Effects of Vehicle Mass and Other Parameters on Driver Relative Fatality Risk in Vehicle-Vehicle Crashes 2013-01-0466

Regression models are used to understand the relative fatality risk for drivers in front-front and front-left crashes. The field accident data used for the regressions were extracted by NHTSA from the FARS database for model years 2000-2007 vehicles in calendar years 2002-2008. Multiple logistic regressions are structured and carried out to model a log-linear relationship between risk ratio and the independent vehicle and driver parameters. For front-front crashes, the regression identifies mass ratio, belt use, and driver age as statistically significant parameters (p-values less than 1%) associated with the risk ratio. The vehicle type and presence of the ESC are found to be related with less statistical significance (p-values between 1% and 5%). For front-left crashes the driver risk ratio is also found to have a log-log linear relationship with vehicle mass ratio.

The driver risk ratio relationship to the vehicle mass ratio is consistent with conservation of momentum and can be used with the literature reported relationship between absolute risk and velocity change, to predict the dependence of the driver fatality risk on its own mass when crashing into other vehicles in a fleet. For front-front crashes, the model indicates that when the subject vehicle mass is at half the fleet mean mass, the risk for the driver is about 3 times that of a driver in a vehicle at the fleet average mass, and it is about 50%, when the vehicle mass is twice the fleet average.

The difficulty in defining independent effects from size and mass using existing field data is discussed for the front-front crash data. Results show that out of the vehicle mass ratio, wheelbase ratio, and footprint ratio, the mass ratio is the best single predictor of the risk ratio: combining these “size” ratios with the mass ratio degrades the fit due to the high correlations between them. Examination of conditional risk shows that, for front-front crashes, the driver fatality does not show a significant dependence on vehicle wheelbase when the mass ratio of the two vehicles is already taken into account.