Mini-roundabouts - good practice guidance
Department for Transport & County Surveyor's
|The only difference between a small or mini-roundabout should
be whether all vehicles can circumnavigate a solid central
part or whether this cannot be provided. If not, then it is a mini-roundabout. That then
requires the different signing and road marking.
BUT THERE SHOULD BE NO OTHER DIFFERENCES
The table below identifies other points of concern.
This states the dimensions, legality and signing of a mini-roundabout. A strong emphasis on the differences as in Photos 2.1.3-2.1.6 on P5.
concern as it overemphasises a difference that should not be
there. The overrun area of a small roundabout may be any
diameter. The absence of a solid centre should not force the
overrun area to be 4m or less.
There is lots on this all over
this site but see in particular:
Several experimental sites are needed and legislative changes.
|2. This section looks at the individual reasons why mini-roundabouts are installed.||There is an emphasis on the separate issues but a lack of integration. I was always taught to include the various aspects of safety, convenience, access, capacity and general functionality as essential parts of the scheme. Try to look on the whole aspect of the scheme and you will probably not go very far wrong.|
|2.2 "mini-roundabouts were initially developed as a method of improving safety at existing junctions".||This is not true. Mini-roundabouts developed from larger roundabouts that could be reduced in overall size once the offside priority rule became established in 1966. It was found that the offside priority rule could be used at small junctions with nominal central islands and therefore roundabouts became used in urban areas at several small junctions. It was then found that at many sites, safety had improved. It was understood that keeping or getting speeds down was a key factor in this and the need for the correct deflection became apparent.|
|2.3 "Mini-roundabouts are used to replace.... conventional roundabouts..."||This is not true. I know of no such sites. In fact the document identifies one site where the reverse took place in Cornwall and I know of one in Torquay similarly. The only possible link is in the use of ring junctions at former large roundabouts such as at Hemel Hempstead and Swindon where mini-roundabouts in a ringed network replaced the single large roundabout circulation. Similar schemes using small roundabouts have been used at Denham (A40/M40), and Essex. See networks for information and links to satellite views of these layouts..|
|2.3 & 3.9 "Mini-roundabouts should not be used at junctions with five or more arms."||This has been assumed because of the difficulty in providing enough deflection for all traffic streams and the potential for driver confusion. A proposed five-arm mini-roundabout came into a seminar a few years ago and more recently I found the site at Otley Road, Eldwick, near Shipley in West Yorkshire. The fifth arm is a very short cul-de-sac but the site appears to work very well. The document assumes that these will not work but provides no evidence. The availability of larger central islands would greatly help any such sites to work more safely and efficiently.|
|2.5 "Careful consideration should be given to introducing mini-roundabouts as part of a new development."||Somewhat
ambiguous. New developments will have low flows for a period
and may have low flows always. I was asked to look at a site
where a mini-roundabout served just 12 houses. It failed to
operate correctly. I do not normally recommend mini-roundabouts for new
junctions but accept that they may be appropriate in some
cases. But generally avoid their use unless the "side-road" flow
will eventually come up to an acceptable level.
UK DfT now recommend daily side-road flows should be a minimum of 500. A criterion here was needed
|Photo 2.6.2 This and other photos... sign upside down.||A point, but a little petty. Far more important is that these signs are often placed out of sight because the position rule was incorrectly interpreted. The sign should be within 15m of the give-way line not 1.5 which often places the sign too far around the radius of the approach, especially on the crucial approach where the "side-road" is on the left..|
|3.1 ... the initial design of the mini-roundabout can be modified...||My book sets out design features in detail and it should be possible to get these right first time in most instances.|
|3.2 unlikely to be appropriate... dual carriageway||The key exception is single lane dualling - see Shenley Road Borehamwood and Yarnton Way, Bexley (this latter has four small roundabouts).|
|3.2 unlikely to be appropriate... speed||Although covered later, it is worth pointing out that some of the most successful schemes lie in areas of higher speed limits. Provided all the right precautions are taken the site should be safe. Binfield crossroads in Berkshire has an excellent safety record.|
|3.3 A mini-roundabout should not be considered a simple lining and signing exercise.||There will be some sites where this is all that is required. Naturally symmetrical shapes such as Y-junctions have built-in deflection and can work well with just the priorities changing to those of a mini-roundabout. A site in rural Berkshire fell into this category and was introduced so that all drivers should give way to the right. With any priority layout at this junction some drivers would have to give way to the left and high hedges were making that difficult. The site on the front cover of Mini-roundabouts - getting them right! cost under £1,000 to install and was similar with inbuilt deflection.|
Lots of general comments regarding visibility.
are several issues here. But the key issue is to get crossing
speeds well down (under 20mph). This will ensure that
conflicts can be minimised and required visibility distances for
yielding purposes can be minimised too.
