Comments on Accidents at urban mini-roundabouts TRL Report 281 by J V Kennedy (TRL) and R D Hall (University of Southampton)

The TRL sites showed accidents of many kinds which I did not seem to get; in particular:

• Merging - I had none

• Shunt accidents on approach - I had very few

• Single vehicle accidents - I had very few

• Pedestrian accidents - I had very few

These accidents are often associated with failure of drivers to appreciate the presence of the mini-roundabout which points strongly to the comments I make on the design of the approaches on my detailed design page.

The TRL report indicates that accident types at 3-arm mini-roundabouts were as follows:

Note the significant numbers of crossing accidents, but over the 200 sites as a whole the variety of accidents is surprising. The single vehicle accidents often involve buses stopping abruptly with consequent injury to passenger(s). Merging accidents tend to occur where drivers following a relatively straight kerbline fail to realise the presence of the junction. This approach is best split into two narrow lanes to control approaching traffic in sufficient time. If you cannot do this consider laying a buff coloured anti-skid surface, but try the two-narrow-lane idea if you can; it really does work. Similarly shunt accidents often involve one driver reacting severely having noticed the mini-roundabout very late on approach. [The zero thickness wedge represents crossing movements which appear very significantly in the next chart below. Both charts were derived from the same table.]

 Below is the equivalent plot of accident types at 4-arm mini-roundabouts:

Note the high proportions of crossing accidents mostly right angle crossing but also other right turn accidents.   Otherwise the accident pattern is basically the same as for 3-arm mini-roundabouts - the overall proportions of the accident types being less as the accident types involving right angle and other crossing predominate; (the right angle crossing type not being represented at all in the three-arm scenario). The central island at crossroads must be large enough to deflect all crossing traffic especially where there is potentially a straight path across the junction.  To achieve this it will often be necessary for the central island to be larger than the 4m currently prescribed in the UK TSRGD (signs regulations). Some authorities will not install 4-arm mini-roundabouts because of their high accident potential. This is a pity. make sure that the approaches make the junction presence clear and then ensure good deflection.

I have asked DfT to consider removing the 4m central island diameter constraint in the next TSRGD revision. Still no go (Nov 2004) See my page on mini-roundabouts at crossroads & The MIDI-roundabout.

NOTE: These are RIGHT HAND DRIVE diagrams but apply just the same to UK layouts...
The conflicts illustrated here represent the various risk factors in vehicle to vehicle collisions.
The right-angle or broadside ones matter the most and these are represented by the larger red circles.

Other observations from the report:

Approach lanes: Just 6 of the 618 approaches at 3-arm sites had 2 lanes on the approach, 232 arms had more than one lane on the approach leaving 386 approach arms with only one lane marked.  At 4-arm sites there were 420 approaches of which just 87 had more than one lane marked leaving 333 approaches in single lane.

My comment on this:  I have repeatedly found that an increase in the number of approach lanes on approaching a (mini-) roundabout can be a crucial safety factor as well as a capacity benefit. Such changes are recommended in current DfT advice but this then goes on to prohibit lane widths less than 3m at the give-way line. The result is that many approaches are marked in one wide single lane which could so easily have been marked in two. At a roundabout a large vehicle occupying more than one lane does not "baulk" traffic as it almost certainly would at signals, so narrower lanes are useful; they are good for cyclists too as cyclists can occupy such a lane without drivers trying to push past.

Involvement of 2-wheelers (powered and cycles)

At 3-arm sites 39.9% of accidents involved 2-wheelers; the majority of these accidents were of the entering/circulating type. At 4-arm junctions the proportions were even higher.

The vulnerability of 2-wheelers at mini-roundabouts is well known, but there remains much concern about the proportions of drivers who enter a mini-roundabout either failing to appreciate its presence or expecting little to yield to or being able to "straight-line" owing to lack of deflection; and this makes 2-wheel riders particularly vulnerable. Pedal cycles and motor cycles were 7 & 8 times more likely to be involved.

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