals for amini-roundabout, particularly one with adomed central island, in a location that mayaffect them on a regular basis.

Due to their size, many emergency servicevehicles cannot manoeuvre around centralislands and wish to avoid overrunning domedislands (domed central islands can causedifficulties for patients travelling byambulance). Mini-roundabouts may thereforebe perceived as affecting response times.

Photo 3.10.2: Fire engine negotiatingmini-roundabout

3.11 Vulnerable Road Users

CYCLISTS

Cyclists are vulnerable at all types of roadjunction although roundabouts poseparticular problems. Cyclists are particularlyvulnerable when circulating and enteringvehicles fail to yield, especially during hoursof darkness, due to their lack of size andconspicuity.

Photo 3.11.1: Cyclists on mini-roundabout

Mini-roundabouts present fewer problems tocyclists than small conventional roundaboutswith flared entries and large conventionalroundabouts, which may result in highspeeds.

Mini-roundabouts can provide usefulassistance for cyclists turning right and wherespeeds need to be reduced.

Mini-roundabouts should be designed to becycle-friendly, especially where they are ondesignated cycle routes, or on other roadsused regularly by cyclists.

Photo 3.11.2: Mini-roundabout with off-highway cyclefacilities

EQUESTRIANS

At junctions where there is regular use byequestrians, it is not felt a mini-roundabout isan appropriate form of junction.

However, provided speeds can be sufficientlyreduced, an equestrian crossing facility on anapproach has been shown to worksatisfactorily.

PEDESTRIANS

If a mini-roundabout is being considered incongested urban areas with large flows ofpedestrians, particularly children or elderlyand disabled people, controlled crossingfacilities should be considered.

Photo 3.11.3: Zebra crossing close tomini-roundabout

Note: The Zebra crossing does not havetactile paving to assist blind and partiallysighted pedestrians.

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Consider centralisland dome heightson bus routes tominimise discomfortto passengers.

A mini-roundaboutmay causedisruption in anUTC area.

Photo 3.11.4: Pelican crossing close tomini-roundabout

Signalled crossings on the approach to amini-roundabout should be used with care toavoid confusion from the green signal and toensure vehicles queuing back from thecrossing do not cause conflict at the junction.

The positive control offered by traffic signalsmay be a better junction alternative,particularly where co-ordination betweenjunctions and crossings can be provided.

3.12 Consultation

Local consultation will help identify groups ofusers that may be disadvantaged by theintroduction of a mini-roundabout.

3.13 Road Network

In addition to a road’s specific characteristics,the character of the local road network will beinfluential in deciding that a specific junctionis a suitable location for a mini-roundabout.

The following network characteristics shouldtherefore be considered:

• existing traffic systems;

• local bus routes and bus priorityschemes;

• local cycle and pedestrian routes;

• traffic calming schemes; and

• traffic management schemes.

EXISTING TRAFFIC SYSTEMS

Mini-roundabouts may not be compatible withlocal junctions if the area has an UrbanTraffic Control (UTC) system that relies onthe platooning effect of signals or thecreation of green waves, or in areas with ahigh number of unlinked traffic signals.

This issue is particularly relevant at locationswhere the conversion from a traffic signaljunction to a mini-roundabout is beingconsidered.

Conversely, at locations with mostlyroundabout junctions in the vicinity, a mini-roundabout may be a more appropriateoption than a traffic signal junction, asconsistency can help drivers negotiate aseries of junctions safely.

LOCAL BUS ROUTES AND BUS PRIORITYSCHEMES

The location of the central island, and heightof dome, should be carefully considered if amini-roundabout is installed along a bus routedue to potential overrunning of, or groundingon the central island by buses.

Photo 3.13.1: Bus at mini-roundaboutwith severe deflection

It is also important to consider the effect ofinstalling a mini-roundabout within a buspriority scheme as the change may affect busjourney times.

However, in some circumstances mini-roundabouts can be integrated into apackage of bus priority measures. Oneexample, at a 3-arm T-junction, is to create abus bypass lane across the junction if thecarriageway width is available.

Photo 3.13.2: Bus bypass at 3-arm small roundaboutshows a similar application

Where a bus route involves making a difficultright turn then an option could be to introducea mini-roundabout to assist this movementand reduce bus delays. The photo aboveshows a similar application for a smallroundabout.

LOCAL CYCLE AND PEDESTRIAN ROUTES

Mini-roundabouts may be considered forjunctions that form part of local cyclenetworks or strategic pedestrian routesincluding Safer Routes to School. It isimportant that cycle and pedestrian routesare protected and consideration should begiven to providing adequate pedestrian andcyclist facilities as part of the junctionimprovements.

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Cyclists are particularly vulnerable at anyroad junction and the safety of all vulnerableroad users should not be compromised byalterations to a junction.

