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Mini-roundabouts -
Getting them Right!
by Clive
Sawers MA MICE CEng,
Traffic Engineering Consultant |
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Large
roundabouts |
| I am
adding more about large roundabouts to cover as far as
possible the issues of trucks overturning. We believe
that the centrifugal force effect is not the only
cause; in fact it is virtually impossible to overturn
any vehicle just by driving it in a circular path at
speed. Sudden changes in radius and crossfall and load
shifting are far more likely to be the cause of truck
overturns. |
Why
is this site necessary?
Mini-roundabouts
are being used where they should not be:
Mini-roundabouts
are not a good traffic calming tool unless justified by
the turning movements at the intersection concerned. If traffic calming is the
main reason for a scheme think carefully.
Mini-roundabouts
are being used where they should but are poorly designed.
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Poor
signing
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Lack
of deflection for crossing streams
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Little
done to change layout on approaches
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Lack
of respect for central island - too low or too easy to
overrun, often not large enough.
See the remainder
of this site for information on all the design details.
Mini-roundabouts
are not being used where they should be:
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Busy
junctions
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Turning
traffic
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Queues,
congestion
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Accidents
Why?
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Lack
of confidence in potential
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Alleged
need for "better" pedestrian facilities.
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Fear
of innovation e.g. central island over 4m diameter at
crossroads.
There seems to be
a serious lack of confidence in the mini-roundabout. Cayman
Islands for example took over three years to implement a
scheme that I knew would work well and could have been
installed in a day! But this happens. This work is designed to
encourage you to introduce these schemes where they will work
best. Correctly done they will reduce numbers and severity of
accidents and cut congestion.
Here you will find
lots of information about when & where to use a
mini-roundabout, how to improve the safety of an existing
mini-roundabout and lots more about making the geometric
layout for your whole urban area more pedestrian and cycle
friendly without resorting to traffic signals. The
main issue with the new TD is that it failed to put
engineering before law. The UK TSRGD defines mini-roundabouts
with central islands not larger than 4m. I have been pressing
for this to be changed for many years. This was the golden
opportunity to do this but we failed to grasp it. This site and the
seminars represent a follow up
to my book Mini-roundabouts - getting them right! The
early research work indicated the uses for small diameter and
overrunnable (i.e. mini-) roundabouts, but this was never well
written up and the official UK guide-lines contained
originally in
TD16/93 are hopelessly out of date and inadequate. The new
guidance in TD54/07 diverges much from my experience.
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The
Mini-roundabout
Seminar/Workshop
Comprehensive seminar/workshops
IN-HOUSE on all aspects of the design, use and
installation of mini-roundabouts developed over
several years, which can be tailored to meet your
needs.
Team up with the Traffic Calming
seminar (started in 2000)
and have a double bill!
The
seminars have been very popular.
For more information see the
Seminar
details page.
The
Workshop aspect is that we can look at your problem sites as
part of the day. This has proved to be one of the most
interesting parts of the day when real issues get a thorough
airing at real sites.
KANSAS
2008
USA National
Roundabout Conference
Download the
powerpoint presentations that I did for the Ourston Workshops.
These have explanatory text added to the slides Right
click then save as...
Ourston1
General aspects of
site selection and design
Ourston2
Flows and Capacity
A further
presentation illustrating actual design procedures is in
preparation
Visit Ourston
Roundabouts website (opens
in a new window).
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