| During 2009 and 2010 the New South Wales Centre for Road Safety conducted a trial of advisory ISA in the Illawarra region of New South Wales. Data from this trial was analysed to:
• estimate the reduction in serious and fatal crashes due to advisory ISA in Australia, and to do so at different levels of penetration.
• determine if the effect of advisory ISA is sensitive to the amount of time the driver is exposed to the advisory ISA device.
• predict the number of serious and fatal crashes that could be saved by advisory ISA in future years.
The reduction in crash risk was calculated by applying Kloeden’s risk curves for travel speed to the distributions of speeds found in the trial. The risk curves were applied to the ‘before ISA’ speed profile and the ‘during ISA’ speed profile. The reduction in risk was then estimated in terms of the difference in the in total crash risk produced by these speed profiles. It was calculated that advisory ISA would reduce Australian serious crashes by 19.3% and fatal crashes by 18.9%.
To determine if the effect of advisory ISA is sensitive to the amount of time the driver is exposed to the advisory ISA device, the data from the period in which ISA was activated was grouped by week. Two measures of the effect of ISA were used; changes in mean speed and changes in percentage of distance travelled over the speed limit. The percentage of distance travelled over the speed limit was disaggregated into three groups; percentage of distance travelled speeding by 5 km/h or less, percentage of distance spent speeding by more than 5 km/h up to speeding by 10 km/h, and speeding by more than 10 km/h. The change in these measures over the 12-week ‘during ISA’ period was determined by calculating the slope of a fitted linear trend line for each vehicle. This process was conducted for the sample of drivers as a whole and also by disaggregating the drivers in several ways. No evidence was found that the effect of advisory ISA was sensitive to the length of time it had been installed in the driver’s vehicle.
The crash savings attributable to all vehicles having advisory ISA in future years was determined by predicting the number of fatal and serious crashes in future years and applying the percentage reductions stated above. The number of serious and fatal crashes in future years was calculated by fitting an exponential trend line to previous years data and therefore determining a percentage change in these crashes per year. In 2015 this would represent 3,873 serious crashes and 219 fatal crashes, in 2020 4,190 serious crashes and 204 fatal crashes and by 2030 4,905 serious crashes and 176 fatal crashes. | 