Summary
The main aim of this project was to establish a mathematical curve that defines the relationship between free travelling speed and the risk of involvement in a casualty crash, for sober drivers in an urban setting. Data collected in a case control study (Kloeden, McLean, Moore and Ponte, 1997) were reanalysed using logistic regression modelling. The speeds of passenger vehicles involved in casualty crashes were compared with the speeds of passenger vehicles not involved in crashes but travelling in the same direction, at the same location, time of day, day of week, and time of year.
Both absolute travelling speeds and speed differences were used in the modelling process and allowance was made for uncertainties in the reconstructed case speeds.
An absolute speed curve was found to provide a good fit for speeds between 60 and 80 km/h whereby the risk of casualty crash involvement approximately doubled for each 5 km/h increase in travelling speed. Although the data were relatively sparse outside this speed range, we assumed that the curve could be used for speeds down to 26 km/h and for speeds above 80 km/h in our hypothetical analysis, since the curve modelled the available data and its general shape (exponentiated second order polynomial) is not unexpected given the physics of road crashes and injury biomechanics.
Such considerations also indicate that speed is a risk factor in and of itself. That is, the observed differences in crash risk between vehicles travelling at different speeds is primarily due to the actual travelling speeds and not other factors such as the type of drivers who choose to travel at different speeds or with the variance in travelling speeds.
A speed difference risk curve was also fitted to the data and found to produce comparable results to the absolute speed risk curve.
The secondary aim of the project was to examine the effect of hypothetical speed reductions on this set of crashes and urban crashes in general, using the derived mathematical risk curves, to allow some insight to be gained into the possible effects of changing the speed behaviour of urban drivers (although a number of unproved assumptions, as stated, had to be made to do this).
It was estimated that illegal speeding in Adelaide 60 km/h zones accounts for around 25 per cent of all casualty crashes in those zones. That is, if we could reduce the maximum speed of all vehicles in Adelaide 60 km/h speed zones to 60 km/h, we would expect casualty crashes in those zones to fall by around 25 per cent.
Moreover, nearly 60 per cent of the benefit of eliminating speeding would be achieved by eliminating speeding among those travelling between 61 and 75 km/h. This is because there are many more drivers who travel in this speed range than at faster speeds. Their relative risk of casualty crash involvement is lower than those travelling above 75 km/h, but their contribution to the total number of casualty crashes is the product of the number of these drivers and their relative risk of involvement in a casualty crash.
Examination of the estimated hypothetical effects of slowing all vehicles down by the same amount indicate that very small reductions in travelling speed (even 1 km/h or less) can be expected to have a meaningful impact on casualty crash numbers.
Estimates were also made for a hypothetical reduction in the general urban area speed limit from 60 km/h down to 50 km/h using two sets of assumptions. Casualty crashes in these speed zones would be expected to drop by around 21 per cent using a speed fine avoidance method and by 28 per cent using a speed distribution movement method. While similar reductions on local streets would be expected from a reduction in the speed limit to 50 km/h, if the speed limit reduction was limited to local streets, the relatively small proportion of casualty crashes on local streets means that the effect on all casualty crashes in the metropolitan area would be much smaller than a change in the general urban area speed limit.
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Type: Research and Analysis Report
Sub Type: Consultant Report
Author(s): Kloeden CN, McLean AJ and Glonek G
Topics: Risk, Speed
Publication Date: 04/04/02