of a large roundabout
under auto-route at Calais
(measuring the outward crossfall)
large roundabouts (UK):
speeds much too high
to enter - due to high speed circulation
crashes of large vehicles
Take up too
to maintain drainage systems
roundabouts at key nodes now signalised
not the intention on this page to go into depth concerning the
capacity and design details of large or normal roundabouts. This work
has been done extensively elsewhere. What I am trying to do here is to
point out problems associated with large roundabouts (rotaries) and
how these problems may be avoided. The French have broken away from UK
design rules and achieved considerable success with neat and tidy
layouts that function well. We should all learn from what they have
done. The Americans have compromised between the two styles.
|| They have
gone so far as possible for single lane roundabouts even having to add
truck aprons to allow the largest vehicles to circulate. The truck
apron is a very useful tool as it forces light vehicles to
circumnavigate at larger radius so restraining circulation speed; but
the truck apron itself may be responsible for the overturning of
trucks at some American sites, a problem mostly not happening in
France. Also the Americans sometimes use crowned circulatory roadways
which I have always considered to be hazardous. However, all the
roundabouts at Carmel (installed since 1999) have outward
drainage and a superb accident record.
|In my seminars we have looked at the
operation of a number of large roundabouts and
the problems associated with them. I have
remarked already that large roundabouts perform
poorly with traffic circulating at excessive
speed resulting in loss of capacity and safety
problems. There comes a point when large
roundabouts are simply inefficient at best and
downright dangerous at worst.
||The giant roundabouts associated
with our motorway and trunk road interchanges
leave so much to be desired and many of them have
failed to handle the traffic demand placed upon
them without extensive installation of traffic
signals to enhance safety and/or capacity. We (TRL) knew of this as
long ago as 1971 - no more excuses.
The French are
not falling for our mistakes on this;
our worst problem is designing the roundabout allowing
excessive speed on the circulatory roadway:
of lack of entry deflection; although guidelines introduced in
1984 have improved this
Because the whole roundabout is
Because inward drainage crossfalls encourage speed.
An accident study in France
indicates that outward sloping circulation is much safer.
The table below illustrates the risk of various accident
types at sites which slope either way.
safety of inward/outward circulating carriageways
Circulating carriageway slope
accidents per roundabout year
to loss of control at entry
to loss of control on circulation
to failure to give-way/yield
the total absence apparently of loss of control
in circulation - in the UK we are very worried
about this regarding HGVs (trucks) and in icy
is no information on nose-to-tail accidents
(shunts) on entry - a serious problem at some
large UK roundabouts that I have studied.
Modern Roundabout practice in the United States - A Synthesis
of Highway Practice - Transport Research Board National
Research Council - 1998.
think we need to learn more from the French here
and I regularly visit some of their sites in
northern France. Here all new roundabouts are
drained away from the central island. This has a
significant effect upon circulating speeds and
according to a safety study reduces accidents
too. But they are making one mistake that is
causing some problems and that is their adherence
to single lane entry. I have been advocating two
lane entries at mini-roundabouts for some years
for safety reasons let alone to handle traffic
demand. My trip during a busy period in August
1999 (the solar eclipse) highlighted this problem. Several
roundabouts in key areas on the network had very
long queues (over 1km) because of single lane
||I have studied some of the A40
grade separated roundabouts in West London. A
particular problem there seemed to be large
numbers of nose to tail shunt accidents. As many
as 50 accidents in a three year period at each
site involved mainly shunts with a few
entry/circulating accidents. The approach
configurations seemed to be designed to encourage
high speed right up to the give-way line with
super-elevated curves on the approaches which
were often single wide (4m) lanes before the
seem to have missed out badly on medium sized
roundabouts, where there is plenty of room to
achieve high capacity but where the layout still
imposes sufficient deflection to prevent
excessive speeds in circulation. Probably one of
the best in this category is the A26/A275
roundabout in Lewes, East Sussex, a three arm
roundabout where the circulatory roadway has been
drained outwards ensuring that the central island
remains high and visible. It also includes a nice
piece of environmental art. See below:
normal and large roundabouts
are views of two roundabouts near Boulogne in
northern France. The outward draining
carriageway keeps speeds noticeably slower than
UK roundabouts of the equivalent diameter.
