Travelling speed and the risk of crash involvement : the South Australian experience

Abstract

The relationship between free travelling speed and the risk of involvement in a casualty crash was explored using a case control study design in two studies conducted in South Australia by the Road Accident Research Unit; one in 60 km/h speed limit zones in metropolitan Adelaide; and one in 80 km/h or greater speed limit zones in rural South Australia. Exponential increases in crash risk associated with high free travelling speeds were found in both studies and no evidence was found for an increase in risk at low free travelling speeds. Hypothetical calculations on the effect of lowering free travelling speeds in both these studies indicated that significant reductions in casualty crashes could be expected from even small reductions in free travelling speeds. The results of an alcohol case control study conducted by the Road Accident Research Unit in metropolitan Adelaide were compared to the free travelling speed studies and it was found that driving with a blood alcohol concentration of 0.05 g/lOOmL equates to roughly the same increase in risk (about double) as travelling 5 km/h faster in a metropolitan area and 10 km/h faster in a rural area.