Domed centralislands can causenoise and vibration.

Designs shouldtake account ofpossible U-turns.

Photo 3.13.3: Off carriageway cycle facilities at amini-roundabout

TRAFFIC CALMING SCHEMES

Mini-roundabouts are often introduced aspart of a wider traffic calming schemealthough whether they work as a speed-reducing feature will depend on the design.

The 1990 Road Hump Regulations referredto mini-roundabouts as a speed-reducingfeature but the current Regulations are silenton the specifics of a mini-roundabout.

Photo 3.13.4: Traffic calming on approach tomini-roundabout

TRAFFIC MANAGEMENT SCHEMES

It is important to consider how theintroduction of a mini-roundabout will affect,or may be affected by, an existing orproposed traffic management scheme.

Careful consideration should be given toinstalling a mini-roundabout within trafficmanagement schemes in which banned rightturns in or out of side roads are located nearthe junction in question and a mini-roundabout would provide the opportunity fordrivers to U-turn (which may or may not bedesirable or safe).

Although U-turns are legal at mini-roundabouts, for which possible turningmovements are the same as at conventionalroundabouts, such movements are infrequent

and therefore usually unexpected. Theperceived wisdom is that U-turns at mini-roundabouts are inevitable, and should notbe prohibited, but the nature of the junctionarrangement often precludes designs thatcan accommodate U-turns by all but thesmallest vehicles, although this will dependon the space available at the junction.

3.14 Noise and Vibration

The overrunning of domed central islands bylarge vehicles can create noise and groundvibrations. Equally, the extra stopping andstarting activities of vehicles caused by aroundabout can create additional disturbance.

Although in some areas additional (andvariable) vehicle noise caused by a mini-roundabout may not be intrusive, it is likely tobe a cause of complaints in residential areas.

In addition, some soil types are prone tovibration and therefore the type of soil in anarea may need to be investigated if vibrationis perceived to be a potential problem.

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4. EXISTING PRACTICE

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4. Existing Practice

4.1 Introduction

In developing this document consultation wasundertaken with 23 local authorities acrossEngland and Wales with comments alsoreceived from other organisations.8 Thedocument authors have also met with somelocal authorities and their officers to discussissues and sites. This has provided a rangeof views on mini-roundabouts and examplesof sites where they have been implemented.

4.2 Responses to Consultation

“It is easier to design a poor mini-roundaboutthan a poor set of traffic signals”.9 Due to themany factors and options involved inintroducing a mini-roundabout there is morescope for poor design.

The following are key issues arising fromuser experience of which designers shouldbe aware.

4.2.1 Speed Limits

In accordance with current guidance themajority of mini-roundabouts are installed onroads with a 30 mph speed limit. Mini-roundabouts should not be installed on highspeed roads, i.e. 85th percentile speed of 35mph or more.10 One local authority hadexamples where the national (60 mph) limitwas in force and in both instances theaccident record had led to the subsequentprovision of a small solid island. It is worthnoting that in both cases the original kerblines were unaltered.

Where limits were greater than 30 mphmeasures were introduced to change thespeed of approach. Some local authoritieshave successfully installed a limited numberof mini-roundabouts on 40 mph roads.

Consideration should be given to the vehicleapproach speeds and ensuring that thedesign will assist in influencing driver speeds.Reliance should not be placed on the centralisland itself as a speed-reducing feature.

4.2.2 Number of Arms

Generally mini-roundabouts have beenimplemented on 3-arm junctions with apreference for this number of arms. Equallymost local authorities had examples wherefour had worked. No local authorities hadexamples of five arms or more, insteadrelying on double or even triple mini-roundabout junctions to handle suchcircumstances.

8 Responses to the questionnaire received from Cycling TouringClub, British Horse Society and North East Ambulance ServiceTrust.9 Cambridgeshire County Council.10 Two examples have been found of a mini-roundabout onderestricted road, one in Essex and one in Stirlingshire.

4.2.3. Position of Central Island

This is one of the most critical aspects of thedesign process. The most usual practice wasto determine the position of the central islandby the swept paths of turning traffic. Thissometimes resulted in an island not at thecentre of the inscribed circle and, wheretraffic might be tempted to pass the ‘wrong’side of the island, use was made of splitterislands or build-outs to encourage ‘correct’behaviour.

Photo 4.2.1: Driver passing over central island

Photo 4.2.2: Driver passing over central island

Good practice should ensure that drivers arenot encouraged to either consistently passover the central island, or drive the wrongside.

