How dangerous is speed? The ABD'S lonely 'factoid' and the real world
The Association of British
Drivers (ABD) likes to cite a
Transport Research Laboratory
(TRL) report as a source for
the true contribution of speed
to road crashes and casualties.
ABD members use the TRL
report to contradict the
'mainstream' figure indicating
that at least one-third of
crashes are speed-related.
According to the ABD, the TRL
report proves that the true
figure is under 5%. This is the
only source of such a low
figure. The ABD and a few
motor lobby journalists are the
only people to use it, generally
to support the argument that
"it is not speed but bad driving
that is dangerous". The ABD
especially likes to use the figure
in letters to local papers where
highway authorities are
implementing speed control
measures in response to deaths
and serious injuries or local
demands for safer communities.
(Their preferred technique is
for one or two writers to flood
papers with pseudonymously
penned letters to make it
appear they have widespread
public support.)
The Slower Speeds Initiative
wrote to the Transport
Research Laboratory
concerning the ABD's use of
the study. The TRL referred us
to reports on speed. This is
because the TRL study cited by
the ABD, TRL Report 323,
concerns "A new system for
recording contributory factors
in road accidents". TRL 323 is
not a study of crash causation.
It is a study of how to collect
data. It was not designed to
draw statistically reliable
conclusions about the causes of
road crashes. The accidents
included in the three month
study were not a statistically
representative sample of all
accidents. There is no basis for
using the study to generalise
about the speed-crash
relationship.
The very low figure quoted
by the ABD comes from a table
which showed pairings of
factors: In 4.04% of crashes
recorded in the study, the
person filling in the form paired
'excessive speed' (nowhere
defined) with 'loss of control of
vehicle'. 4.04% is only a subset
of all speed related crashes
recorded in the study. This use
of statistics has been described
by a professional statistician as
"extremely naughty" and by the
Department of the
Environment, Transport and
the Regions as "mischievous".
DETR go on to say "it is
interesting that none of the
many other TRL reports on
speed and accident risk have
been mentioned by those using
this report as the basis for their
argument."
The TRL 323 methodology
for recording contributory
factors simply does not ask the
questions which would reveal
the inherent dangers of speed:
- Would the factor still have
been present if the driver,
and/or all the other drivers
involved, had been driving
more slowly?
- IF YES, Would the factor still
have resulted in a crash?
- IF YES, Would the crash still
have been so severe?
It is obvious to almost everyone (with the exception of
libertarian motorists with a soap box to drive) that higher
speeds reduce the amount of time any driver has to respond
to the unexpected and that higher speeds increase the force
of any impact. The importance of reduced speeds to crash
prevention and reducing crash severity is no mystery. In fact,
the TRL study beloved of the ABD and its fellow-travellers, indirectly acknowledges the
overriding importance of speed:
"Virtually the only factor
that road accidents have in
common is that all would have
been avoided if those involved
had known with certainty, a
few seconds in advance, that
an accident was about the
occur."
Lower speeds provide those
few extra seconds.
Among the TRL reports the ABD does not like to cite is TRL
421, "The effects of drivers' speed on the frequency of road
accidents" published in March 2000. Unlike TRL 323, this
study was designed to discover the speed-crash relationship.
The authors looked at 300 sections of road, made 2 million
observations of speed and got 10,000 drivers to complete
questionnaires. They found that
- the faster the traffic moves on average, the more crashes there
are (and crash frequency increases approximately with
the square of average traffic speed)
- the larger the spread of speeds around the average, the
more crashes there are
Significantly for the ABDs argument, and for the rest of
us, they also found that:
- drivers who choose speeds above the average on some
roads tend also to do so on all roads
- higher speed drivers are associated with a significantly
greater crash involvement than are slower drivers
For these reasons they conclude that the speed of the
fastest drivers (those travelling faster than the average for the
road) should be reduced. The study confirmed what is
described as a 'robust general rule' relating crash reductions
to speed reductions: for every I mph reduction average speed,
crashes are reduced by between 2-7%. More specifically, the
crash reduction figure is around
- 6% for urban roads with low average speeds
- 4% for medium speed urban roads and lower speed rural main roads
- 3% for higher speed urban roads and rural main roads
To put the dangerousness of speed into perspective, how
many drivers care about or would notice a 2mph reduction in their average speed? Yet,
averaged across the entire road network, a mere 2mph
reduction in average speeds would prevent more than 200
deaths and 3,500 serious casualties a year. The authors
of TRL 421 suggest that this target (about a sixth of the
overall speed related casualty figure) is a 'reasonable
minimum' to aim for. More importantly they use it to show
"the sensitivity of accident numbers to a small change in
average speed". In other words, speeds that might not seem
excessive. Speeds that TRL323's methodology wouldn't even
record. Thanks to Stephen Plowden, Rosamund Weatherall and DETR
