||Those who know that they don't know, all know more than those who think they do.|
||LTSA & Police Myth|
|A car travelling at 65 kph takes 5 metres further to stop than one travelling at 60 kph.
||If you drive at 5 kph over the speed limit you are a major danger.|
The LTSA is currently running scare advertisements sourced from Victoria, Australia, which purport to show that a car travelling at 65 kph will have a serious crash when one travelling at 60 kph will have only a minor one.
(For a bit of sardonic humour, try this Unshown Advt (660KB).)
Take a look at this theoretical braking curve chart from LTSA's own website:
This shows that a car travelling at 65 kph will stop in 55 metres where one travelling at 60 kph will stop in 50 metres. The kindest interpretation of the advertisements is that the braking distance was carefully selected to within 2 metres in order to obtain the desired dramatic comparison and that if that exact distance had not been chosen a completely different impression would have emerged.
However, I doubt that such kindness is warranted by the facts. The theoretical curves above are critically dependent on many variables, including the precise reaction time of the driver, the car braking system, tyres and road conditions. It seems unlikely that these could be predicted in advance or even reproduced sufficiently accurately on successive runs. There must be a strong suspicion that the filmed comparison was deliberately faked for the camera.
Moreover the gentleman purporting to be an objective university scientist is nothing of the kind. He is the head of the Monash University road accident unit, the High Church of rigid enforcement policies, and a bureaucrat who has built his career on advocating governments should bully motorists into submission.
Let's investigate a few more facts. The chart above suggests a driver reaction time allowance of 1.4 seconds. Others have used 0.7 seconds. That variation alone would make a difference of 12 metres at 60 kph.
In an indepth review of many driver reaction time studies, Green reports3 that the mean driver reaction time varies from 0.75 seconds for expected events to 1.5 seconds for unexpected events. Moreover, the standard deviation is typically 0.6 seconds so that roughly only two thirds of drivers will stop within 20 metres of each other at 60 kph. (The distribution is strongly skewed with a long tail in the direction of bad performance so this is only a rough guide.)
The following chart is from a study by Strandberg in Sweden using Volvo cars with varying tyres and icy surfaces and 66 ordinary drivers4.
As can be seen from the chart, there were huge variations in braking performance, depending on driver, tyre, road condition and car braking system.
Or look at this table, adapted from UK traffic engineer, J J Leeming1 (converted to metric):
Expected Braking Distance (metres)
UK Road Code|
Leeming comments that the UK Road Code figures in that table are based on a driver reaction time of two-thirds of a second, with the car, tyres and road all in excellent condition. His CERB formula makes better allowance for practical conditions. The LTSA chart formula is generating numbers roughly in the middle of the two estimates in the table.
What is apparent from all of these studies is that the range of variability due to factors other than speed vastly outweighs minor changes in speed.
Again, LTSA and the Police prefer to promote misleading scare propaganda rather than accurately inform the public.
The most serious consideration regarding braking however is that blaming and punishing drivers takes preference over maintaining and monitoring road surface quality.
Leeming1 points out in great detail that improving the surface quality of roads where accidents occured has by far the greatest impact on reducing injury and fatal accidents. On 31 sites where this was done the accident rates were reduced by a factor of over 7. In contrast, on 56 sites where speed limits were imposed there was no significant change in accident rates and possibly even a slight detrimental effect.
The political emphasis on speeding simply diverts attention from real causes and opportunities (and responsibilities) to reduce accidents effectively.
1Road Accidents - Prevent or Punish, J.J. Leeming, Cassell - London, 1969, Fig 3, p32.
2Civil Engineering Reference Book, J.J. Leeming, Butterworth - London, 1951.
3How Long Does It Take to Stop? - Methodological Analysis of Driver Perception-Brake Times, M. Green, TRANSPORTATION HUMAN FACTORS, 2(3), 195-216, 2000.
4Winter braking tests with 66 drivers, different tyres and disconnectable ABS, L. Strandberg, International Workshop on Traffic Accident Reconstruction, Tokyo, 1998.