One of my most interesting sites was at the A421/A4095 a converted T-junction in Bicester where the accident record had increased considerably following resurfacing and changes to the layout.  It became apparent that there had originally been two approach lanes on the critical approach from the south (arms at south, west and north) but that with re-surfacing this had been dropped in favour of a single lane approach.

This photo shows the scars of the original approach layout beyond the limit of the resurfacing (just behind the red car).

Problems:

• Drivers "running" the junction from the south,
• Sharp increase in accidents.

The skid marks indicate clearly the failure of drivers to assess the junction in sufficient time.

This site has proved what I strongly recommend that the approach layout has to give drivers sufficient "information" about the imminent approach onto the mini-roundabout and two narrow lanes do that better than one wide one.

• Re-instate the two lane approach as before re-surfacing
• Ensure central island is sufficiently raised to deter over-running

  Lessons to be leant:

  • Take very seriously what your approach layout is saying to drivers - ALL drivers!
    It is understood that the two lane system has been re-instated, but the central island remains flat.

The development served about 12 properties.  The flow in and out of this development would hardly be sufficient to justify the scheme and it was apparent that "through" drivers were learning to ignore the roundabout and some were even observed driving to the wrong side of the islands altogether and it was this problem that brought the site into the seminar.

The original scheme was welcomed initially as the "major" road was straight and there had been some speeding problems; also the developer was unable to provide sufficient sight-line for a priority junction so the offer of a mini-roundabout seemed a good idea at the time particularly as it was thought that it would deal with the speeding problem.

Problems:

• Wilful disregard by some drivers for the mini-roundabout - passing right of the islands as looking in the direction of this image.

• Speeding

• Re-locate approach island to "capture" approaching vehicles better.
• Re-locate mini-roundabout sign
• Consider additional measures to traffic calm the road along its length if possible.

Lessons to be learnt:

• Avoid using isolated mini-roundabouts where not justified by side-road flow,
• If you must, consider using additional speed controlling features.

The site comprises a mini-roundabout to serve a development but originally built as a small roundabout.  Problems:

• The design might be much more suitable for a larger junction but this is only 20m ICD,
• The central island has been demolished by the HGVs over-running it.
• The kerbs on the overrun area are beginning to break out.
• The drainage towards the central island means that it is not very conspicuous.
• The exit direction signs (still to be erected on the splitter islands) will be poorly sited and only visible by a driver already circulating which is too late.
• A kerbline build-out by the red car is regularly hit.

• Make central island traversable anyway - if using kerbs be sure that they are designed for the purpose - most are not;
• Re-profile surface to eliminate crowns and hollow thus raising the central island;
• Remove or reprofile kerbline build-out;
• Re-design direction signing, left and right flags opposite entries.

Lessons to be learnt:

• Don't try to design a mini-roundabout like a reduced version of a larger roundabout. They don't work that way.