MIDI-roundabouts at Crossroads

Overseas 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!

Time to drop to the maximum central island size prescribed in the UK regulations...

It was 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 summarised below:

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 or less.

Here is a drawing of a symmetrical layout based on a typical crossroads
(Note: the overrunnable splitter islands may now be illegal in the UK.
One illuminated bollard on each guarantees legality.)

Note 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 lane only;
Signs will have to be on the nearside, but place on the straight;
Hatching and arrows not shown.

Special authorisation required in the UK.

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I urge all UK authorities to do two things:

1. Examine all of your existing crossroads mini-roundabouts and see if they could be improved
by enlarging the central island and associated alterations;
2. Look out for poorly performing priority junctions or signal control at crossroads and
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, Berkshire

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:

  • Deflection provided by large raised central island

  • Split on approaches into 2 narrow lanes

  • Signs very well placed

  • Lots of turning traffic

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 611.1).

A closer view of the central island which too needs refurbishing.
Had this been just 4m there would have been easy
straight through paths.

Feature Notes
1. Deflection 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.
2. Flows 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.

* * * * * * *

Halfway St/Willersley Ave, Bexley (Nov 2004)

Here 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.

INSTALLED - 5 June 2005 - working well.

Here 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.

Image courtesy of Bexley Council.

Update. A 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 were necessary.
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.

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Penntraff - January 2012
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