apologies to overseas viewers who can install
mini-roundabouts of any size. Here in the UK we
have a rule that limits the size of the central
island to just 4m diameter. This has caused
serious problems and lost many potentially useful
Some authorities will not now install
mini-roundabouts at crossroads as a matter of
|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, too small for a solid central
Drawings of idealised layouts to illustrate
|Here is a series of five drawings
of symmetrical idealised crossroads. Each
illustrates central island sizes required to
provide deflection at 60m radius for approx. 2m
wide vehicle paths. The island size is determined
by the deflection required which in turn is
determined by ICD and other
factors such as road widths on entry and exit,
||In this series of drawings with
reducing ICDs the first is the only one with a
solid centre. In the following four layouts it is
only the truck apron which remains, and it is
always designed to be large enough to deflect
light vehicles, and that will often mean a
diameter larger than 4m.
What we are seeing in
effect here are two roundabouts! One designed for
trucks, coaches etc. and one designed for light
In the UK
this would be legal.
The size of the
truck apron here is determined mainly by the road
widths. Reducing these locally would help
(See Millennium Vision page)
There are many
cross-roads like this in the UK and thousands in
traffic by the central island alone requires
quite a large island.
would be assisted here by the use of
build-outs on the approaches but avoid
significant reduction of the ICD.
In my seminars
we look at a possible way of dealing with one of
the sites at Hove which is remarkably similar.
the UK this might be illegal!
or require special authorisation.
This is a relatively
extreme case where a mini-roundabout might be
tried on a crossroads with tight corner radii and
wide approach roads. As with the previous layout
narrowing the approaches will make a better
layout altogether but which may still require
over 4m for the central island.
this might be illegal in the UK!
Medium-size flared crossroads
7.3m is the typical width of many
UK roads so this illustrates well the need
for a central island larger than 4m. Many highway
authorities will not now install mini-roundabouts
in these circumstances which is a pity because
they can work well provided that there is
sufficient deflection, achievable using a
larger central island. In addition I would
recommend raised splitter islands at a site like
technically illegal in the UK!
crossroads - little flare
I would not normally
recommend a mini-roundabout for this geometry
unless speeds are already very well controlled by
other means than the mini-roundabout lateral
deflection. This drawing is really to illustrate
the position compared with the other ones.
UK such a mini-roundabout would be legal
(but hardly practical).
seeing the Americans using extensive truck aprons that
convinced me of the need for re-assessing what a
mini-roundabout central island is and does. This is
effect a mini-roundabout is a truck apron without
a solid centre.
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
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.
* * * * *
I 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
try to square up an obviously staggered or
scissored crossroads to install a single
mini-roundabout; install a double mini-roundabout
Binfield Crossroads, Bracknell,
Here are three views of Binfield
Crossroads which is very close to layout 4 above.
This has an exceptionally good safety record of
just one slight injury accidents since 1/1/1991! The
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
straight through paths.
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.
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
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
St/Willersley Ave, Bexley (Nov 2004)
is a plan which illustrates exactly the problem
at a real site. Formerly a crossroads, the roundabout will 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.
for this scheme to be safe a 4m centre would be
insufficient allowing drivers to
- 5 June 2005 - working well - images on www.midi-roundabout.co.uk.
(Sorry, I STILL have not been to
the site; as soon as I have more images I will
Belt Road/ChurchBrae, Derry
Double Mini-roundabout in widened outline
illustrates a design for a straight crossroads in
Derry, not right-angled, but with around 50º
between the axes. One of the earliest schemes at
Upton Cross in Dorset was similar, installed in
1970 on the (then) A35/A350 intersection
previously controlled by traffic signals. The
double mini-roundabout always ensures better
deflection than a single mini-roundabout. In any
case the latter at such a crossroads would fail
with drivers making the acute right turns unable
to pass to the correct side of one central island.
Designers should not be afraid to use designs
like this; they are particularly effective and
examples: Arch Hill crossroads Truro (Mini-roundabouts
- A Definitive Guide Fig 5, p22), Maidstone
Road, Wigmore Road, Gillingham, Kent (photo 10, p7).
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© Penntraff - March 2008