4.2.4 Domed Central Islands

There is a general practice to dome centralislands, although preference was expressedin some cases not to dome in residentialareas to avoid noise nuisance from heavygoods traffic. The purpose of the dome,where provided, was for improved conspicuitybut it is generally used to encouragecirculatory behaviour although in someinstances this had resulted in traffic passingthe wrong side. However, the few instances ofdomes being removed resulted from thecomplaints over noise rather than thebehaviour of traffic.

Regulations allow for a maximum domeheight of 125mm. In practice the maximum

Good designpositions the centralisland so drivers arenot encouraged todrive over, or on thewrong side, of it.

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Deflection is a keypart of good mini-roundabout design.

height is normally taken as 100mm to reduceunnecessary noise, vibration, and scuffing. Insome cases especially where low-floor busesoperate this should be reduced to 60mm orless to avoid grounding. In some cases it maybe necessary to use flat central islands.

4.2.5 Deflection, Build-Outs andSplitter Islands

All respondents of the contact group felt thatdeflection was important. In practice manysites reviewed failed to provide adequatedeflection. This is a key area, which shouldbe addressed when a mini-roundabout isintroduced.

Where deflection was provided this was doneby up to three different methods:

• a nearside build-out provided before theGive Way line;

• a nearside build-out within the circulatoryarea;

• using central splitter islands.

Any combination may be used with the use ofsplitter islands the most common.

Where a build-out occurred prior to the GiveWay line it was observed that overrunning ofthe central island was likely to be morecommon and could cause difficulty forcyclists.

Photo 4.2.3: No deflection on approachor through junction

Photo 4.2.4: Deflection through alignmentand traffic island

Photo 4.2.5: Approach deflection with kerb alignmentand traffic islands

Photo 4.2.6: Approach deflection through alignmentand traffic islands

4.2.6 Visibility

Appropriate visibility to the right, and ofvehicles within the junction, was regarded asimportant by the entire contact group.Visibility to the left for turning traffic did notseem to be as critical a factor to satisfactoryoperation. Nor did approach visibility, withviews of the roundabout or give way signingvery limited in some examples.

Photo 4.2.7: View from side road at sitewith poor visibility

Some guidance on visibility design criteriahas been developed; in particular the visibilitydistance to the right and forward visibility.

The visibility distance to the right (D) could beconsidered the minimum sight distancerequired by a road user approaching theroundabout forward of a point at distance “F”from the give way line, measured along thecentre of the offside approach lane. Itenables the driver of an entering vehicle to

(Note: TSRGD diagram 611.1 incorrectlyplaced above TSRGD diagram 602)

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observe a vehicle coming from the right for 2or 3 seconds, as appropriate, before itreaches the conflict point. Distance ‘D’ varieswith the 85th percentile ‘dry weather’approach speed on the arm to the right of anentering vehicle at 70 metres before the giveway line. See Figure 4.1 and Table 4.2.1below.

Table 4.2.1 Visibility Distance to the Right

The ‘F’ distance should usually be 9 metresso that the first two vehicles in the queuehave visibility of traffic from the arm to theright. In difficult circumstances, the ‘F’distance may be taken as a relaxation from 9metres to 4.5 metres on an arm where thetraffic flow is less than 300 veh/hr. Inexceptionally difficult circumstances, a furtherrelaxation to 2.4 metres is the minimumpermissible ‘F’ distance, as it enables a roaduser who has reached the give way line tosee approaching vehicles withoutencroaching past the give way line. This will,however, allow only one vehicle at a time toenter safely and requires following driverslikewise to be prepared to stop and look. An‘F’ dimension of 2.4 metres must only beused on an arm with a flow of 300 veh/hr orless and where there is no entry arm to theleft. In such cases the mandatory give waymarkings and upright sign (diagrams 1003,1023 and 602) must be used to require roadusers to give way to circulating traffic.

Excessive visibility between arms may resultin approach and entry speeds greater thandesirable, with a tendency for approachingdrivers to take a decision too early aboutwhether to give way. Road users approachinga mini-roundabout need to be able to stop ifvehicles are circulating or if there is anobstruction on the junction. Although the ‘D’distance should always be provided, if this isexceeded it may induce high approachspeeds and take the driver’s attention awayfrom the immediate junction conditions.Consideration should be given to limiting thevisibility to the right of adjacent entries to amaximum ‘F’ distance of 15 metres back onthe approach and to no more than the ‘D’distance.

Table 4.2.2 provides suggested forwardvisibility distances ‘E’.

Table 4.2.2 Minimum Stopping Sight Distance

4.2.7. Use by Vulnerable Road Users

Moderate use by pedestrians and cyclistscaused little concern to the contact group, butlarge numbers of cyclists, such as occurredat times in university towns, were found tocause safety problems and this has led to thereplacement of at least two roundabouts bysignals. In those instances, at least threequarters of the accidents involved cyclists.However in these situations the unusuallylarge number of cyclists may have increasedtheir exposure to such accidents.

