Speed limit reductions in 2011 on South Australian high speed roads

Abstract

In 2011 the speed limit on 864 km of South Australian rural roads was lowered from 110 km/h to 100 km/h. This study examined the number of injury and fatal crashes on the affected roads and on all other 110 km/h roads in the five years before the speed limit reduction and the five years after. On the roads where the speed limit was lowered, the raw number of injury crashes decreased by 6.9 per cent and the number of fatal crashes decreased by 20.0 per cent after the speed limit was changed. However, neither of these results was statistically significant. On 110 km/h roads where the speed limit was not changed, crashes over the same two time periods were examined. Injury crashes decreased by 16.4 per cent and fatal crashes increased by 14.6 per cent on these roads. If the changes on these roads are assumed to represent the general background changes on all roads then the net effect of the change in speed limit on the roads where the speed limit was lowered were a 9.5 per cent increase in injury crashes and a 34.6 per cent reduction in fatal crashes. However, neither of these results was statistically significant. More sophisticated modelling of yearly injury and fatal crash numbers for the roads where the speed limit was changed similarly failed to find any statistically significant effect of the speed limit change. Incorporating crash numbers on roads where the speed limit was not changed into these analyses also did not find any statistically significant effect of the speed limit change. These results do not imply that the change in speed limit did not affect the underlying risk of injury and fatal crashes on the roads where the speed limit was lowered. The change in speed limit would certainly have had some effect and the large body of research on the subject suggests that it would likely be to lower the underlying risk. In the current study, there are just too few crashes on the roads where the speed limit was lowered to be able to discern even a large underlying effect of lowering the speed limit on the risk of injury and fatal crashes. It is suggested that this study be repeated once 10 years of pre and post speed limit change crash data is available in 2022 when the 2021 crash data is finalised. Note that the analysis for this study was carried out in 2018 using the data available at that time.