Mini-roundabouts & Traffic Calming

Well justified, a mini-roundabout will, in itself, act as a useful traffic calming feature. Drivers will expect to give way at the site and this will result in speed reduction. Problems arise when traffic engineers assume that a mini-roundabout can be installed just to control traffic speeds when there may not be much turning traffic. In this circumstance drivers tend to "run through" the mini-roundabout not expecting to yield much, in some cases very little.

If you expect problems with your site as there is low flow and relatively little side-road traffic, try making the central island a feature in itself. I have recommended the use of a mini-roundabout of up to about 8m diameter in a crossroads of ICD 20m just to ensure adequate deflection. The roundabout would operate on the basis of single file entry and probably using overrunnable splitter islands. I now recommend using shallow kerbs as a deterrent raising the whole centre by about 75mm, ensuring that drivers cannot cut straight across the central island and so have to treat the roundabout with respect.

It is important to ensure that drivers will expect to give way. An exception to this may be where there are several mini-roundabouts along a route which is being traffic calmed - safety in numbers may work out. I have seen a few such schemes and they seem all right. It is the isolated mini-roundabouts that were never justified in their own right that so often seem to come into my seminars as problem sites.

Main road traffic calming

I am looking forward to getting much better control of speeds on main roads through villages and towns using mini-roundabouts and differential slope vertical deflections, (i.e. speed cushions, H and S-ramps) where the steepness of the vertical deflections is dependent upon the effective wheel track of the approaching vehicle. Buses and larger vehicles including most emergency vehicles suffer less vertical acceleration than cars and light vans and if the design is right should be able to traverse at about the same speed.

Here is the form of H-hump on a wide two-way road that many will have seen demonstrated in my seminars. I have seen one in use at Feltham, a pair on a bus route on the outskirts of Belfast and a series in Glenrothes, Scotland; let's get using these, perhaps at some trial sites first.

Drawing of H-hump

The H-hump. This device has great potential for use on main roads where there are problems with speeds, but genuine pedestrian facilities are needed at footway level. While pedestrians often demand light control, the safety of Pelicans and signal junctions is doubtful. Some H-humps have been installed in Scotland in Fife but there they incorporate the dug-out area on the exits ramps from the table which I consider unnecessary. Also they assumed that drainage gullies will be needed which will rarely be the case. On most reasonably level roads the side-fall is usually more significant than the longitudinal fall. In Borehamwood traffic speeds along Shenley Road are controlled by mini-roundabouts, single lane dualling and ordinary speed tables. The buses are at some disadvantage and need a facility such as the H-ramp to reduce their vertical accelerations, while increasing the vertical accelerations for light vehicles.

An interesting example of main road traffic calming with mini-roundabouts occurs at Craven Arms in Shropshire. This site came into my seminar at Shrewsbury in July 1999 and is reported in DETR Traffic Advisory Leaflet 2/97 - Traffic calming on major roads: A49, Craven Arms, Shropshire.
See also TA 1/98 - Speed Cushion Schemes.

Various views of A49 Craven Arms, Shropshire
(The site has not been visited since these images were taken in July 1999.)

A49 Craven Arms looking S
A49 Craven Arms northern mini-rbt
Note in these two views the smooth alignment of the carriageway and note in particular the misleading effect of the bifurcation arrows which really must not be used at mini-roundabouts. Note the visual clutter of the hatching; the use of different colour surfacing may be effective but the whole thing would be better if we would make our painted islands solidly marked and not hatched.
A49 Craven Arms - southern mini-rbt looking north
A49 Craven Arms southern mini-rbt from vehicle
In this view approaching the southern mini-roundabout from the south, note the superelevation, the lack of indication by the street lighting of the mini-roundabout and the false impression that the hatching could just lead to a refuge! There are virtually no clues on the ground that this has to be the approach to a mini-roundabout! This view is no better; although I can see the mini-roundabout (just), it is not that conspicuous and the approach layout fails to attract my attention or slow me down. If we are going to be successful in reducing accidents at mini-roundabouts the approach layouts really must do better than this.

Concern was expressed by Shropshire CC that the northernmost mini-roundabout had very low side-road flow and was hardly justified - I am inclined to agree. But I am also concerned about a number of design faults along the route through the village that I think should be rectified:

1. The use of bifurcation arrows Although extremely close to the mini-roundabout give-way lines, bifurcation arrows have been used on several approaches, but these are misleading to drivers giving the false impression of a priority junction. These arrows should be removed.

2. The street lighting This fails to highlight properly the presence of the junction; at least three units are needed over each junction, one on each corner.

3. The superelevation  Probably one reason why the scheme was installed in the first place - the southern mini-roundabout is placed on a slight curve in the A49 and it is noticeable that the carriageway is super-elevated. No attempt has been made to remove this - probably too expensive, but observers should note that the need for super-elevation in urban areas must be very rare indeed and it is extremely important not to create new problems by including this in designs which sadly I have noticed all too often.

4. The clutter  The different surface colours and the hatching all conspire to hide rather than clarify the junction layouts.

Lessons to be learnt:-

  • Just "dropping in" one or more mini-roundabouts is rarely enough;

  • Clearer layouts are needed - avoid all that hatching;

  • Think twice before using mini-roundabouts with low side-road flows;

  • Don't build super-elevated highways in the first place!

General Comment

I am very pleased to see villages where mini-roundabouts are being used to deal with excessive speed in such an environment, and I am pleased too to see the use of vertical deflections. In time we will develop the use of more sophisticated vertical deflections such as H- & S- humps which are more helpful to pedestrians and, combined with mini or small roundabouts, will secure good speed and environmental control in villages and urban main roads currently plagued with excessive and intimidating traffic.

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Penntraff - Dec 2014
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