No such problem has been noted withpedestrians although concern has beenexpressed that pedestrians could dominate ajunction in town centres and signals wouldoffer better positive control in such cases.

Equestrians are not normally present in thesort of situations where mini-roundabouts areused but there is an example in a horse-training town where a signal-controlledequestrian crossing operated satisfactorily onthe approach to a mini-roundabout.

4.2.8. Use by Large Vehicles

The use of mini-roundabouts by heavy goodsvehicles does not cause any particularproblems except the overrunning of thecentral island at smaller sites. There is somereluctance to use domes on bus routes orwhere there are large numbers of emergencyvehicles and where a noise nuisance couldresult in residential areas. In addition, a largenumber of turning manoeuvres by HGVs canlead to the rapid wear of road markings.

Photo 4.2.8: HGV driving through a mini-roundabout

D distance (m)85th percentile

speed of arm

to right (mph)

For anacceptance gap

of 2 seconds

For anacceptance gap

of 3 seconds

35 40

35

25

55

30 50

25 40

85th percentile speedof arm to right (mph)

Minimum “E”Distance (m)

35 80

30 70

25 50

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4.2.9. Overrun Areas and Surfacing

The use of overrun areas to permit turns bylarge vehicles whilst providing guidance forlight vehicles was observed on a fewoccasions. These were constructed of avariety of materials, most commonly redtarmac or anti-skid and most often wereapplied on the left-turn radius of large orawkward junctions.

Photo 4.2.9: Mini-roundabout with overrun area

Similarly anti-skid, usually red, or buff, wasapplied on the approach to a roundabout asa result of anticipated rather than recordedproblems.

Photo 4.2.10: Mini-roundabout with red anti-skid

4.3 Example Sites

As the process of deciding the suitability of asite for a mini-roundabout is a complex onewhere a range of variables comes into play, anumber of sample sites have been collectedto identify the issues involved. These samplesites will assist practitioners understanding ofgood and bad design issues, includingseveral examples of mini-roundabout signserected incorrectly.

Figure 4.1 Mini-roundabout Visibility Distances

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Site: Treffry Lane – B3268

Location: Bodmin

Highway Authority: Cornwall County Council

Before:

After:

Site Description

• 4-arm junction

• Derestricted road

• Rural location

• Large junction

• No street lighting

Issues

The County Council hadproblems with the mini-roundabout layout shown above.

These were:

• Unrestricted road leading tohigh speeds

• Unlit creating conspicuityproblems at night

• Lack of deflection on approaches

• Forward marked give way

• Vehicles driving over central island

The mini-roundabout was removed and replaced with a conventional roundabout.

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Site: A1134 Brooks Road - Brookfields

Location: Cambridge

Highway Authority: Cambridgeshire County Council

Brooks Road (north) approach Brookfields (west) approach

Site Description

• 4-arm junction

• 30 mph speed limit

• Urban location

• Street lighting

• Junction located in university town with large number of cyclists

Issues

The County Council were concerned with the safety of this mini-roundabout due to the highnumber of accidents involving pedal cyclists.

The mini-roundabout was removed and replaced with traffic signals.

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Site: Poppyfields

Location: Alsager

Highway Authority: Cheshire County Council

Site Description

• 3-arm junction


• Access to new housing estate

• Street lighting

This mini-roundabout was installed by a developer as the access to a residential development.

Design Characteristics

• The mandatory give way marking and associated road sign have been used on theapproach to the right turn.

• A ‘Slow’ road marking on red surfacing precedes the mandatory give way.

Issues

• The County Council have concerns over the design of this mini-roundabout due to the poorvisibility both for and of vehicles emerging from the new estate road.

• The mini-roundabout road markings were repositioned following its installation.

Site Description

• 4-arm junction


• Rural location

• Street lighting

DesignCharacteristics

• The mini-roundabout islocated on araised table.

• The central islandhas an outer‘overrun’ areasurfaced in red.

Issues

• The visibility at this mini-roundabout is very good / excessive and the raised table is used toassist slowing vehicles.

• Right-turning vehicles often pass the wrong side of the roundel.

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Site: B5259 / B5260 junction

Location: Wrea Green, Kirkham

Highway Authority: Lancashire County Council

28

Site: Colchester Road - Freebournes Road

Location: Witham

Highway Authority: Essex County Council

Site Description

• 3-arm junction


• Access to industrial estate

• Street lighting

Issues

• HGVs form a high proportion of the traffic using this mini-roundabout.

• Large vehicles driving over the central island have caused the white reflectorised surface ofthe roundel to deteriorate and lose conspicuity.

