Reduction of speed limit from 110 km/h to 100 km/h on certain roads in South Australia: a follow up evaluation

In July 2003, the speed limit on approximately 1,100 km of rural arterial roads in South Australia was reduced from 110 km/h to 100 km/h. An earlier study, conducted in 2006, found that this speed limit change was associated with a 19.7 per cent reduction in casualty crashes. However, this finding was not statistically significant, potentially due to the limited quantity of crash data available at the time. This paper details a follow up investigation using more crash data (10 years before and 10 years after the speed limit change). The number of casualty crashes on the subject roads since the speed limit was lowered was found to be 27.4 per cent lower than would have been expected if the subject roads had simply matched the control road (roads that remained at 110 km/h) reductions. This reduction was found to be statistically significant with 95% confidence limits of ±12.4%. While the methodological design of the 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 by a large amount.