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
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
Highway Safety, Design and Operation. Report 93-7.
Subcommittee on Investigations and Review.
Committee on Public Works, U.S. House of Representatives
What is a MIDI-roundabout?
MIDI-roundabout is a mini-roundabout but with the
following special characteristics:
central island larger than 4m diameter
central island to be raised with a deterrent edge, flush but not so shallow
that drivers casually drive onto it*.
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 15º (1:4).
Why is it necessary to introduce
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
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?
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?
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
Deflection - the forcing of
curvature on vehicle paths is paramount as the
accident patterns below illustrate.
Accidents at mini-roundabouts
|Note the distribution of
Crossing accidents involve about one third.
Source: Accidents at
J V Kennedy (TRL), RD Hall & SR Barnard
(University of Southampton)
TRL Report 281
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
all the sites in the study were obviously
some were K or other shapes.
required please on Research Projects
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
In Germany these
larger MIDI-roundabouts have an excellent safety
(better than small normal roundabouts). Search
for "compact" roundabout.
If you have or know of any
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!
BUT kept up to date here and on
A Definitive Guide
Mini-roundabouts - A Definitive Guide
may be placed NOW:
- By phone
+44 (0)1626 830225
(mobile) 07967 148794
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Newton Abbot, Devon, UK
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|Clive Sawers MA
Traffic Engineering Consultant
2 Colehayes Park Cottages, Bovey Tracey, Newton
Abbot, TQ13 9LD
+44 (0) 1626 830225
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