The slopes that I have measured appear to
be about 1:40 but although the roundabouts are on
a slope themselves the radial outfall appears to
|| this means that there are no,
or very slow, changes
to the lateral G-forces as a vehicle circulates
the roundabout. But the roundabout MUST be
circular - no ellipses or other shapes. The entry to these
roundabouts needs to be radial; it is here that the sharpest
turn, smallest radius must occur.
roundabout lies on a gentle slope and has not
been profiled other than to follow that slope. It
is at a rural location and the lighting unit on
the centre is the only street light in the area.
This is a classic "pillar of cloud by day
and pillar of fire by night" example of the
use of street lights on the central island.
clock-towers seem to have ended up on
roundabouts; this one by Torquay harbour operates
very well. Large pedestrian flows are
accommodated at a split Pelican crossing through
the splitter island just off the right of the
Here is a graph that indicates the relative
G-forces at a 60m radius roundabout with various circulatory speeds
radius this is quite a large roundabout. It is generally accepted that
the limit of comfort is 0.2g so around the 23-27mph area. Although the
adverse crossfall as used in France (above) looks alarming, it is
clear that this is a steady state condition and dynamically nothing
awful is happening.
Rollover - What is going on?
we were all misled by a spectacular demonstration of truck
rollover at TRL open days in 1971. A rigid truck was fitted with
outriggers and was driven round a small diameter roundabout of
about 10m radius at 13mph and then at 16mph.
At the lower speed the truck
was well down on its outer suspension but stable. At 16mph the
inner wheels began to lift and the truck would have rolled but
for outriggers that prevented it. So what was going on?
lateral g- force keeping a truck on a circular path is
determined by the formula:
F = V2/R
where V is the speed
in metres/sec and R the radius of the truck path. In this case F
= 4.25 m/s2 or 0.43G at 15mph. This is already over
twice the comfort limit understood to be about 0.2G. I believe
this demonstrated what can happen but only if a driver is
prepared to push way beyond acceptable limits. In
short the test would not be possible without the driver being
strapped in a full harness seat belt system and so is NOT
representative of typical truck rollover.
|This second graph of a
much smaller roundabout indicates the relatively little effect of the
crossfall. At 0.2g the speeds are in the range 14-17mph. Looking at
the cross section of a truck it takes a lot more than 0.2g for the
effective weight to fall outside the wheel-track. All
the research that I have done recently (Oct 2010 onwards) indicates that
several experts have looked into truck rollover and have shown
that centrifugal forces cause rollover at certain speeds
upon the height of the Centre of Gravity (CoG) in relation to
the axle width. While this is a useful guide to the relative
risk of rollover I remain to be convinced that this is the
correct issue. But the reason I reject centrifugal force as the
principal cause needs some explanation.
In all cases of rollover
crashes reported that I have found, there is a common thread
were NOT aware that the rollover was about to occur.
suggests strongly that centrifugal force is not the issue - why?
Because centrifugal force can be felt! The driver in a cab of a
truck or in a car or any other vehicle is keenly aware of the
centrifugal force acting on a vehicle while it travels around a bend. At the
kind of speeds associated with roundabouts, particularly on the
continent it is clear that quite severe discomfort can be generated
by the outward drainage causing lateral G-forces to gradually
build up; long before danger is reached the drivers ease off
because they can feel the lateral G-force starting to act. So
what is going on and why do the continental designs have lower crash
rates on the circulatory roadway? It is time to look at the next
dynamic and that is angular momentum about a longitudinal axis.
have mostly failed to look at this although one study of dump
trucks identified instability caused by driving over a bump on
rough ground at a lateral angle that should have been well
within stability but became unstable due to the bump. The
dynamic that happens on UK roundabouts is a relatively
sudden rotation about a longitudinal axis from front to rear of
the trailer unit, specifically from the pin at the front to the
base of the rear wheels - the axis about which rollover is
that you are following a truck circulating a UK roundabout; you
will notice that it is tipping to the right perhaps up to 4°.
As the vehicle exits the driver will start to straighten the
steering then turn to the left to exit. Once on the exit the
truck may now be leaning 3-4° to left. It is this rotation that
has to be achieved somehow and initially if the manoeuvre is
taken at some speed there will be a tendency for the sprung part
to remain at the angle it was and then to suffer angular
acceleration anti-clockwise as you see it from behind. This
could result in a lesser reactive load on the nearside (left)
wheels and they might leave the ground for a short period. The
trailer is now gaining anti-clockwise angular momentum rapidly
and reaches the point when the road is now at a steady slope to
the nearside say 3°. But the angular momentum gained is trying to be
conserved and will continue the rotation unless resisted. But
the trailer is already leaning 3° in the direction that this momentum is trying to rotate it and it is now dependent upon
there not being sufficient momentum to lift the offside wheels
to the point that complete rollover occurs. It is known that
most of the UK roundabout rollovers occur on exit.
scenario is illustrated graphically below:...
of drawings to illustrate how rollover is generated from
camber reversals. Imagine that you are following this truck on a
roundabout. Initially it is circulating to the RIGHT. But the
starts to exit LEFT and the truck rolls over the crown.
