Reduction of speed limit from 110 km/h to 100 km/h on certain roads in South Australia: A preliminary evaluation

Abstract

The speed limit on certain rural arterial roads in South Australia was reduced from 110 km/h to 100 km/h at the beginning of July 2003. The speed limit reduction was applied to 73 separate sections of 48 roads outside of the Adelaide metropolitan area, spread across southern South Australia. Casualty crash data were used to evaluate the change in speed limit by comparing the number of casualty crashes and casualties on these roads for two years before and two years after the change in speed limit. There was a decrease in casualty crashes after the reduction in speed limit of about 32 per cent. However, there was also a decrease in casualty crashes of about 12 per cent for the remaining 110 km/h speed limit road network. Thus, it seems likely that a reduction in casualty crashes of about 20 per cent can be attributed to the changed speed limit. Similarly, a reduction in casualties of about 19 per cent can be attributed to the change in speed limit. A very limited dataset on speeds has also been examined, comparing average speeds before and after the speed limit change at six sites. This analysis showed that there was an average reduction in mean speed of 2 km/h for the measured sites. Considering the observed changes in casualty and casualty crash numbers, it must be noted that there are, however, significant uncertainties associated with the estimates. These stem from both unavoidable random variability, due to the short time period since the change and hence the small amount of data for comparison, and from the methodology, as most of the roads were selected for the speed limit reduction due to their condition or configuration and were not randomly selected, which also made the choice of appropriate control roads difficult.