Colchester Road (south) approach Colchester Road (north) approach

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Site: Fox Lane - West Paddock

Location: Leyland

Highway Authority: Lancashire County Council

Before:

After:

Fox Lane (east) Fox Lane (west)

Site Description

• 3-arm junction


• Street lighting

Issues

• Private driveways within junction

• Visibility too good / excessive

• Failure to give way to right

The County Council revised the design of this mini-roundabout after it was installed, originallayout shown in before photos.

The following features were added:

• Mandatory give way on Fox Lane (East) approach

• Traffic islands with illuminated ‘keep left’ bollards

• Advisory cycle lane on Fox Lane (west)

• Additional blue mini-roundabout signs

• Wide central hatching on approaches

Comment: Note diagram 611.1 in bottom left photo is incorrect

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Site: Treswithian Road - Weeth Road

Location: Camborne


Before:

After:

Siite Description

• 3-arm junction• 30 mph speed limit• Street lighting

Design Characteristics• domed central island• traffic islands with illuminated bollards• carriageway realignment / build-outs to provide deflection

IssuesThis mini-roundabout was installed as part of a local safety scheme with theintention of:• calming traffic speeds• assisting right-turners

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Site: Castle Road - Phillpotts Avenue

Location: Bedford

Highway Authority: Bedfordshire County Council

Phillpotts Avenue approach Castle Road approach

Site Description

• 4-arm junction (onearm is one-way)


• Urban location

• Street lighting

DesignCharacteristics

• Uncontrolledcrossing facilitieson each arm.

• Build-outs.

Issues

• Very low side roadflows.

• Some drivers fail togive way but there have been no accidents.

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Site: The Avenue (north) - The Avenue (south) - St Swithuns Road

Location: Kennington

Highway Authority: Oxfordshire County Council

The Avenue (north) approach The Avenue (north) approach

The Avenue (south) approach St Swithuns Road (approach)

Site Description

• 3-arm junction


• On a hill

• Street lighting

• Part of traffic calming scheme


• Road humps on The Avenue (north)

• Mini-roundabout located on raised table

• Mandatory give ways are used on The Avenue even on the south approach.

Issues

• Poor visibility on The Avenue approaches due to gradient.

Comment: Give Way (diagram 602) should not be used where traffic approaches from the left

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Site: Westgate - Sherborne Road

Location: Chichester

Highway Authority: West Sussex County Council

Sherborne Road approach Westgate approach

Site Description

• 4-arm junction


• Street lighting


• Buff-coloured overrun areas are used instead of build-outs to encourage deflection.

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Site: High Road - Falkers Way (east)

Location: Trimley

Highway Authority: Suffolk County Council

High Road (east) approach Falkers Way (east) approach

Site Description

• 3-arm junction


• Street lighting

• Part of traffic calming


• Segregated cycleway/footway across High Road (east).

• Domed central island.

• Use of mandatory give way on High Road (east) approach.

• Build-outs and overrun areas to provide deflection.

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Site: A414 Main Road - Well Lane

Location: Danbury


Main Road (west) approach Main Road (east) approach

Well Lane approach

Site Description

• 3-arm junction


• On a hill

• Outside a primary school

• Street lighting

Issues

• High traffic flow on side road

• Main Road very busy in peak periods

• Junction is within 50 metres of bus stop outside school on Main Road (west)


• Anti-skid surfacing on Main Road (east) approach.

• Mandatory give way on Main Road (west) approach.

• Zebra Crossing on Main Road (west) approach

• Traffic island with keep left signs / illuminated bollards on Main Road (west) approach.

36

Site: Kennington Road (north) - Kennington Road (south) - Upper Road

Location: Kennington

Highway Authority: Oxfordshire County Council

Upper Road approachKennington Road (south) approach

Site Description

• 3-arm junction


• Street lighting

• Commencement of traffic calming scheme

This mini-roundabout has replaced a priority junction.


• Very small roundel

• Original give way line has been retained.

• Mini-roundabout is located on a raised table

• Mandatory give way on Kennington Road (north) approach

Comment: Non-prescribed sign erected below diagram 611.1

37

Site: The Glebe - Manor Road

Location: Camborne


Manor Road (east) approach Manor Road (west) approach

Site Description

• 3-arm junction• 30 mph speed limit• Street lighting

Design Characteristics• 2-lane approach on Manor Road (east) approach• Mandatory give way on Manor Road (east) approach

Issues• Vehicles overtaking cars parked on the Manor Road (west) approach often drive

over the central island of the mini-roundabout• The two lanes on the Manor Road (east) approach appear to be too narrow for two

vehicles and therefore this approach tends to be used as a single lane• Road markings becoming worn by continual overrunning of traffic

38

Site: A12 off-slip - Shell Garage Access

Location: Colchester


A12 off-slip approach

Site Description

• 4-arm junction


• Street lighting


• 2 lanes on A12 off-slip approach

• Domed central island

• No circulatory arrows

Shell Garage access approach

39

4.4 Post Implementation Monitoring

It is important after implementing a mini-roundabout that post implementationmonitoring is undertaken. This is usual whereaccident remedial schemes are introducedbut should be extended to all schemes.

