Monash university accident research centre report documentation page

5.2 Secondary safety estimates

Crashworthiness represents the risk of a vehicle’s occupants being killed or seriously injured in a crash. Figure 5.2a shows a clear trend of this risk reducing over time for all taxi and hire car types as the vehicle fleet is regenerated. The declining risk is driven by improvements in crashworthiness of newer vehicles entering the fleet.

Aggressivity represents the risk of death or serious injury to a third party with which the vehicle collides (vehicle occupant, pedestrian, cyclist or motorcyclist). In contrast to crashworthiness trends, trends in aggressivity have been relatively flat only showing some downward movement in the two most recent years of data.

Total secondary safety combines crashworthiness and aggressivity, representing the risk of being injured to come degree in the event of a crash. Figure 5.2c shows a clear trend to improving total secondary safety by year of crash. This is driven by the improving safety of new vehicles entering the fleet and the removal of older, less safe vehicles as the fleet regenerates. This is similar to the trend found for all light passenger vehicles in the Australian fleet by Newstead and Scully (2009).

Improvement in total secondary safety with crash year has been consistent between taxi and hire car types with the absolute level of safety being similar between taxi and hire car types reflecting the similar types of vehicle used in each vehicle fleet type.

Exponential regression analysis was used to examine the trend in vehicle secondary safety by age of vehicle within each crash year. Separate models were fitted to each of the three secondary safety measures considered. The regression analysis was able to assess the statistical significance in the difference in average secondary safety between taxi and hire ca types, the trend in average secondary safety by age of vehicle and whether the age based trends differed by taxi and hire car type.

A summary of the results of the statistical significance tests are present in Table 5.4. Tests are based on the chi-squared statistic generated from the regression model with the significance probability calculated from the chi-squared statistic and degrees of freedom. Results in Table 5.4 show that each secondary safety measure does not differ between taxi and hire car types (as indicated by the significance probabilities above 0.05) nor are there significant differences in age based secondary safety trends between taxi and hire car types. For each measure, however, there was significant overall age based trends in secondary safety. As noted in the methods section, this does not mean the secondary safety of vehicles deteriorates with age, rather it means that at a point in time, older vehicles have significantly inferior secondary safety due to their earlier year of manufacture dictating generally poorer safety design and specification.

Identifying trends in the total secondary safety of vehicles at time of crash is important in the context of examining the effects of vehicle age limits on road trauma. The significant improvement in average secondary safety for each newer year of manufacture means that limiting the age of taxis will have a positive impact through reducing road trauma. Trends in relative secondary safety presented in Figure 5.3, and in particular the trend in total secondary safety shown in Figure 5.3c form a key input to the scenario model considered in the next section. In applying this result it has been assumed that secondary safety will continue to improve with year of manufacture as it has in the past. This is considered likely given improvement has been constant over a 40 year period.