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Large or "normal"
roundabouts
Part
of a large roundabout
under auto-route at Calais
(measuring the outward crossfall)
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| In my seminars we look 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 the capacity usually from an
overloaded slip road. We (TRL) knew of this as
long ago as 1971 - no more excuses. |
The French and the Americans are
not falling for our mistakes on this;
our worst problem is designing the roundabout allowing
excessive speed on the circulatory roadway:
Outward draining
normal and large roundabouts
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
installed.
There are many of a diameter around 6-8m but few much
larger than that.
The Clock-Towers at Torquay and Exeter feature.
I have some more images and will add them soon.
| A37 |
Dorchester
northern bypass, nr Charminster, Dorset |
| A250 |
(Tesco site)
Sheerness , Kent |
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Sea Street,
Herne Bay, Kent |
| A26 |
(north of
tunnel entrance) Lewes, E Sussex |
| A379 |
The Clock
Tower, Torquay Harbour (image below) |
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The Clock
Tower, New North Road, Exeter |
| A367 |
Bath (about 2
miles out on the Exeter Road |
| A31 |
A350 (I spotted
this one evening on the way back from the
seminar
at Chichester; It was too dark to take any
photos. |
| A379 |
B3192
Churchstow, S Devon (image below) |
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| Here
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
be consistent; |
this means that there are no changes
to the lateral G-forces as a vehicle circulates
the roundabout. But the roundabout MUST be
circular - no elipses or other shapes. |
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| This
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.
Excellent! |
Several
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
picture. |
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.
Relative
safety of inward/outward circulating carriageways
(France)
Circulating carriageway slope
|
Inward
sloping |
Outward
sloping |
| Accident
type |
42
roundabouts |
21
roundabouts |
| Total
accidents per roundabout year |
0.50
|
0.28
|
| Accidents due
to loss of control at entry |
0.12
|
0.06
|
| Accidents due
to loss of control on circulation |
0.09
|
0.00
|
| Accidents due
to failure to give-way/yield |
0.14
|
0.09
|
| Note
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
weather. |
There
is no information on nose-to-tail accidents
(shunts) on entry - a serious problem at some
large UK roundabouts that I have studied. |
| I
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 highlighted this problem. Several
roundabouts in key areas on the network had very
long queues (over 1km) because of single lane
entry. |
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
approach flare. |
We
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: |
A26/A275 roundabout - Lewes,
East Sussex
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Note the outward fall on the
circulatory roadway,
the conspicuous central island and the
environmental art. |
Note the three lane high capacity
entries
and the absence of any crowns. |
| By
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; problems start
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. |
| The
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. |
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© Penntraff - March 2008
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