Designers should monitor not only if thescheme is successful in accident terms but interms of capacity, priority, etc dependent onthe original scheme objectives. Experiencehas shown that it may be necessary to makeamendments to a junction where works havebeen limited.

4.5 Maintenance

The ongoing success of a mini-roundabout isdependent on continued maintenance toensure the lining and signing and othermeasures are kept to a good standard. It isclear that maintenance is an issue in someareas and a continued problem with wear canindicate a problem with the design of a mini-roundabout.

Photo 4.5.1: Poor lining maintenance indicatingfrequent central island overrunning

Photo 4.5.2: White paint wearing on blockworkcentral island

4.5.3: Well-maintained mini-roundabout

(Note: TSRGD diagram 611.1 incorrectlyplaced above TSRGD diagram 602)

4.5.1 Carriageway joints

When introducing a mini-roundabout to anexisting junction consideration should begiven to the existing joints and camber. Thepath of vehicles negotiating the new junctionmay direct them over the carriageway joint asshown below leading to maintenanceproblems. This can be resolved throughresurfacing when the mini-roundabout isintroduced.

4.6 Driver Behaviour

It is apparent that many drivers are eitherconfused as to how to use a mini-roundaboutor do not attempt to use them correctly.Through consultation with designers andgeneral site observations it has been shownthat both road users and Local Authoritiesare unclear as to the give way rulesgoverning mini-roundabouts.

Although it is an offence for a car driver todisobey the mini-roundabout traffic sign,many drivers:

• overrun the central island;

• do not give way;

• do not indicate;

• do not slow down; and,

• drive to the right of the central island.

It is common for drivers to continue tonegotiate a junction according to its layoutprior to becoming a mini-roundabout. This islikely to be particularly true when a mini-roundabout is new or where there are few in

Post implementationmonitoring shouldinclude not justaccidents butoperational factors.

Figure: 4.5.1: Carriageway joints

40

the surrounding area. Drivers on the majorroad often do not treat a mini-roundabout asa junction control mechanism but as anobstacle that must be negotiated.

The Highway Code states that mini-roundabouts should be approached in thesame way as normal roundabouts and that‘all vehicles MUST pass round the centralmarkings except large vehicles which arephysically incapable of doing so’. It alsoreminds drivers that there is ‘less space tomanoeuvre and less time to signal’.

The mini-roundabout sign (611.1) tells driversto give way to vehicles approaching from theirright, as at a conventional roundabout, thetransverse road marking (1003.3) informsdrivers to give way to circulatory traffic.

The smaller scale of a mini-roundaboutmakes the ‘give way to the right’ rule lessstraightforward due to the short distancesbetween vehicles. In practice, mini-roundabouts usually function well due tonegotiation between drivers of vehiclesapproaching and on the circulatorycarriageway.

Although any driver confusion resulting inhesitation is not considered to be a safetyproblem, it may lead to decreased capacityand longer queues.

Local Authorities appear to be particularlyconfused as to the use of mandatory giveway signs, which are known to be sited at allapproaches of some 4-arm mini-roundaboutjunctions. The correct siting and design ofmini-roundabouts may be compromised bymisunderstandings as to the correct use ofthe signs by practitioners. This is explained indetail in Chapters 3 and 5 of the Traffic SignsManual.

4.7 Road User Education

In areas where a mini-roundabout would be anovel feature, or where a school or oldpeople’s home is nearby, it is appropriate toconsider road user education and contactshould be made with the relevant road safetystaff. Budget provision of a small percentageof the construction cost is likely to beadequate.

Local awareness campaigns may also beconsidered advantageous in order to educateall road users about mini-roundabouts.

4.8 Frequently Asked Questions

As a result of the consultation workundertaken in compiling this document it isclear that there are a number of issues,which require clarification. We have compileda response to a number of frequently askedquestions below.

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Question

Construction of Mini-Roundabouts

Must the central island be white?

Can the central island be constructed ofgranite setts, block paving or othertextured material?

Can the central island have raisedkerbs?

Can setts be placed around theperiphery of the central island?

Can reflective road studs be placedaround the periphery of the centralisland?

Can the central island have an outer‘overrun’ area/apron?

Can concentric circles/annular rings bemarked on the road surface around thecentral island?

Can a mini-roundabout be located withinan area of carriageway with colouredsurfacing?

