The MIDI-roundabout

If a tool is the wrong shape for the task, who can say how a tool of the correct shape will perform and
what its limitations will be...?
Clive Sawers

Much of what has gone on in highway design and operation has represented activity without sufficient thought..... Deficiencies are products of incipient institutional and individual failure. This failure is aided and abetted by the indifference of policy makers at all levels.... We cannot remain wedded to the past or embrace uniformity when there is just reason to suspect that certain practices are uniformly bad.
Highway Safety, Design and Operation. Report 93-7.
Subcommittee on Investigations and Review.
Committee on Public Works, U.S. House of Representatives
July 1973.

What is a MIDI-roundabout?

A MIDI-roundabout is a mini-roundabout but with the following special characteristics:
  • A central island larger than 4m diameter
  • The central island to be raised with a deterrent edge, flush but not so shallow that drivers casually drive onto it*.

*Note: In the UK we understand that the central island of a mini-roundabout is an overrun area as defined in the Traffic Calming Regulations. The side-slope must not be steeper than 15 (1:4).

Why is it necessary to introduce the MIDI-roundabout?

The need for larger central islands became apparent when examining the safety records of certain mini-roundabout junctions especially crossroads. Drivers have often been able to drive in a (nearly) straight line across the junction and so have failed to reduce speed sufficiently. It is now generally accepted that safety is governed mostly by speed control. Deflection is measured by the vehicle path curvature that the layout forces. A raised central island acts as the main point of deflection.

A series of drawings illustrates the sizes of central island needed for given intersection geometry.

See drawings page...

Why were mini-roundabouts in the UK restricted to 4m?

When mini-roundabouts were first tested, there was no particular specification. The layouts of small roundabouts were tested using turning circles of the largest known vehicles (articulated trucks). If the design vehicle could pass around the central island without encroaching on it or the outer kerblines, it was deemed to be a small roundabout and could be constructed fully kerbed. If it couldn't the central island had to be made traversable. At the point where this occurred, the inscribed circle of the junction was found to be around 28m diameter and the central island around which the test vehicle would fit was around 4m; at this point it was assumed that any island larger than 4m would be associated with an inscribed circle of larger than 28m and therefore solid; and any island less than 4m would be associated with and inscribed circle less than 28m and so had to be traversable.

Why was the assumption wrong?

The assumption failed to take deflection into account. Some designs even of small roundabouts in the early experiments did not have sufficient deflection. An experimental scheme introduced in Cardiff in 1969 showed a sharp increase in numbers of minor accidents. The central island had been reduced from a typical large diameter island to approx. 8m (still solid). The overall scale of the original roundabout meant that drivers now had some relatively straight paths across the junction.

Deflection - the forcing of curvature on vehicle paths is paramount as the accident patterns below illustrate.

Accidents at mini-roundabouts

Note the distribution of accident types.
Crossing accidents involve about one third.

Source: Accidents at Urban mini-roundabouts
J V Kennedy (TRL), RD Hall & SR Barnard
(University of Southampton)
TRL Report 281

At 4-arm crossroads the distribution of the left pie chart is replicated but reduced overall as the right angle crossing accidents appear. Now over half are crossing accidents.
It is clear that controlling crossing speeds at these sites is vital, and this can only be achieved in most instances using larger central islands.

Not all the sites in the study were obviously crossroads,
some were K or other shapes.


Help required please on Research Projects
Crossroads probably in urban areas and therefore subject to a 30mph speed limit, where there is either an existing mini-roundabout which could be enlarged or where a new MIDI-roundabout could be installed which would be over 4m diameter to test the effectiveness of using the larger size.

This is vital work and so far not many authorities seem willing to give this a try!

In Germany these larger MIDI-roundabouts have an excellent safety record
(better than small normal roundabouts). Search for "compact" roundabout.

If you have or know of any such sites
please contact me
(Think laterally - these potential sites may currently be right on your doorstep but you have not thought about them!)


Getting them right!

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A Definitive Guide

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Mini-roundabouts - A Definitive Guide
may be placed NOW:

  • By phone +44 (0)1626 830225
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    2 Colehayes Park Cottages,
    Bovey Tracey,
    Newton Abbot, Devon, UK
    TQ13 9LD

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Links to other pages:
| Crossroads | Drawings | Millennium Vision

Links to other sites

German Institute for Transportation and Traffic Engineering

Some of the images on this site are high resolution and may take some time to download.
Please be patient!

While the information and advice on this website are given in the best of faith, Engineers and Designers must use their own discretion regarding their validity and application. All sites differ in layout and traffic composition, and what may work well at one location may not be as appropriate at another.

The author cannot be held liable for actions taken by others based on the information contained within this site.

(This site is subject to continual development)

Clive Sawers MA MICE CEng,
Traffic Engineering Consultant
2 Colehayes Park Cottages, Bovey Tracey, Newton Abbot, TQ13 9LD
+44 (0) 1626 830225

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