||MIDI-roundabouts at Crossroads
designers can install mini-roundabouts of any
size. Here in the UK we have a rule which limits
the size of the central island to just 4m
diameter. This has caused serious problems and
lost many potentially useful sites. Some
authorities will not install mini-roundabouts at
crossroads as a matter of policy.
So UK designers will be
particularly interested in the crossroads at Binfield
where I ignored this rule in 1988 and installed one of
the most successful schemes ever (images below). Here the
central island is about 6m diameter. I have recommended a
mini-roundabout at a crossroads where the central island
would be nearly 10m to achieve the necessary deflection
at a site where the ICD is about 23.5m. The
MIDI-roundabout was born!
seeing the Americans using extensive truck aprons that
convinced me of the need to re-assess what a
mini-roundabout central island is and does. This is
In effect the
mini-roundabout central island is a truck apron
without a solid centre.
It should therefore be designed
and installed to be as large as necessary to
deflect all light traffic streams forcing all
crossing traffic streams to a path of 60m radius
is a drawing of a symmetrical layout based on a typical
overrunnable splitter islands may now be illegal in the
One illuminated bollard on each guarantees legality.)
the larger overrunnable central island with
shallow kerbs, raised but nearly flat;
Overrunnnable splitter islands with similar
kerbs; (These may not be lawful in your area)
Approaches are below 4m wide so will be single
Signs will have to be on the nearside, but place
on the straight;
Hatching and arrows not shown.
authorisation required in the UK.
* * * * *
urge all UK authorities to do two things:
||Examine all of your existing
crossroads mini-roundabouts and see if they could
by enlarging the central island and associated
||Look out for poorly performing
priority junctions or signal control at
see if this sort of solution might work better.
|A word of caution...
|Don't try to square up an
obviously staggered or scissored crossroads
to install a single mini-roundabout; install a
double mini-roundabout instead.
Binfield Crossroads, Bracknell,
Here are three views of Binfield
Crossroads which is very close to layout 4 on the
drawings page. This has an exceptionally good
safety record of just two slight injury accidents
in 10 years! The crucial factors:
Other options: I would look now at
the street lighting which seems inadequate. Lamp
columns would be better placed on each corner
such that four units would illuminate the
junction. Standard lighting drawings fail to do
this and can be dangerous as the layout can give
the impression of continuity across the junction
"hiding" the crossroads.
View from the west. The crucial
lane-split is now worn out and should be
refurbished. The central island is about 6m
diameter although it looks smaller.
View from the east. The red
surfacing under the hatching is new. I would
change the splitter island colour infill and
renew the lane-split markings. Note the use of
the off-side only mini-roundabout sign (dia
A closer view of the central
island which too needs refurbishing.
Had this been just 4m there would have been easy
straight through paths.
||I insisted that the central island
had to be large enough to deflect the four
crossing movements. This meant about 6m diameter.
It is slightly over-height but is certainly a
deterrent. The ICD is around 20m so the central
island has to be over-runnable. I now recommend a
shallow kerb for these larger central islands.
||There are plenty of turning
movements at the junction - so over time all
drivers have learnt high expectation of the need
to give way/yield. The former side-roads carry
significant traffic volumes.
|3. Two-lane approaches
||The approaches from the former
major road are split into two narrow lanes - for
drivers this is a powerful visual tool just in
case they missed everything else.
* * * * * * *
St/Willersley Ave, Bexley (Nov 2004)
is a plan which illustrates exactly the problem
at a real site. Previously a crossroads with
priority east-west, the roundabout involved some
widening (the blue lines) but not to affect the
BT boxes (yellow). Existing refuges are shown
with my larger ones superimposed. ICD approx.
20m. The brown areas are overrunnable, the
central island is just over 7m to the edge with
approx 600mm horizontal climb to a height of
50-75mm. Quite a lot of buses turn from E->N
& vice versa.
Clearly, for this scheme
to be safe a 4m centre would be insufficient
allowing drivers to "straight-run" it.
- 5 June 2005 - working well.
is the best general view of the scheme. The
chevrons are on the slope which is gentle; at low
(drivers') level they are not clear. I would
prefer the area to be white all over. Note also
the street lighting which probably should be
altered to include a unit on each corner. During
hours of darkness the street lights currently would appear
to line up across the junction with nothing at
the node - this can contribute to overshooting.
courtesy of Bexley Council.
revisit in February 2011 indicates that the markings are
severely worn out and need renewing. The chevron pattern is
useless and the whole island should be white. Because the
central island is too gently profiled there is little incentive to pass
around it especially the heavy right turn from the right to the
top of this image. Steeper edges (1:4) might deter the abuse
without hopefully causing the buses to lurch if they traverse
it; BUT the site was designed such that normal 12m buses should
be able to stay on the main circulation area, thus avoiding the central island.
Satellite view, © Google Images,
illustrating well the layout; while the chevrons look fine
in plan, they are almost useless on the ground. TRL realised the
value of a plain circle
as long ago as 1974 but the official view was that the arrows
The scheme helped to complete the network in the area reducing
traffic flow past two
schools, but its safety has been disappointing, mainly, from
observation, owing to the casual attitude by drivers to the
central island which is very gently profiled.
Links to other pages:
Home | Crossroads | Drawings | Millennium Vision
© Penntraff - January 2012
Pages designed by: