TRL Report 323

TRL report 323 (1998) is a report from the Transport Research Laboratory. It is often quoted by pro-motorist pressure groups as the source of "ABD's lonely factoid"

It is often misrepresented as attributing a uniquely low proportion of collisions to excess speed, but the study was not considering speed and collisions, it was a study into "A new system for recording contributory factors in road accidents".

The abstract says:

The factors involved include, as TRL have pointed out to ABD and others on numerous occasions, more than one causation which would relate to speed. For example, Loss of control of vehicle (22.6%) and Failed to avoid vehicle/object in carriageway (22.0%), the top two "precipiotating factors", will both be strongly influenced by speed.

The variability between the police forces surveyed is quite significant. For example, in the contended area of Excessive speed, one force attributed 19% while another attributed 5%. Driver distracted has even wider variation, between 23% and 1%.

Additionally, factors such as 11 Behaviour - in a hurry (2.1%) and Site details - bend/ winding road (2.8) are both strong indicators of speed in excess of what is safe for the conditions - the speed imperative as I tend to call it.

The report is available as a free PDF download.

TRL response to ABD disniformation
The following was printed in TRL News, September 2002:

Speed and accidents - let's put the record straight!

A statement by the Transport Research Laboratory

There is a vast amount of evidence demonstrating the strong link between vehicle speed and road accidents. So why does material keep appearing in the media suggesting the effect is small?

The issue is so important we feel it is time to reiterate the true position. In the 1990s a number of police forces conducted a limited trial of an experimental accident reporting system. The results were reported clearly in TRL Report 323 but they have frequently been misquoted.

Speed increases the impact of many of the factors which contribute to accidents. For example, "aggressive driving" or "driving too closely" are both much worse at speed. Such factors were recorded in the system separately from speed; but speed plays a big part in their effect on accidents. The system also allowed speed to be recorded in its own right. The total effect of speed on accidents is obviously the sum of both types of factor.

Misunderstandings in the press appear to have resulted in two ways. First, speed identified as a separate factor in its own right was present in 15% of accidents, not the 7.3%, or lower figures, that are often wrongly quoted. Secondly, the 15% is only one part of the total effect of speed on accidents. When allowance is made for all the other speed-dependent factors, the contribution is, we believe, much greater.

This means that speed is far more important in causing accidents and increasing their severity than the misquoted figures suggest. Importantly, other TRL studies have directly examined the relationship between speed and accidents. These are summarised in TRL reports 421 and 511. They avoid the inevitably subjective judgements associated with studies of contributory factors which, for example, involve estimating what an appropriate speed is in each situation.


 * Studies of individual drivers have examined how drivers' speed choice affects their likelihood of accident involvement. Accident records of more than 10,000 drivers were related statistically to their observed speeding behaviour. These showed clearly that accident risk rises the faster a driver travels: at 25% above the average speed, a driver is about 6 times as likely to have an accident than a driver travelling at the average speed. (see graph)
 * Road-based studies looked at how speeds on a given road affect accidents occurring there. Several hundred thousand observations of vehicle speed on almost 300 roads of different types were related statistically to the numbers of accidents on those roads. These showed clearly that the faster the average speed of traffic on a given type of road, the more accidents there are. Injury accidents rise rapidly as average speed increases, if all else remains constant.

Many 'before and after' studies of measures which slow traffic and result in substantially fewer accidents have also been reported. These measures include, for example, traffic calming schemes in 20mph zones - where injury accidents were more than halved (TRL Report 215).

These studies together provide extremely robust evidence of how speed affects accidents. They are large-scale studies, of real traffic on real roads, involving rigorous statistical analyses. The conclusions are unambiguous. Remember, 10 people die and 100 are seriously injured on our roads per day. Improvements in driver behaviour have the potential to cut these statistics dramatically; reducing drivers' speeds will play a vital part in this.

Reproduced, with permission, from TRL News, September 2002 enquiries@trl.co.uk