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TitleReduction of speed limit from 110 km/h to 100 km/h on certain roads in South Australia: a follow up evaluation
AuthorsMackenzie JRR, Kloeden CN, Hutchinson TP
AbstractIn 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. A previous CASR study 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. However, the reduction in casualty crashes was not statistically significant; potentially due to the limited quantity of crash data available at the time. This report details a follow up investigation using more crash data (10 years before and after the speed limit change) and longer term speed data (up to 11 years after the speed limit change). The number of 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 just followed 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%. 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.
Report NumberCASR115
PublisherCentre for Automotive Safety Research
Publisher CityAdelaide
SponsorDepartment of Planning, Transport and Infrastructure
ISBN978 1 921645 53 2
Page Count28

Mackenzie JRR, Kloeden CN, Hutchinson TP (2015). Reduction of speed limit from 110 km/h to 100 km/h on certain roads in South Australia: a follow up evaluation (CASR115). Adelaide: Centre for Automotive Safety Research.

Files Available for Download
CASR115.pdfPDF version of published report
CASR115-FAQ.pdfResponses to common public criticisms of the report.