In July 2003, the speed limit on approximately 1,100 km of rural arterial roads (73 road sections) in South Australia was reduced from 110 km/h to 100 km/h. A previous CASR report investigated the change in casualty crashes on these road segments, as well as on control road segments where the speed limit remained at 110 km/h, using data from two years before and two years after the speed limit reduction. The investigation found that the speed limit change was associated with a 1.9 km/h reduction in average speed and a 19.7 per cent reduction in casualty crashes. A 19.0 per cent reduction in casualties was also identified, which was calculated as an injury cost saving to the South Australian community of $9 million per year. However, these results were not statistically significant; potentially due to the limited quantity of crash data available. This report details a follow up investigation of the effect of the speed limit reductions on the identified road segments. Casualty crash data on the subject road segments from ten years before and ten years after the speed limit reduction were analysed. New control road segments, where the speed limit has not been changed from 110 km/h during the longer analysis period, were also selected and analysed. Long term trends in speeds were analysed by examining two long term speed sites located on the changed roads. The average number of crashes on both the subject roads and the control roads declined over time. However, the decline was greater on the subject roads. The number of crashes on the subject roads in the after period was 27.4 per cent lower than would have been expected if the subject roads had just followed the control road reductions. This reduction was found to be statistically significant with 95% confidence limits of +/- 12.4%. The number of people injured was also found to have fallen by a similar amount resulting in a rough, dollar equivalent, estimate of $6.7 million per year in economic savings. The limited long term speed data suggests that speeds, and particularly high speeds, have continued to fall on the subject roads in the 10 years since the speed limit was lowered. While the methodological design of this study was not ideal: the size of the effect, the consistency of the various elements and agreement with other research provides rather convincing evidence that the lowered speed limits were effective in reducing casualty crashes and injuries by a large amount. (Author/publisher)
Abstract