The Design of Mini-roundabouts

County Surveyor's Society
Transport and Environment Commitee
Traffic and Safety Working Group

This is a most useful document that looks at the publications to date (2002) and compares and contrasts the various points of advice given. The Documents and their respective use are:

Document Percentage use always Percentage used sometimes
TD 16/93 Geometric Design of Roundabouts 58 33
Clive Sawers - Mini-roundabouts - Getting them right! 13 64
IHT - Transport in the Urban Environment 9 46
TRL - The Design of Roundabouts 9 33
TRL Report 281 - Accidents at urban mini-roundabouts 3 25

In particular there is a section that highlights points of general agreement and areas of contention.

The table below represents an attempt to clarify the points of contention between the various opinions that have been expressed on these issues. In many instances, e.g. deflection, the point made in CSS report failed to differentiate between crossing and merging conflicts which appears to be very important. I hope that these notes will enable more definitive answers to be reached helping to streamline recommendations on the design and use of mini-roundabouts and their great road safety potential at modest cost. This is a long page - take your time over it!

Issue My comment ACTION
Speeds 30mph max.
Should mini-roundabouts only be installed when all approaches are subject to a 30mph speed limit, or less (some publications refer to approach speed instead of speed limit).
No!. Obviously it is necessary for traffic to be constrained to a safe speed particularly for crossing movements. But I think the key issue is what is done on the approaches. Do drivers have sufficient time and distance to take in the presence of the mini-roundabout and therefore to respond correctly and reduce speed? If so, then I suggest that a higher approach speed is not in itself an objection to a mini-roundabout installation. The excellent scheme at Binfield Crossroads in Berkshire is in a 40mph zone. Advise only. If approach speeds are higher than 30mph then special care is needed.

Very small villages may not have a speed limit but a mini-roundabout may be OK.

New Junctions
Should mini-roundabouts only be used for the improvement of existing junctions with road safety or capacity problems and not new junctions?
Where possible, yes. New junctions are usually provided by developers who can expect to provide a junction to full standard unless it is quite unreasonable to fit one in. It must be remembered that a mini-roundabout remains a sub-standard junction when compared with a good small roundabout. Developers may offer to fund a mini-roundabout as an improvement at a junction nearby as happened at Binfield Crossroads.
Gradients
Can mini-roundabouts be safely installed on steep gradients (over 2%), at summits or in road dips?
I am aware of a number of such sites and there seems to be little problem. There is presumably an existing control at this junction (or it is uncontrolled, i.e. a side-road - T or X-roads) A major scheme installed as a double mini-roundabout in Cornwall near Truro has been in operation for over 30 years and has a remarkably good safety record. It has uphill slopes of at least 8% and lies in the side of a col. A site over a sharp convex hill will have limited forward visibility and care will need to be taken that drivers are made aware of the imminent presence of the mini-roundabout by good signing and approach road layout. See approach visibility below.
Visibility
Should maximum and minimum visibility standards be applied for mini-roundabout approaches and at entries.
There are several issues here.
1. The minimum forward visibility distance to the mini-roundabout should be based on approach speeds and the approach configuration, ensuring that drivers know what they are approaching.
2. The point on the approach at which a driver can assess whether or not there is anything to give way to. At a T-junction side-road, this point may be close to the give-way line or in the case of approaching from the right across the top of the T could be a long way back. It seems generally preferable that drivers should not make the give-way decision too early. Indeed at some larger roundabouts anti-visibility fencing has been installed to ensure that drivers slow down properly before attempting to make this decision. 15 metres is the distance between the give-way line and the point at which drivers are allowed to see to the right. Where it is not possible to prevent drivers from seeing early it remains particularly important that the deflection is adequate to prevent such drivers from accelerating towards the roundabout while knowing that there is nothing to give way to.
3. How far to the "right" can drivers see once they reach the point of being able to see to the right to assess the need or otherwise to give way? The yield rule is to traffic "from the immediate right". If this distance is relatively short, it is important that such traffic is well constrained. Taking the T-junction situation, I prefer that drivers from right to left (left hand drive) are well deflected with entering drivers having less visibility, than for the give-way line to be brought forward to increase visibility distance but allowing higher speeds across the junction.
Deflection
Is physical deflection required through the junction to constrain vehicle speeds in the interest of road safety or is there no need for deflection if mandatory Give Way signs and road markings are used.
Again, clarification of the issues is required here. I always distinguish between the crossing and non-crossing or merging movements. The crossing movements must be deflected (usually to 60m curvature on entry or more usually across the junction) so that there speeds are well controlled. For the merging movements the problem is that the "deflection" is rarely effective, more a lateral displacement without actually bending the vehicle path. The use of kerbline build-outs seems to cause more accidents as drivers hit the kerb or pinch cyclists. Include CROSSING stream deflection criteria in future guidance.
Traffic Calming Schemes
Should mini-roundabouts be used in traffic calming schemes simply to act as physical obstructions for drivers to negotiate or is this ineffective and creating road safety problems (including wider disregard of mini-roundabouts by drivers)?
I believe that it is fundamental to the safety of mini-roundabouts generally that drivers respect them. Sites that do not have sufficient turning traffic lose drivers' respect and this may spread to other sites. Mini-roundabouts that are too easily traversed at speed in such schemes represent a hazard in themselves. Some criteria for "side-road" volumes/proportions recommended.

