Evaluation of the Crash Effects of the Changes in Speed Zones in Victoria during 1993-1994 (excluding 100 to 110 km/h)
Monash University Accident Research Centre - Report #98 - 1996
Authors: S. Newstead & N. Mullan
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 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 which had undergone speed zone changes were 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.
Analysis of the effects of speed zone changes on casualty crash frequency in metropolitan Melbourne showed an overall increase in casualty crash frequency of 6.9%, although this result was of marginal statistical significance and should be interpreted with caution. This estimated increase represents in the order of 200 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 an 8.7% casualty crash increase (with marginal statistical significance) when zone speed was increased. For particular speed zone changes, the change from 60 to 80 km/h showed a statistically significant casualty crash reduction of 47%, translating to a saving of approximately 70 casualty crashes per annum across Melbourne. Increased speed zoning from 75 to 80 km/h showed a marginally statistically significant casualty crash frequency increase of 10.5%, representing an increase of approximately 150 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. The overall casualty crash frequency change for all speed zone changes combined in the rest of Victoria was estimated as a 32.9% reduction (representing a saving of approximately 150 casualty crashes per annum across the rest of Victoria), however the statistical significance of this result was marginal.
The net effect of the speed zone rationalisation over Victoria as a whole was no statistically significant change in overall casualty crash frequency. Whilst the results presented here are generally based on limited quantities of data after implementation of speed zone changes, this study has established a framework under which the evaluation could be easily repeated at a later date to more precisely estimate the effects of speed zone rationalisation.