Getting them Right!
Sawers MA MICE CEng,
Traffic Engineering Consultant
to Mini-roundabouts - Getting them Right!
a tool is the wrong shape,
who is to decide what the correct shape is,
how it will then perform and
what its new limitations will be?
website was first developed before the government
produced relevant guidelines on mini-roundabouts.
These have now arrived and I have commented on them on
this site. Sadly these documents have failed to deal
with some of the most critical aspects of
mini-roundabout design which should have been covered
in the review. In particular the research team were
not allowed apparently to make recommendations that
meant altering the regulations - in particular the
size of the central island at mini-roundabouts. The
guidelines for deflection have been watered down and
there seem to be some strange criteria that have been
introduced regarding maximum flows from certain arms
of mini-roundabouts which I have yet to discover the
background for! These are contradictory!
adding more about large roundabouts to cover as far as
possible the issues of lorry rollover. I 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 in particular crossfall
are far more likely to be the cause of truck
rollovers. They happen far less where the circulatory
roadway drains outwards as continental statistics
proved over ten years ago. But yet again our rule
makers still insist that roundabouts must be drained
inwards to "resist" rollover when the slope
differences themselves probably cause it.
is this website necessary?
are being used where they should NOT be:
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 your scheme think very carefully.
are being used where they should but are poorly designed.
signing, or signs in the wrong place
of deflection for crossing streams
ineffective build-outs in the name of
done to change the approach layouts or appearance
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.
are not being used where they should be:
of confidence in potential
need for "better" pedestrian facilities.
of innovation e.g. central island over 4m diameter at
design rules in official UK technical directives leave a
lot to be desired.
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.
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 they failed to grasp it.
This site and the
seminars represent a follow up
to my books Mini-roundabouts - getting them right! and
Mini-roundabouts - A Definitive Guide. Early research indicated the uses for small diameter and
traversable (i.e. mini-) roundabouts, but this was never well
written up and the official UK guide-lines contained
TD16/93 are hopelessly out of date and inadequate. The new
guidance in TD54/07 diverges much from my experience. TD16/07
updates normal roundabouts but still insists on inward
drainage! I think we are the only country left draining
roundabouts towards the centre.
Indiana all of the roundabouts built over the last 10 years or
so drain outwards.
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
their time the original whole day seminars proved very popular.
I have reduced this now to a half-day.
For more information see 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.
powerpoint presentations that I did for the Mini-Roundabout
These have explanatory text added to the slides Right
click then save as...
time to time members of the public contact me through
the website about a mini-roundabout that is causing them
problems. It is not good for the local highway authority
or for residents that a site is poorly designed or
therefore prepared to make brief investigations using
the satellite views that are now available through the
internet to comment on a junction layout that might
appear to be hazardous. That does not usually give a
definitive answer but often there are several clues that
I can identify as to the likely operation of the
junction even remotely.
be aware that I do not represent the highway authority
in this assessment but I may contact them to advise them
of the position and possibly to make recommendations.