7. Traffic Calming using Mini-roundabouts

It is worth considering the reasons I believe (mini)-roundabouts have worked so well, at this early stage of looking at any device or systems for constraining drivers’ speeds. We are all too aware of junctions where drivers fail to give way (some crossroads), or where drivers with priority lose control (junctions with the priority stream taking a turn (left or right hand shoulder junction), and bends or corners. I believe that the success of roundabouts lies in the fact that drivers have more than one reason to slow down:

  • They have to be prepared to give way, and
  • They have to negotiate the physical geometry of the junction.

Because drivers have to consider both yielding and negotiating the physical nature of the junction at the same time, they are much less likely to push either one to the limit, particularly the speed with which they negotiate the physical layout. All this introduces safety and thousands of roundabouts world-wide testify to the relative safety of these devices. Of these two factors, only one normally features at most traffic calming sites and that is negotiating the physical layout. So you will observe many drivers pushing through traffic calmed areas as fast as the physical obstructions allow.

So what has all this got to do with traffic calming? - it has to be a pointer to engineers and designers of one highly successful method of slowing traffic down that really works, so if you can devise more than one reason for drivers to slow down then your scheme is likely to work much better.

I have written extensively about mini-roundabouts in my book and elsewhere on the site so there is no need to recap here, but there are several comments that need to be said in the context of traffic calming.

First I am aware that many authorities wish to install mini-roundabouts specifically to deal with speeding but where they are not required to help turning traffic or even to reduce turning accidents. So my rule about avoiding kerbline build-outs may need some modification here; the point being that drivers will get to know that there is little turning traffic at the junction and the scheme will be dependent upon the physical obstruction of the mini-roundabout layout to bring about speed reductions.


This drawing illustrates the principle of the scheme I devised for Tunbridge Wells BC to deal with this problem. The drawing illustrates four areas of the junctions which are raised but which can be fully overrun. "Ahead" vehicle paths are illustrated from where it can be seen how difficult it is to force a curved path along a straight kerbline. One of the schemes will be illustrated in a photo. Unfortunately TWBC declined to make the build-out an overrun area and instead this feature was fully kerbed at each of the three sites concerned. They are often hit.

Second, as your current junctions may not have a significant record of accidents it is very important that your new junction control does not make matters worse, so I remain sceptical about using mini-roundabouts in this way as indicated in my book.

I have developed ideas on the detailed design of mini-roundabouts from studying what the Americans have achieved with more normal larger roundabouts and in particular their use of quite large overrun areas around the central island (truck aprons to use their terminology). This has clarified my understanding that a mini-roundabout is a truck apron/overrun area where restricted size prevents the use of the solid centre. The size of the truck apron is dependent upon the deflected geometry of the paths of light vehicles and so will be highly site specific. This often means that the centre will need to be of a diameter larger than 4m (currently the UK maximum permitted in TSRGD &DMRB). It may even point to the need for elliptical shapes as different axes may have entirely different geometry. For more details on the designs see the mini-roundabouts crossroads page. I have further developed the idea of using mini-roundabouts at crossroads with one or more speed tables (usually incorporating H-ramps), narrowings to shorten pedestrian paths, and refuges. A typical layout of a large crossroads is illustrated in the Millennium vision page.

But the main use of mini-roundabouts with traffic calming is to provide safe high capacity junctions as part of network management in and surrounding the area and so reduce the need for drivers to seek to cross the area where traffic calming has been introduced. In conjunction with main road traffic calming, mini-roundabouts have an extensive role to play at key junctions.

While mini-roundabouts have generally been designed to increase capacity one traffic calming feature in particular limits it and that is the subject of my next page - the pinch-point.