A 50 km/h default speed limit was introduced in built-up areas in South East Queensland in March 1999 with the primary aim of reducing the incidence and severity of casualty crashes. An evaluation of this initiative by Queensland Transport (QT), using a control chart style of statistical analysis, revealed significant reductions in the number of both serious and fatal crashes, translating into substantial social cost savings on projected outcomes. QT, in accordance with its adoption of a recommendation by the Parliamentary Travelsafe Committee to obtain ‘independent external evaluations of Queensland Transport’s major road safety countermeasures’, commissioned MUARC to analyse the program effectiveness and assess the internal evaluation undertaken by QT. This report details the results of this analysis, performed using Poisson log-linear regression, and compares the outcomes of the two evaluations. The MUARC evaluation found that the implementation was associated with statistically significant average yearly reductions of 88%, 23% and 22% for fatal crashes, all casualty crashes and all reported crashes, respectively. Crash reductions appear to have increased with time after program implementation for each crash severity level considered. Analysis of speed survey data associated with the program implementation suggested these crash reductions stemmed largely from a reduction in excessive speeding in 50km/h zones rather than large reductions in mean speeds at the affected sites. These results showed that QT internal evaluation underestimated the impact of the 50 km/h implementation. This resulted from a poorly specified treatment area that consisted of roads other than those that had changed to 50 km/h. This study also affirmed that the assumptions underlying the control chart method used by QT did not meet the theoretical properties of the crash data and formal statistical evaluation of program effects was better undertaken using Poisson regression techniques. However, the control chart method was endorsed as a satisfactory method for internal monitoring of program performance. (Author/publisher)
Abstract