Four-wheel Drive Vehicle Crash Involvement Risk, Rollover Risk and Injury Rate in Comparison to Other Passenger Vehicles: Estimates Based on Australian and New Zealand Crash Data and on New Zealand Motor Vehicle Register Data

Recent analyses have established that 4WD vehicles are aggressive in crashes, causing comparatively more harm than other passenger vehicles when in collision with other road users, and have a high risk of rollover crashes. To assess the safety of these vehicles more completely, this study produced two analyses of risk: firstly, estimates of crash risk and injury rates using NZ data; and, secondly, induced exposure estimates of rollover risk using Australian and NZ crash data. Crash risk and injury rates were estimated using NZ data, controlling for a number of relevant owner and vehicle characteristics. Nevertheless, within given locations, age bands and gender groups, drivers wanting to drive in a particular manner may choose types of vehicles to suit their driving style, leading to differences in risk between vehicle types. This is an explanation why Sports cars, which often have highly developed braking and handling systems, were found to have a high risk of crash involvement and a high rate of resultant injury. It is likely to be the manner in which these vehicles are driven that leads to these elevated rates. Similarly, despite previously estimated high rollover rates and high aggressivity for 4WDs, crash involvement risks estimated for 4WD vehicles were generally low, likely to be related to the manner in which they are driven. This estimated low crash involvement risk in combination with established generally good overall protection 4WD vehicles provide their own occupants in a crash meant that 4WD vehicles were estimated to impose relatively low injury risk on their own occupants, to other roads users and occupants of other vehicles per registered year adjusted for distance driven, despite their established high aggressivity. The one situation identified with unusually high risk for 4WDs compared to other passenger vehicle types was that of Large 4WDs owned by teenagers. The induced exposure risk estimation involved two steps: identifying the most appropriate crash type to represent exposure (in terms of amount of driving, driving environment, and driver characteristics) using existing data on distance travelled in NZ; then fitting a model to crash data from Australia and NZ to estimate risk. The best set of comparison crashes was found to be multi-vehicle crashes in which the vehicle type analysed was damaged in the rear. Estimates of risk consistent with previous studies were generally obtained, showing higher rollover risk for those vehicles with a high centre of gravity compared to the width of the wheel track (4WDs and People Movers). As found in a previous study, higher rollover risk was found for young drivers. Higher rollover risk was also found for older vehicles: a 2% higher rollover risk (95% CI 1% to 3%) for a vehicle that is a year older than another vehicle. Female drivers were found to have a 35% lower rollover risk than male drivers (with 95% confidence interval 42% to 27%). The results overall warn that parents who are 4WD owners – and, to a lesser extent, owners of People Movers – need to be wary of allowing their novice family members to use such vehicles (keeping in mind that for young drivers, regular cars present significantly less rollover risk than 4WDs and people movers), and reinforce the importance of electronic stability technology as a highly desirable risk-reducing feature for these relatively unstable vehicles.