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The
MIDI-roundabout
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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
deterent 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 15º.
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Why is it necessary to introduce
the MIDI-roundabout?
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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
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...
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Why were mini-roundabouts in the
UK restricted to 4m?
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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 vehicle could pass around the central
island without encroaching on it or the outer
kerblines, it was deemed to be a small
roundabout. If it couldn't the central island had
to be made overrunnable. At the point where this
occurred, the inscribed circle of the junction
was around 28m diameter and the central island
around which the test vehicle would fit was
around 4m; hence the assumption that any
island larger than 4m would be associated with an
inscribed circle of larger than 28m and therefore
solid. |
Why was the assumption wrong?
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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 an 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 relatively straight paths across the
junction. Deflection - the forcing of
curvature on vehicle paths is paramount as the
accident patterns below illustrate.
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Accidents at mini-roundabouts
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Note the distribition 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
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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.
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| 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 are 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.
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If you have or know of any
such sites
please contact me
(Think laterally - these sites may
currently be on your doorstep but you have not
thought about them!)
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Disclaimer:
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.
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(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
07967 148794 |
© Penntraff - February 2007
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