I understand that future guidance will include such criteria.

Flow balance
Do mini-roundabout traffic flows need to be reasonably balanced to prevent dominant traffic flows disregarding priority?
This is linked to several previous issues; dominant traffic streams my include drivers who come to disregard the need to give way as there may be very little to give way to. Balance is less of an issue; the virtual absence of a right turning manoeuvre (may be caused by a one-way street involved) may cause this syndrome from the direction concerned.
Safety at 4-arm mini-roundabouts
Can 4-arm mini-roundabouts be designed to operate safely?
Yes, undoubtedly. The single scheme at Binfield Crossroads already mentioned is a supreme example and more are coming online that have larger central islands. Because the central island usually is the only effective deflection at a straight crossroads it needs to be as large as necessary. The UK 4m diameter limit must be removed.
"Before" accident rates
Should minimum accident levels be used before considering mini-roundabout control at existing junctions and if so, what are they for 3 and 4-arm junctions?
No. Accident rates are not the only criteria relating to the need for a mini-roundabout. Accident information should be reported with any proposal to install a mini-roundabout, both the existing record and precautions taken in the design to prevent future accidents.  
Central island always raised?
Should the central island always be raised to assist deflection or does this have no significant effect on safety and cause operational problems for buses and HGVs and noise etc?
The only legal requirement at standard mini-roundabouts is that drivers must circumnavigate them. Raising the central island helps to ensure this. The design guidance given in my book and seminar helps to ensure that drivers are directed to the correct side of the central island. In almost all cases raising the central island promotes more respect for the mini-roundabout operation. I recommend that mini-roundabouts should be steeper at the edges (not more than 1:4 to comply with the traffic calming regulations. The central island is deemed to be an overrun area.)
Lane widths
Should narrow lanes, less than 3metres wide, be used on mini-roundabout approaches to increase the awareness of drivers and improve capacity at the Give Way line or does this cause problems for HGVs and other road users, e.g. cyclists on the approach or circulation (being obscured)?
Yes. This device has proved to be most effective, yet least used. At the majority of sites far too little is done on the approaches and narrow lanes starting at 2m wide (as recommended in the UK Traffic Signs Manual) are extremely effective. Cyclists can use a 2m lane very comfortably. The restriction in DMRB that lane widths shall be not less than 3m should be removed in the case of any approach of not more than two lanes at the give-way line, and re-considered for all other cases.
Diameter of central island
Should the central island diameter be increased above 4m at some sites to improve operation and safety?
Yes, I have absolutely no doubt about this and the original reason for the 4m restriction is flawed. THIS RESTRICTION MUST BE REMOVED FROM UK TSRGD AT THE EARLIEST OPPORTUNITY
Low (kerbed) deflection islands
Should low kerbs be used to assist deflection or do they create a maintenance problem and potential hazard for two wheeled vehicles?
Any raised but overrunnable area represents a potential hazard for two-wheeled vehicles, but these are the ones best able to avoid them! I believe that elevated areas remain useful for road safety. Perhaps there should be a long lead-in using flat markings first. The area should NOT be constructed using kerbs that might easily break out; perhaps the American system of using screed concrete might be considered along with our more conventional system of using blacktop areas coated with thermo-plastic. Anti-skid materials should be used on such areas. A possible need for specific advice may be appropriate here.
Central island white?
Should the central island always be painted white and reflectorised or are there situations where dull materials can be used e.g. setts?
I recommend that the central island should always be painted white. But there is a problem with the three arrows which are almost indecipherable to drivers. I think these could be dropped. Retain central island in TSRGD allowing any size. Make arrows optional. White is already a compulsory requirement as the marking must be used with the sign 611.1.
A minimum ICD?
Is a minimum ICD required to take account of the legal requirements in S164 of the Highway Code, S36 of the RTA and S10(1) of TSRGD that all vehicles MUST pass around the central marking except large vehicles which are physically incapable of doing so and what design vehicle should represent the "large" vehicle?
No, but it should be realised that the smaller the ICD the more vehicles there will be that are unable to keep entirely off the central island. Enforcement should take this into account, not the specific regulations.  
Central island on IC centre?
Should the mini-roundabout be sited at the centre of the inscribed circle or take account of vehicle swept paths.
Those who advocate the former clearly have not worked extensively with mini-roundabouts! In some cases the island will fall on the centre of the IC but in non-symmetrical layouts it will almost certainly not for good operational and safety reasons.  
Give way lines on ICC?
Should the Give Way lines be located on the inscribed circle circumference or brought forward to assist visibility in certain situations.
The exact placing of the Give Way lines is usually crucial, especially the deflection point at the right hand end of the give way line. This should be placed where it releases drivers correctly onto the roundabout circulation area. Too far back and drivers may be able to cut across the central island too easily. Moving it forward just to increase visibility is not usually appropriate or necessary.
Distinctive hatching?
Should hatching or solid painted islands be used on mini-roundabout approaches (the latter distinguishing mini-roundabout splitter islands from pedestrian refuges)?
Our UK markings are very fussy. The continental use of more solid areas clarifies what is going on and is recommended in the UK. Hatching for pedestrian refuges should be different from that used for mini-roundabout approaches. A new form of hatching may need to be prescribed.
Offset Centre lines?
Should the centre-line be offset to the right on mini-roundabout approaches to increase the awareness of drivers on the approach and improve traffic capacity at the entry.
Yes, if there is space and the configuration seems to work, then add the approach lane-split line.

