- Getting them Right!
was always taught not to separate issues
pertaining to safety and capacity; yet I
has to be a trade-off between capacity
again and again!
Safety is a
constant theme throughout the book and on the website. It
is important to distinguish between real safety issues
and perceived safety. The TRL study Accidents at Urban
Mini-roundabouts Report 281 tracks down the many factors
that lead to accidents at mini-roundabouts and I have
reviewed some of it.
The key findings
at all mini-roundabouts are that:
Pedestrians are relatively safe,
Two-wheeled vehicles are relatively vulnerable.
are open to interpretation, but a common theme concerning
accidents seems to be a failure by drivers to appreciate
the presence of the mini-roundabout in good time, more
than likely caused by layouts that fail to grasp their
attention, particularly on the approaches. Too much
emphasis is also placed on signing and insufficient
attention to road layout and correct deflection.
It is the
vehicles which cross one another's paths which get
involved in almost all vehicle/vehicle accidents at
mini-roundabouts. In effect these are the right turners
(UK); their speeds must be controlled by the imposition
of 60m radius or less on vehicle paths across the
junction. This applies to the the right (UK) turn/ahead
(crossing) movements, but as an alternative, vertical
deflections may be used.
For the merging
conflicts left turns run at slower speed anyway but the
movement along a straight kerbline can run quickly.
Bulging out the kerbline is rarely an answer as drivers
can still pass at speed, only this time closer to the
mini-roundabout island. There is mounting evidence that
kerbline bulges can create accidents.
(mini-) roundabouts would work better!!
|Many Highway Authorities waste huge sums by
installing, operating and maintaining traffic signals at
sites where (mini-) roundabouts would be much cheaper to
install, run and maintain, have greater capacity, reduce
delays to pedestrians, and be safer all round including
for pedestrians. At a time when financial constraint is
severe, local highway authorities should be looking more
carefully at this issue distinguishing between perceived
safety of signals and real safety of roundabouts.
It is a
myth that signal junctions guarantee safety for
pedestrians. Recent research shows that the provision of
red/green man pedestrian crossing stages does not enhance
safety; in many cases such facilities actually make
details click on the links)
simple layout of junction - avoid clutter
- Conspicuous central island
- Approaches must NOT
resemble any other form of junction
Mini-roundabout give-way (yield) signs (dia 611.1
UK) where best seen
- Never sink
island in a hollow - drain outwards
lines follow outer swept paths
NOT normally ICC (inscribed circle
island position based on equally
deflected inner swept paths
- Speed of
crossing streams must be closely
- Speed of
merging streams need not be so controlled
kerbline bulges, they rarely slow the
traffic down but just get hit
- Ensure all
visual continuity across junction is
broken e.g. centre lines shouldn't line
- Light the junction
of these rules apply to normal roundabouts too.
I am finding quite a number of sites in the
UK where the designers have avoided draining down
to the central island of quite large roundabouts
and it is noticeable how circulation speeds are
reduced. The central island is relatively
conspicuous and there are many advantages to this
form of construction. Such adverse camber is not
thought to be related to the problems of HGVs or
trucks overturning on roundabouts.
Excessive speed, rapid steering switching
combined with sudden changes in the crowns, often
on non-circular roundabouts, all seem to add up
to this risk.
Far too many UK small roundabouts
look like this, just because the designer has to
install inward drainage and it falls to a gully;
excuses for not installing a (mini-) roundabout
have to be in balance"
first public road mini-roundabout had right
turning flows in the ratio 1:5:11 and worked
will merely transfer to another arm"
will depend entirely upon the relative turning
movements and demand flows but the junction
operation is so different that this is rare.
streams will develop"
can happen in the virtual absence of a turning
movement, but the junction operation should still
be improved. Check each arm's operation.
flows won't work"
tidal flows can be disruptive - examine flows and
apply any formulas sensibly to see the likely
effects; quite a severe degree of tidality may
still be acceptable.
passage of an HGV compromises the integrity of
the site concerned a long vehicle turning would
overrun the whole junction and both sides of an
approach road. An HGV will usually occupy the
whole junction anyway; the mini-roundabout will
be likely to have reduced queues and therefore
allow the HGV a better chance to complete its
enable [the Traffic Engineer] to maintain
of what? Yes, it may be possible to cause
longer queues on one arm than another during peak
periods by using signals. But off-peak this
is virtually impossible and it would be wrong to
deliberately cause a long queue on one arm when
other arms are significantly free of traffic as
some form of deterrent.
about pedestrians; surely they are better off
because pedestrians do not get priority this does
not mean that they are worse off. Pedestrian
accidents are much lower at roundabouts. Delays
to pedestrians using a signal system at a busy
junction are much higher than at a busy
roundabout where splitter islands allow the road
to be crossed in two stages. Better designs which
keep speeds down will further enhance the safety
of pedestrians and cyclists.
cyclists are vulnerable at roundabouts, this is
true of most forms of non signal junction
control. The use of outward crossfalls on
circulating carriageways and relatively narrow
lanes on the approaches to mini-roundabouts all
help cyclists to be part of the traffic rather
than obstructions that "get in the
way". For mini-roundabouts in particular,
the repeated failure at many sites of drivers to
appreciate the presence of the roundabout in the
first place put circulating cyclists in great
danger - ensure your designs get ALL drivers out
and Traffic Calming
There has been an upsurge in the use of
mini-roundabouts in the UK for traffic calming. While it
is true that mini-roundabouts are effective at traffic
calming this has been because they have generally been
justified in their own right at the site concerned.
is not satisfactory to install a mini-roundabout at any
junction just to reduce speeds.
If the side-road at a site has very low
flows then the mini-roundabout will not perform well and
drivers will start to treat it merely as an obstruction
to be negotiated. At one site I came across
recently a series of mini-roundabouts had been installed
along a road that needed traffic calming but there was
very little side-road flow. One of the minis was a
two-arm mini-roundabout!! Avoid this.
mini-roundabouts and traffic calming.
The long term aims and
will come on line.
|TRL studies similar to existing
studies on links and other junctions will clarify
gradually learn not to worry about adverse
crossfalls on larger roundabouts.
management using calming devices with roundabouts
at all key junctions will become the norm in
large areas especially residential and small
towns; these areas have the poorest UK child
vertical deflections with mini-/small roundabouts
will bring effective traffic calming to main
road and village environments; especially abroad...
international design standard will emerge.
casualty reductions possible if roundabouts
(Annual savings of about 8000 deaths and 100,000
casualties in USA alone.)
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© Penntraff - August 2018