Mini-roundabouts – Getting them right! – UPDATE – Please read carefully…
07/11/18
Mini-roundabouts - Getting them Right! was published in October 1996 and much has happened since then. Summarised below are the main points that have emerged as being critically important. Please note that I run seminar/workshops on mini-roundabouts where all of these issues get a good airing. Details of the seminars and latest updates are on my website:
and on my discussion group:
- The size of the central island especially at crossroads
The current maximum size permitted for central islands in the UK is 4m (TSRGD & DMRB). But this has proved inadequate at many sites (page 31). At most crossroads layouts it is only the central island that can deflect traffic so it is important to make this large enough to be effective. If necessary, UK designers should seek approval from their nearest Government Office or DfT headquarters who have indicated that they will authorise such a departure from the regulations where this appears to be justified. Note that in America their roundabouts make extensive use of overrun areas (truck aprons) to ensure adequate deflection for light vehicles – see their web sites – I have some links to them from my web site. The mini-roundabout central island is in effect a stand-alone truck apron.
(See for illustrative drawings.)
UK designers please note that the central island and truck aprons are overrun areas as defined in the UK Traffic Calming Regulations and must be designed accordingly with no vertical upstands exceeding 6mm and no slopes exceeding 15º. Recommended universally.
- Mini-roundabouts used for traffic calming
The use of mini-roundabouts just to control vehicle speeds may not be very effective. Drivers must have good reason to slow down to negotiate a mini-roundabout, so each site should normally be justified in its own right. There should be plenty of turning traffic; sites with less than about 10% (page 7) seem to have overrunning problems where regular drivers get used to not yielding as there is often little or nothing to yield to. A possible exception to this appears to be where several mini-roundabouts are used together along a route but even then drivers can often be seen to just “negotiate the obstructions”. Avoid causing this syndrome where possible.
- Crossfalls/drainage at normal roundabouts
Studies in France indicate that draining roundabouts away from the centre is safer and they have adopted this practice. The Americans also design some roundabouts like this and the UK DfT and HA are now recommending this for small and mini- roundabouts. In my view these should always be drained outwards to make the layout conspicuous. This also reduces the risk of drivers crossing the junction too fast. Remember that the criterion regarding speed control of 0.2g sideways force is achieved by forcing a circular path of approx. 60m on level ground; but favourable crossfalls (as in the UK) allow or may even encourage excessive circulatory speeds.
Raising the roundabout with outward falls overcomes many visibility difficulties that drivers experience when approaching them. There is no substitute for being able to see the roundabout layout properly on approach. On many UK sites it is quite impossible at normal drivers' levels to see very much at all and too many Engineers think it is sufficient to "cure" this problem by installing a clutter of signs. At normal roundabouts, good entry deflection is essential.
- Lane widths
In the UK TD16/93 (incorporated into the DMRB) requires lane widths of not less than 3m at the yield lines. In many circumstances that will be restrictive. I have been very successful at using narrower lanes; these are quite safe too, in particular for cyclists. Splitting the approach into two narrow lanes in good time has a remarkable effect on drivers who rarely overshoot such approaches. I am pressing for this restriction in the UK DMRB to be removed altogether - it rarely makes sense especially at a mini-roundabout. TD16/93 is now being revised in the light of these and many concerns.
Another way of helping drivers to identify the presence of the mini-roundabouts on the approaches is to change the surface colour. Combined with anti-skid properties this has proved effective. Do not use anti-skid surfacing on the circulatory area.
- Deflection along straight kerblines at T-junctions
Many designers use kerbline build-outs at T-junction mini-roundabouts. There appears to be mounting evidence that they can cause accidents and may be ineffective at reducing vehicle speeds unless they are so large that they compromise the layout altogether. While it is desirable to reduce speeds for this movement, it can usually be done as effectively by the use of two narrow lanes on approach as above and illustrated on page 40. This can be a very powerful tool in getting drivers out of “auto-pilot”. Safety auditors need to consider this carefully as so often they look for “deflection” without really knowing what is meant by it; see the definition on page9 but be prepared to apply it now at 60m radius.
- Schemes funded by Development
Developers should not normally be providing mini-roundabouts to service their developments. In some cases there will be insufficient flows generated to justify the scheme especially in the early days, but if a roundabout would be useful for traffic calming purposes, use a small roundabout. There is no objection of course to developers contributing towards the cost of a mini-roundabout where this is already planned at a nearby junction and where it will not be possible to install a more normal roundabout (e.g. Binfield Crossroads - p31).
- Roundabouts near railway level crossings and tram lines
(Mini-) roundabouts can be installed quite satisfactorily near railway level crossings provided that the barrier system is manually controlled (page 77). In Europe tram and rail tracks often run through roundabouts. In some ways this is easier to control as there will not normally be queues around a roundabout which could otherwise delay the lowering/signalling sequence.
- Carriageway markings and hatching
There is a tendency (in the UK particularly) to make road markings very "fussy" and so make the overall appearance cluttered. Try using more solid areas to distinguish them from the hatching used with pedestrian islands. Many mini-roundabout splitter islands resemble pedestrian islands because of this; try to avoid it.
- Street Lighting
The introduction of a mini-roundabout should be accompanied by alteration to the street lighting to help drivers identify the new junction operation at night. It is important to highlight the presence of the new layout and not just to provide sufficient light levels. The general vista of the street and lighting should at all times highlight the presence of the (mini-) roundabout.
- Pedestrian and Cycle facilities
It continues to be assumed that pedestrians and cyclists are best accommodated by using traffic signals at junctions and at light controlled crossings elsewhere. Evidence does not generally support this; although vulnerable users rightly demand help and protection, traffic speeds are usually much higher through signals than through correctly designed (mini-) roundabouts and this is reflected in relative accident rates. There remains great scope for crash reduction in many urban centres by using traffic calming techniques as for example at Borehamwood, Herts.
See and
(Transport 2000)
- The Seminar/workshops
I have run well over 50 whole day events covering the design and use of mini-roundabouts since 1997. These events are unique to each organisation. Held “in-house” I am often able to get to sites identified for discussion in the seminars and record video and still images.
The seminar is an extremely effective way of getting the information over and exchanging ideas. See website for more details.
Clive Sawers (February 2007)
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07/11/18