It is good that capacity and safety issues are not always in conflict!

 
Location of mini-roundabout sign
Should the mini-roundabout sign (dia 611.1) be sited approx. 1.5m back from the give Way line or would it be preferable to site it further back to improve its conspicuity at certain junctions?
The only principle here surely is that the sign should be seen correctly and in good time. 15m was always intended and the advice should reflect this but with an overriding requirement to make the sign visible. Advice needs to be modified here.
Uses of give way sign
Should the mini-roundabout Give Way line (dia 1003.3) be allowed with the Give Way sign (dia 602)?
The intention is that there should be some enforcement of the need to give way when drivers have a straight path. At present it is permitted to add the Give Way triangle (dia ) with the standard mini-roundabout Give Way line. My preference is to use the mini-roundabout give way line at all times at a mini-roundabout regardless of the signing - this marking is MORE conspicuous. This would require a change of regulation.
Use of concentric rings
Should concentric rings be allowed around the mini-roundabout instead of circulatory arrows?
Concentric rings have been found to be confusing as some drivers have tried to avoid them altogether. One concentric ring on the flat area increases the visible "target area" but this should not be used to replace the arrows. The arrows in themselves are not effective at drivers' eye height and could usefully be dispensed with. Regulatory changes might be appropriate.
Guard-rails
Should guard-railing be an integral part of mini-roundabout schemes where there are significant numbers of pedestrian crossing movements?
This should be left to the discretion of the Engineer responsible. Each site will be different.  
Zebra Crossing location
Should Zebra Crossings be allowed close to a mini-roundabout or are there situations where they cause operational and safety problems.
This should be left to the discretion of the Engineer responsible. Each site will be different. Crossings will affect the operation and pedestrians can take their share of the road capacity.  
Visibility distance for pedestrians
What minimum standards of visibility should be provided for pedestrians at designated crossing points towards all vehicular approaches?
 No specific comment except that visibility distance towards a mini-roundabout from an exit should cover all directions. Site distances will need to be much greater if pedestrians have to cross both streams (2-way) in the same crossing task.  
Cycle Safety
How can road safety be improved for cyclists circulating at mini-roundabouts?
In most instances cyclists are struck by entering vehicles that are crossing their paths. This points to a clear need for correct deflection to be applied. In either case, crossing or merging, cyclist safety is largely dependent upon drivers' appreciation of the presence of the mini-roundabout in sufficient time to slow down correctly and make all the usual observations. Therefore the approach layouts must be well laid out defining the imminent presence of the mini-roundabout. Good lighting at night will help make cyclists conspicuous at these times.
The above should help clarify many issues, but please feel free to comment to me.

Links to other pages:

(This site is subject to continual development with new pages added in February 1999, December 2000, May & October 2002 and several updates throughout 2004, 2005. MIDI-roundabouts site added 2006.)

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Penntraff - Sept 2007