Evaluation of the crash effects of the changes in speed zones in Victoriaimplemented during 1992-93 (excluding 100 to 110 km/h): update including 1990-1997 crash data

Monash University Accident Research Centre - Report #136 - 1998

Authors: S. Newstead & S. Narayan

Abstract

During late 1992 and early 1993, a rationalisation of speed limits on Victorian roads was undertaken in order to achieve credible speed limits which were uniform with the rest of Australia. As part of this rationalisation, many speed zoning changes occurred across Victoria, with some of the most notable being the phasing out of 75 km/h speed zones and the introduction of 50, 70 and 80 km/h zones. Under the rationalisation, posted speed limits on some road sections were increased while on other road sections the posted speed limits were decreased.

This study updates an earlier one which evaluated the casualty crash effects of the speed zone changes implemented in Victoria for speed zone changes other than 100km/h to 110km/h. Effects are estimated for the program of speed zone changes as a whole as well as for each particular type of speed zone change. Results are presented for the whole of Victoria as well as for metropolitan Melbourne and the rest of Victoria separately. Estimated effects of speed zone changes on casualty crash frequency are further related to changes in crash type as well as results of speed monitoring.

Results of analysis in metropolitan Melbourne showed an overall increase in casualty crash frequency of 4.7% with marginal statistical significance whilst no statistically significant change in casualty crash frequency was found in the rest of Victoria. The net effect of the speed zone changes over Victoria as a whole was a statistically significant increase in overall casualty crash frequency of 5.4%.

Recommendations for further research are made.

Executive Summary

During late 1992 and early 1993, a rationalisation of speed limits on Victorian roads was undertaken in order to achieve credible speed limits which were uniform with the rest of Australia. As part of this rationalisation, many speed zoning changes occurred across Victoria, with some of the most notable being the phasing out of 75 km/h speed zones and the introduction of 50, 70 and 80 km/h zones. Under the rationalisation, new criteria for speed zoning of roads was developed resulting in posted speed limits on some road sections being increased whilst on other road sections the posted speed limits were decreased. This study evaluates the casualty crash effects of the speed zone changes implemented in Victoria for all speed zone changes other than 100 km/h to 110 km/h.

A pseudo experimental study design was used for the evaluation, examining changes in casualty crash frequency before and after speed zone changes. Only a sample of sites that had undergone speed zone changes was used in the analysis. The analysis also incorporated the use of control sites to represent parallel changes in casualty crash frequency due to other factors. Control crashes were selected from all roads where speed zone was unchanged and matched with treatment crashes by local government area and level of urbanisation. Crash data from three years before and three years after implementation of the speed zone changes was analysed.

Analysis of the effects of speed zone changes on casualty crash frequency in metropolitan Melbourne showed an overall increase in casualty crash frequency of 4.7%, although this result was of marginal statistical significance and should be interpreted with caution. This estimated increase represents in the order of 235 extra casualty crashes per annum across Melbourne due to all speed zone changes. Assessment of the general effects on casualty crash frequency of increasing zoned speed or decreasing zoned speed showed no statistically significant change in casualty crash frequency when the zone speed was decreased, and a 9.3% casualty crash increase (with high statistical significance) when zone speed was increased. For particular speed zone changes, the change from 100 to 80 km/h showed a highly statistically significant casualty crash reduction of 46%, translating to a saving of approximately 44 casualty crashes per annum across Melbourne. Also the change from 75 to 60 km/h showed a marginal statistical significance casualty crash increase of 43%, representing an increase of around 151 casualty crashes per annum. Increased speed zoning from 75 to 80 km/h showed a highly statistically significant casualty crash frequency increase of 10.7%, representing an increase of approximately 188 casualty crashes per annum across Melbourne.

The results of analysis of casualty crash frequency in metropolitan Melbourne were generally consistent with the results of speed monitoring.

Most of the speed zone changes which occurred in the rest of Victoria took place on the fringes of country towns in the speed transition zones between 100 km/h zones of the open highway and 60 km/h zones of the built up town area. Analysis of the overall casualty crash frequency change for all speed zone changes combined in the rest of Victoria as well as analysis by specific speed zone changes showed no statistically significant changes in crash frequency.

The net effect of the speed zone evaluation over Victoria as a whole was a statistically significant increase in overall crash frequency of 5.4%, although this result is largely driven by the results in metropolitan Melbourne.

Sponsoring Organisation: VicRoads