CR 181: Vehicle compatibility: analysis of fatal crashes

In recent years, the focus of vehicle designers and policy makers has extended from consideration of attributes of individual vehicles that increase the safety of their occupants to systemic features of the vehicle fleet that determine the safety of the vehicle occupant population as a whole. One important determinant of the absolute level of road trauma in the population is the variance in vehicle size within the vehicle fleet, ie vehicle compatibility.

The Federal Office of Road Safety (FORS) has put in place Australian Design Rules to improve the self-protection of occupants in frontal and side impact crashes. FORS is currently involved in international research programs to address the issue of vehicle compatibility – how to make dissimilar vehicles in the fleet provide the same level of occupant protection when they crash into one another?

While vehicle mass was previously identified as the major factor in determining injury outcomes in a two vehicle collision, more recent thinking suggests that mass is not the only factor involved, and may not be the major factor. Vehicle stiffness and geometric design are other factors that are related to vehicle compatibility.

In the last decade, small passenger cars and large 4WD vehicles have been the fastest growing sectors of passenger vehicle sales. This trend is increasing the incompatibility of the vehicle fleet and potentially raising the risk of harm to the vehicle occupant population.

To help define the degree to which vehicle compatibility is a problem in Australia, FORS has commissioned projects to examine the relative risk of injury and death of occupants in passenger vehicles of different sizes. This study provides estimates of relative injury risk in fatal front and side impact crashes involving passenger vehicles in Australia, complementing the study of casualty crashes by Les et al (1999).

The project objectives were:

  • To provide frequencies of fatal frontal and side impact crashes between two passenger vehicles and between a passenger vehicle and a narrow object. This indicates the size of the potential problem.
  • To provide mortality ratios for passenger vehicles involved in fatal frontal and side impact crashes according to vehicle size. This indicates the level of risk associated with vehicle compatibility.

In order to estimate the relative risk of fatal injury to occupants of passenger vehicles in various impact configurations, crude mortality ratios were calculated for each vehicle size and impact combination. The mortality ratio is defined as the ratio of the total number of deaths in one class of vehicle, defined in terms of size and point of impact, to the total number of deaths in the other vehicle. Mortality ratios were computed for drivers only (referred to as driver mortality ratios, DMR), as well as for all occupants (occupant mortality ratios, OMR). In order to adjust for possible confounding by vehicle size (due to the probable association between vehicle size and number of occupants), occupant mortality ratios were additionally adjusted for the total number of occupants in each class of vehicle.

Total passenger vehicle occupant deaths comprise 59% of the national road toll. Passenger vehicle occupants killed in crashes involving only passenger vehicles, account for 46% of all road deaths and 78% of all passenger vehicle occupant deaths.

Frontal impacts between two passenger vehicles account for 7% of all deaths. Side impacts account for a similar number (8%). Passenger vehicle impacts with a narrow object account for 12% of all deaths (6% front and 6% side). The impacts between passenger vehicles account for more deaths than impacts with narrow objects (since more persons are involved in these crashes).

There were 494 front to front fatal collisions between passenger vehicles in Australia in the four years for which detailed data on fatal crashes were available (1988, 1990, 1992 and 1994). Mortality ratio calculations are based on only 260 of these collisions, (47% are excluded due to incomplete data for size classification).

These 260 crashes resulted in 335 deaths, including the deaths of 226 drivers. A subset of 188 crashes involved vehicles of different size classes. Among these 188 crashes, 140 driver fatalities occurred in the smaller vehicles compared with only 28 driver fatalities in the larger vehicles. This results in a driver mortality ratio (DMR) of 5.0 (140/28). The corresponding occupant mortality ratio (OMR) is 4.1, based on a total of 196 occupant fatalities in the smaller vehicles and only 48 occupant fatalities in the larger vehicles. Since the total number of occupants in the smaller vehicles (373) is similar to the total number of occupants in the larger vehicles (366), adjustment for occupants doesn’t substantially change the OMR(4.0)

The mortality ratios increase with increasing differentials in size. For example, for collisions involving a small car, the driver mortality ratios increase from 3.6 to 6.3 and 17.0 for collisions with medium, large and 4WD vehicles, respectively. The pattern is similar for the OMRs (2.5, 5.1 and 24.0).

