Accidents at urban mini-roundabouts
TRL Report 281 by J V Kennedy
(TRL) and R D Hall (University of Southampton)
|Abstract (from the
report) The report gives the
findings of a study of accident risk based on a
national stratified sample of 200 3-arm and 100
4-arm urban mini-roundabouts on 30mph single
carriageway roads (2100 personal injury
accidents). Tabulations are given showing
frequencies, severities and rates by type of
central island and by region. The accidents are
also tabulated by accident group, road user
involvement and number of casualties per
accident. The main objective of the study was to
develop relationships between accident frequency
and traffic flow, road features, layout,
geometry, land use and other variables. The
technique of generalised linear modelling was
used to develop such relationships for different
types of accidents.
||My comments: Quite a proportion of the
accidents at mini-roundabouts involve one or more
drivers/riders who have failed to appreciate the
presence of the mini-roundabout in time to
respond correctly. While this problem
should come out "in the wash"
statistically, I am sceptical of this approach
just yet. When drivers fail in this way,
the consequences could vary considerably from nil
to a fatality and the type of accident could also
This is a serious problem at mini-roundabouts and
the statistical charts derived from the types of
accidents at the sites studied seem to vary
considerably from the accidents at the sites
which I designed and installed mainly in
Berkshire (UK). While crossing accidents featured
strongly in the TRL study, these were just about
the only accidents that I observed at my sites.
And they featured strongly at one site in
particular where I knew I had problems getting
the required deflection.
The TRL sites
showed accidents of many kinds which I did not seem to
get; in particular:
Merging - I
accidents on approach - I had very few
vehicle accidents - I had very few
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.
report indicates that accident types at 3-arm
mini-roundabouts were as follows:
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).
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.
accidents often involve one driver reacting
severely having noticed the mini-roundabout very
late on approach.
thickness wedge represents crossing movements
which appear very significantly in the next chart
below. Both charts were derived from the same
the equivalent plot of accident types at 4-arm
the high proportions of crossing accidents mostly
right angle crossing but also other right turn
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.
have asked DfT to consider removing the 4m
central island diameter constraint in the next
Still no go (Nov 2004)
See my page on mini-roundabouts
& The MIDI-roundabout.
NOTE: These are RIGHT
HAND DRIVE diagrams but apply just the same to UK layouts...
illustrated here represent the various risk factors in vehicle to
The right-angle or broadside ones matter the most and these are
represented by the larger red circles.
Strictly, it proved impossible to determine
whether the junctions studied were T-junctions,
Y-junctions or symmetrical (3-arm), or X-roads or
K-junctions (4-arm), so TRL decided to consider
movements between respective arms as the sole
arbiter of direction, i.e. Movement from
arms 1-3 and 2-4 represent "right angle
crossing" regardless of the actual geometry,
and at 3-arm junctions - left turning means
taking the next exit after entry and right
turning means leaving at the second exit.
left hand drive - for right hand drive
countries left and right should be interchanged.]
observations from the report:
accident frequency at 4-arm sites was 1.35 pia
per year about 50% more than average accident
frequency at 3-arm sites of 0.92 pia per year.
more flow at four arm minis and much more scope
for accidents because there is more scope for
weak design. Remember four "ahead"
crossing movements instead of one or at most two at 3-arm
This is an almost
meaningless statistic.Where mini-roundabouts are
installed on busy road junctions the risk will
normally be higher; but "busyness" may
help. The busy Binfield Crossroads has a good
severity was 11.6% at 3-arm sites and 14.1% at
more crossing accidents which tend to be the
serious ones, esp. when vulnerable users (riders
of two wheeled machines) are involved. At most 4-arm
sites the deflection is hopelessly inadequate
owing to the 4m max. UK island size.
accident rate was 12.5 PIA per 100 million
vehicles inflow at 3-arm sites and 22.8 PIA per
100 million vehicles inflow at 4-arm sites.
above - more risk due to poor design, and the 4m
accident frequency Acc/yr
rate PIA/100M vehs
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.
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.
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.
entering/circulating (3-arm sites)
entering/circulating (4-arm sites)
vulnerability compared with cars
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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© Penntraff - Dec 2014