A statistical evaluation of the default 50km/h speed limits in Victoria

Victoria introduced a statewide default 50 km/h speed limit in built-up areas (except where otherwise signed), on January 22, 2001. The purpose of this legislation was to reduce the incidence and severity of crashes involving unprotected road users. The effectiveness of the initiative has been evaluated under a quasi-experimental design framework at various intervals with the last covering a period of almost three years. The first evaluation, covering the first five months of the initiative, showed that the program was associated with statistically significant reductions for all casualty crashes. Results of the second evaluation were unable to show continuing crash reductions for the six to seventeen month post implementation period. Closer inspection of the data suggested that this was not necessarily an indication that the 50 km/h speed limit was no longer effective in reducing crashes, but an issue relating to the study design used. Specifically, it was believed that contamination of the comparison group (as defined by the study design) had occurred due to the introduction of a number of new speed enforcement initiatives. To overcome this issue, an alternative methodology was devised and applied to data covering a 35-month post implementation period allowing the effects of the 50 km/h speed limits to be estimated separately. This alternative methodology is described in detail, as well as the results derived from this study design.