Evaluation of the Adelaide Hills speed limit change from 100 km/h to 80 km/h

In January 2002 the speed limit was changed from 100 km/h to 80 km/h on 18 sections of roads in the Adelaide Hills area. An analysis has been conducted of the effect that these changes had on the number of casualty crashes and casualties occurring on these roads. A before and after comparison was made for the five years before and five years after the changes: an eight per cent reduction in casualty crashes was found. Comparisons were also made with trends in crash data and with relevant control groups, with the aim of accurately representing the effect of the speed limit change while taking into account any unrelated system wide effects. Different comparison methods lead to different estimated reductions in casualty crashes. For example, not using a control group, and assuming a trend of 1.3 per cent per annum increase (based on population growth) implies an estimated reduction of 15 per cent. Whichever method is used, the estimate of the change is not statistically significantly different from zero. The merits and failings of the different methods are discussed.AD Long and TP Hutchinso