Answer

Yes. The central island of a mini-roundabout mustbe white (see TSRGD regulation 11 (1) andreflectorised (see TSRGD regulation 31 (1)).

As the central island must be white andreflectorised (see above), it is not advisable for theisland to be constructed of textured material. Anysetts, blocks, etc used must have a white,reflectorised surface and there are problems ingetting paint, thermoplastic or other materials toadhere successfully and this will create amaintenance issue and potential problems with theconspicuity of the roundabout. Setts or blocks canalso result in longer term maintenance problems ifoverrunning vehicles cause the blocks to move orsubside.

No. TSRGD does not permit raised kerbs to beused in association with the mini-roundabout roadmarking. Kerbing has been used with domedcentral islands as some designers use them as aretaining device for the dome construction, butsuch kerbs must be flush or have a maximumupstand of 6mm. In this form they are notfunctioning as kerbs, more as channel blocks.

Good practice suggests they should not be used.However there are examples and, provided theupstand does not exceed 6mm they are notprecluded by TSRGD. They can presentmaintenance issues as overrunning, etc can makethem shift.

No. TSRGD does not permit the use of road studswith the mini-roundabout road marking 1003.4.See TSRGD regulation 31.

Yes. Overrun areas formed of contrasting materialcan contribute to the conspicuity of a mini-roundabout and this is not precluded by TSRGD.However, the size, colour and construction of suchareas vary considerable across the UK.No standard approach is likely to be developed.See also TAL 12/93.

No. See Traffic Signs Manual, Chapter 5, RoadMarkings 2003 para 2.1.

Yes. Coloured surfacing has no significance exceptincreasing conspicuity. The use of surfacingmaterial of a contrasting colour within the junctionarea can benefit conspicuity of a mini-roundaboutbut that coloured surfacing should not be laid inany shape or pattern intended to convey ameaning as a road marking. However the visibilityof white road markings is greatest whencontrasted with a very dark background, not with alight surface.

Answer

Yes. See figure below:

Yes. Care is needed to ensure give way markingsremain visible to approaching vehicles.

See figure below:

Yes. However, it is recommended that mini-roundabouts are not used solely to provide accessto development where the traffic flows expected touse the development access are <500 veh/day(AADT) or minor road flows are less than 15% ofthe major road flow.

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Question

Is there a correct way to mount TSRGDdiagram 611.1?

Can a mini-roundabout be located on araised table?

In which order should the give way (TSRGD diagram 602) and TSRGD diagram 611.1 be mounted?

Use of Mini-Roundabouts

Can a mini-roundabout be used as anaccess to a new development?

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Question

Are mini-roundabouts cheaper to installthan other junction types?

Location of Mini-Roundabouts

Should a mini-roundabout be installed ata junction with five or more arms?

Can a mini-roundabout be installedwhere the approach roads have speedlimits >30 mph?

What if my local authority has a numberof mini-roundabouts on highways withspeed limits exceeding 30 mph.

Operation of Mini-Roundabouts

What is the give way rule at amini-roundabout?

When should mandatory give way signsbe used?

Answer

Not necessarily. A mini-roundabout is likely to incurcosts as a result of associated measures, such ascarriageway realignment and build-outs, streetlighting, new crossing facilities, carriageway resur-facing etc. In addition, mini-roundaboutsincur ongoing maintenance costs.

No. Although a mini-roundabout with five or morearms is not unlawful, it is not recommended forsafety reasons. Where a junction has five or morearms, a double mini-roundabout may be used,although a signal-controlled junction may be moreappropriate.

Yes. However, the advice is that mini-roundaboutsshould be installed only on roads with a speedlimit of 30 mph or less. Some local authoritieshave introduced mini-roundabouts in 40 mphareas where low approach speeds ensure driversare able to use the junction safely. The installationof a mini-roundabout on a local road where thespeed limit is >30 mph is acceptable only if theapproach speeds are low and can be controlled.See Section 3.6.

They are not necessarily unsafe as actual speedsare more important than the speed limit. Sitesshould be checked and, if operating safely,monitored on a regular basis. A local authoritymay instigate a programme of speed limit reviewsto identify reductions in speed limit or mayearmark mini-roundabouts for future junctionimprovement, using a more appropriate junctiontype.

The regulatory blue mini-roundabout sign (TSRGDdiagram 611.1) requires drivers to cede priority tovehicles coming from the right. The (advisory)transverse road marking to TSRGD diagram1003.3 requires drivers approaching a mini-round-about to give way at or immediately beyond theline to traffic circulating on the carriageway of theroundabout. The TSRGD permits the use ofTSRGD diagram 1023 with TSRGD diagram1003.3 markings.