Truck circulating normally;
the slope here is 4º inwards.
Driver turns to left
Slight lurch of sprung mass
to right. I have allowed 2º.
This will then impart some anti-
clockwise angular momentum
to the sprung mass helped
by the change in camber.
Truck has now passed over
the crown and sprung mass
has rotated about 10-12º
anti-clockwise where the exit
slopes say 4º to the left. Can
the truck stabilise at this
Depending on the speed of
the truck exiting the
roundabout, the angular
momentum gained from 2. to 3.
will try to maintain that rotation
and may lift the offside
wheels completely with
the anti-clockwise angular momentum gained be enough to lift the
This may not be sufficient to roll the truck over but it will be
a close thing. Clearly the crowns are an issue.
Resisting the established angular momentum at 4.depends on many
recent crash on a "square" roundabout over a motorway
involved an articulated truck rolling towards the
inside of the roundabout. This unusual incident requires further
investigation to establish the exact cause and at present no
information is available on account of possible legal
proceedings. However, it is worth looking at the possibilities
assuming that there was no mechanical failure or natural cause.
I suggest that the scenario is
the mirror image of that above; in this case the truck entered
the roundabout possibly at a green traffic signal since the
roundabout had been signalised.
means that entry speed may have been much higher than usual
resulting in the same effect as above but 1. occurring on the
entry curvature, 2. possibly occurring as the vehicle
straightened up, 3. as the truck passed onto the circulatory
roadway slope now to the right while still on slight left lock,
and 4. nothing to resist the clockwise angular momentum now
gained (as viewed from the rear), so unless the speed was kept
down, as it might need to be, rollover could happen quite
An important lesson here is that
it might not be safe to signalise certain large roundabouts
without checking and perhaps adjusting the roadway profiles.
Particularly as this was not a circular roundabout.
A26/A275 roundabout - Lewes,
East Sussex, UK
|Note the outward fall on the
the conspicuous central island and the
|Note the three lane high capacity
and the absence of any crowns.
sloping the circulatory roadway outwards drivers
should be able to visually separate the dome of
the roundabout central island from the remainder
of the vista including in particular from the
splitter island/central reservation.
|The question of
draining the circulatory roadway gets much
discussion in my seminars; obviously a main worry
is that of trucks overturning on roundabouts, but
also that of skidding in adverse weather,
including possible problems for powered two
wheelers. But the truth is that drivers and
riders are very aware of the side-force demands
that they are making between the vehicle and
carriageway and drive accordingly;
to arise when the carriageway suddenly changes
from one slope to another, sometimes causing a
considerable lurch; similar problems arise if
there is an insecure load which moves suddenly as
a result of or also causing a lurch. As always it
is important to secure good entry deflection to
ensure that drivers cannot arrive at the adverse
crossfall section at excessive speed.
advantages which are apparent to designers who
accept the principle of outward drainage are
considerable, not only in design,
construction and maintenance, but for road
safety and capacity too.
Traffic speeds on
entry, and around or across the roundabout are reduced;
There are no
crowns; risk of grounding or overturning is reduced;
island, circulatory roadway and central reservation are
clearly visible and distinguishable;
and maintenance are easier;
In the case of new
roundabouts being built on existing road lines, the provision
of outward drainage can reduce utilities costs by keeping many
of the construction elements of the scheme well above existing
Here is a short list of
sites that I know of in the UK where normal roundabouts,
as opposed to small (less than 8m dia), have outward
drainage. If you know others please let me know as I want to monitor their performance. I have now seen several,
but mostly they are either very old or very recently
There are many of a diameter around 6-8m but few much
larger than that.
The Clock-Towers at Torquay and Exeter feature.
northern bypass, nr Charminster, Dorset
Sheerness , Kent
Herne Bay, Kent
tunnel entrance) Lewes, E Sussex
Tower, Torquay Harbour (image above)
Tower, New North Road, Exeter
||Bath (about 2
miles out on the Exeter Road)
||A350 (I spotted
this one evening on the way back from the
at Chichester; It was too dark to take any
Churchstow, S Devon (image above)
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© Penntraff - Dec 2014