The smallest mortality ratio is for collisions between medium and large cars where the DMR is 2.3 and the OMR is 1.8. This still corresponds to a doubling of the risk of death in the medium compared to the large car.

There were 574 front to side fatal collisions between passenger vehicles in the four years under study. Mortality ratio calculations are based on the subset of only 342 of these collisions for which a size classification could be made for both vehicles.

These 342 crashes resulted in 435 deaths, including the deaths of 212 drivers. As expected, more deaths occurred in the vehicles struck on the side. A total of 191 driver fatalities occurred in these vehicles, compared with only 21 driver fatalities in the vehicle striking the side of the other vehicle. This results in a driver mortality ratio (DMR) of 9.1 (191/21). The corresponding occupant mortality ratio (OMR) is 7.4, based on a total of 383 occupant fatalities in the struck vehicles and only 52 occupant fatalities in the striking vehicles. The adjusted OMR is 6.7.

Although results for side impacts are less consistent than those for frontal crashes, it does appear that the size of both the struck and the striking vehicle are important. As the size of the struck vehicle increases, the mortality ratio tends to decrease. Similarly, as the size of the striking vehicle increases, the mortality ratio increases. 4WD vehicles generate extreme results. There were 50 cases where a 4WD struck a car in the side. This resulted in no deaths in the 4WD vehicles (out of 94 occupants) and 66 occupant deaths (out of 110 occupants) in the cars.

An apparent anomaly related to large cars striking small and medium cars. The DMR for large cars into small cars (22.5) is nearly equivalent to that for large cars into medium cars (22.0). The all occupant adjusted mortality rate is much higher for large into medium than large into small.

Another anomaly relates to small cars into small cars. The DMR is high (18.0). This is similar to the DMR for large cars into small cars (22.5) and much higher than medium cars into small cars (3.3). These differences are not as evident, however, if all occupant deaths are taken into account.

Most of these inconsistencies probably relate to the very small number of deaths in the striking car. The observed mortality ratio is highly sensitive to small changes in these numbers.


Passenger vehicle occupant deaths make up 59% of the national road toll. This corresponds to approximately 1000 fatalities in Australia, annually. The results suggest that occupant protection against a side impact (from another passenger vehicle or narrow object) is as important as frontal protection. The results also suggest that impacts with narrow objects, such as trees and poles, while not resulting in quite as many deaths as collisions between passenger vehicles, are nevertheless substantial, accounting for 21% of all passenger vehicle occupant deaths.

The mortality ratio results suggest that drivers and occupants in smaller vehicles are more likely to be killed in both frontal and side impact collisions with larger passenger vehicles. The pattern of increasing driver mortality ratios with increasing vehicle size disparity was also observed in the corresponding driver injury ratios reported for casualty crashes by Les et al (1999). However, the magnitude of the mortality ratios was considerably larger than the injury ratios. This reflects that differentials in occupant protection become increasingly important in crash situations that are severe enough to cause fatal injury (ie high speed collisions).

The role of vehicle size in fatal and injury crashes is especially relevant to Australia where the sales of small and large cars and 4WD vehicles are expanding, while sales of medium size vehicles are declining. If this continues in the long term, it will promote a vehicle fleet composition with greater size variance and therefore greater risk divergence than in the current fleet.

The current study did not (and could not) distinguish between the influence of vehicle mass, stiffness and geometric design on vehicle compatibility. It is intended to conduct further analysis to establish the relevance of these and other variables to the results reported herein.

Type: Research and Analysis Report

Sub Type: Consultant Report

Author(s): R Attewell, M McFadden, K Seyer

ISBN: 0 642 25543 1

ISSN: 0810-770X

Topics: Occ protection, Risk, Compatibility

Publication Date: 01/06/99