Mandatory give way signs (TSRGD diagram 602)may be used on approaches where drivers maynot be expecting to give way to traffic to their right,e.g. on the main road approach to a former priorityT-junction. Mandatory give way signs should notbe used on approaches that have an entry to theirleft, as it is not intended for drivers at a mini-roundabout to cede priority to vehiclesapproaching the roundabout from the left, e.g. thestem of a T-junction or any arm of a 4-arm mini-roundabout.

Where used the give way sign (TSRGD diagram602) should be placed above the mini-roundaboutsign (611.1).

44

Question

Are vehicles allowed to drive over thecentral island?

Can drivers U-turn at mini-roundabouts?

Answer

Yes, but only vehicles that are physically incapableof manoeuvring around the central island. SeeHighway Code, RTA 1988 Sect 36 & TSRGD16(1). Drivers of vehicles that can manoeuvrearound the central island are in danger of beingprosecuted if they drive across it.

Yes. However, the Highway Code warns roadusers of drivers making U-turns at mini-roundabouts, as this can be an unexpected, andsometimes difficult, manoeuvre. Designers shouldconsider the effect of nearby traffic managementor junction arrangements that may encourage U-turning at the proposed mini-roundabout (forexample prohibited or difficult turning movements)and try to avoid them.

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5. ACKNOWLEDGEMENT AND REFERENCES

46

5. Acknowledgement and References

5.1 Acknowledgements

The development, drafting and production ofthis document was undertaken by FaberMaunsell Ltd (Peter Yendall, Project Directorand Christian Bodé, Project Manager) onbehalf of the CSS and DfT with theco-operation and input of a number ofindividuals and organisations. Particularthanks to the following organisations:

• Bedfordshire County Council

• Blaenau Gwent County Borough Council

• Buckinghamshire County Council

• Cambridgeshire County Council

• Cardiff County Council

• Cheshire County Council

• Cornwall County Council

• Devon County Council

• East Sussex County Council

• Essex County Council

• Flintshire County Council

• Gloucestershire County Council

• Kent County Council

• Lancashire County Council

• Leicester City Council

• Middlesbrough Borough Council

• Nottinghamshire County Council

• Oxfordshire County Council

• Pembrokeshire County Council

• Powys County Council

• Roads Service - Western Division,Northern Ireland

• Suffolk County Council

• West Sussex County Council

• British Horse Society

• CTC

• North East Ambulance Service Trust

• Jacobs Babtie (Bedfordshire CountyCouncil)

• Mouchel Parkman(Essex County Council)

Special thanks to the Steering Group involvedin the development and production of thisdocument, which included: LancashireCounty Council (Terry Carter, MartinGalloway, Adrian Hanks, and Peter Andrews),Department for Transport (David Williams,Brian Lyus and John Cooper), HighwaysAgency (Gordon Heath), and Faber Maunsell(Peter Yendall and Christian Bodé).

5.2 References

Traffic Signs Manual, Chapter 3: RegulatorySigns, 1986, TSO (New edition to bepublished in 2007)

Traffic Signs Manual, Chapter 4: WarningSigns, 2004, TSO

Traffic Signs Manual, Chapter 5: RoadMarkings, 2003, TSO

The Highway Code

Road Traffic Act 1988

SI 2002 No 3113: The Traffic SignsRegulations and General Directions 2002;TSO

British Standards Institution BS5489 RoadLighting: Part 4: Code of Practice for Lightingfor Single-Level Road Junctions IncludingRoundabouts (1992)

SI 1999 No 1025 – The Highways (RoadHumps) Regulations 1999: TSO

SI 1999 No 1026 – The Highways (TrafficCalming) Regulations 1999: TSO

Traffic Advisory Leaflet 12/93 Overrun Areas

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SITE ASSESSMENT FORM

48

Site Assessment Form

Location: _______________________________________________________________________________ OSGR: ____________

No of Junction Arms: _____________ Current Junction Type: Priority � / Traffic Signal � ICD: ______________

Street Lighting: Yes � / No � In UTC Region: Yes � / No �

Area Type: Residential � / Commercial � / Industrial � / Rural �

Arm 1

Road Name: _____________________________________________________________________________ Public Highway: Yes � / No �Classification: ____________ Approach Speed (85%ile):________________ Speed Limit: _______________

Flows: ____________ No of Approach Lanes: __________________

% HGVs: ____________ Carriageway Width:______________________ Gradient: __________________

% Pedestrians: ____________

% Cyclists: ____________ Bus Route: Yes � / No �_________________ Cycle Route: Yes � / No �Pedestrian Crossing Nearby: Yes � / No � Distance: ____________ Toucan � / Puffin � / Pelican �

Zebra � / Central Refuge �Additional Comments: _______________________________________________________________________________________________

Arm 2







Arm 3







Arm 4







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Mini roundabouts: good practice guidance

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