1. Forward Visibility to the junction.
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 has been shown 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. Visibility to the right at the point of giving way.
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 allowing higher speeds across the junction
|3.6 Vehicle speeds||Touched on earlier, it is imperative that speeds are brought down to a safe level in good time. Almost all normal roundabouts have approach layouts that make it clear to drivers that they are coming to something. Mini-roundabouts often fail in this regard. Take a look at the photos in the document and see just how little is actually done (say 10m, 30m and 60m back from the yield line). In most instances the answer is "absolutely nothing"! If you want a safe site, that is NOT good enough.|
|3.7 Gradients||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. For downhill approaches it is necessary to have good forward visibility to the junction so that drivers have more time to make adjustments. See approach visibility above.|
|3.7 Number of lanes||In the 1970s and 1980 we did not use different colour surfaces. I found that squeezing lanes in was helpful at increasing capacity AND slowing down the approaching traffic. At T-junctions this often meant offsetting the centre lines so that across the top of the "T" they did NOT line up. The break in continuity and narrow lanes all helped drivers assimilate the junction well BEFORE the give-way line.|
|3.7 Street Lighting||Care needs to be taken that street lighting highlights the presence of the junction. Too often the "standard" junction drawings are used. These often "hide" the junction.|
|3.11 ...large flow of pedestrians||The double mini-roundabout at Marlow, Bucks carries huge numbers of pedestrians - so many that they would not wait for a green man at signals. The use of the double mini-roundabout at that location ensures that speeds are at the lowest possible. Shenley Road Borehamwood also illustrates an area where pedestrian flows can be high but the extended facilities afforded by the continuous central reservation enable pedestrians to cross freely and easily.|
|4.2.1 ...Reliance should not be placed on the central island itself as a speed reducing feature.||Unbelievable! In the majority of instances it is the central island that is the main deflector (using the forced radius method). In view of the small scale of mini-roundabouts it is far less relevant that drivers are being controlled in the junction whereas at large roundabouts they are controlled by entry configuration.|
|4.2.3 '...use was made of splitter islands or build-outs to encourage 'correct behaviour.'||Build-outs deflect the vehicle path to the offside. Done BEFORE the give-way line this can reduce speed in conjunction with a splitter island. Most of the build-outs that I see deflect vehicles TOWARDS the central island contributing to overrunning. Avoid this! Also the actual deflection achievable is very little in terms of forced vehicle path radius which is the only aspect worth measuring..|
Where deflection was provided this was done by up to three different methods:
|First point - what
about the central island? This is the key deflector!
Build-outs can be a hazard in themselves and have been reported to pinch cyclists. The object of deflection is to control vehicle speeds. A radius of 60m forces speeds down to 20mph or thereabouts. The radius is achieved by bending the vehicle path - see the other pages of this site. This can usually only be achieved by the central island bending vehicle paths and is essential for crossroads sites. Splitter islands have a relatively small effect here. I have repeatedly found build-outs ineffective. They limit capacity unnecessarily and rarely reduce speed.
Another method of deflection is of course the speed table. Vertical shifts are effective based on the change of vertical angle.
The graphic below illustrates the principle of deflection for the crossing streams (shown hatched) - right to left deflected by the central island and the two kerb radii on the corners; and the relative lack of deflection for the left to right stream along the top of the T despite a considerable build-out. With a fourth arm the central island would have to be much larger to achieve the same deflection for all four crossing streams - probably around 7m.
|Include CROSSING stream deflection criteria in future guidance.|
|Check list.||Although I have a
quick instinct for whether a mini-roundabout might be
suitable, I appreciate that others will not be sufficiently
experienced. The check list suggested as the last page is a
good idea. I believe that there should be a little more
Location might include:
What is the diameter of each and separation between the centres?
For each arm include:
Links to other pages:
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Society Report 2002
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(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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