Compatibility of cars in the Netherlands

Within the scope of the European project ‘Improvement of crash compatibility between cars', SWOV carried out analyses of car accidents in The Netherlands, based on data from police registration which were linked to data describing the vehicles involved. The purpose of this study was to detect vehicle factors that influence occupant and opponent safety, in order to evaluate the amount of compatibility between cars in crashes, especially frontal collisions. Compatibility is defined as the capability of vehicles to protect both their own occupants and occupants of opponent vehicles during crashes. Furthermore, compatible vehicles should offer equal amounts of protection to both their own occupants and to occupants of crash-opponent vehicles. Since theoretically occupant safety is strongly related to vehicle weight (the greater the better), the first part of the study was based on statistical data concerning the Dutch car park and its development over the years with respect to vehicle (kerb)weight. The data shows that the kerb weight of cars has increased considerably over the years and will continue to do so. This increase is caused mainly by the autonomous increase of weight of almost every single car model in the fleet. The average car weight has increased from about 900 kg in the mid 1980s to little less than 1000 kg now. The scope of the problem of incompatibility between cars is greater than illustrated by numbers of casualties from frontal collisions alone. Since the front end of cars are also involved in various other collisions types (side impacts, rear-end impacts, fixed obstacle impacts), as well as in collisions involving other vehicles and road users, casualties from these other collision types and vehicle categories are also relevant. Therefore, even if the scope of this specific study is limited to frontal collisions in particular, the full scope of the problem of incompatibility should always be monitored, in order to control for negative interactions between these other collisions. A subset of all available Dutch accident data was selected to analyse effects on injury severity of drivers of cars and vans involved in frontal collisions. This selection narrowed the accident sample to two vehicle collisions only, and to those only involving vehicles with front-end damage. Thus, a total of 5,014 frontal accidents (1992-1997) representing 10,028 vehicles, of which 9% were vans, was studied in more detail. As criterion to evaluate compatibility capability of individual vehicle types, two indices were developed: OS (Occupant Safety) and AI (Aggressivity Index), in which the number of seriously injured drivers (respectively in the vehicle type studied, and in opponent vehicles) was divided by the total number in the sample of the vehicle type studied. First, influence of mass and mass ratio on outcome was analysed, using area (either urban or rural) as control variable. Mass ratio was expressed as the quotient of the kerb weight of each vehicle, and the kerb weight of its opponent vehicle. It was found that both in urban and rural areas, mass and especially mass ratio, had a considerable effect on outcome. In both urban and rural area, the percentage of seriously injured drivers (those killed or admitted to hospitals) increased with more than a factor 5 with increasing mass ratio: driver safety was very low for mass ratios under 0.6 and very high for mass ratios of 1.7 and higher. Area in itself also made a big difference, since the share of seriously injured drivers in rural areas was about three times as high as in urban areas. Sex appeared to be another factor having considerable influence on outcome of frontal collisions. Female drivers showed a far higher proportion of seriously injured and not-seriously injured, and consequently, a far lower proportion of not-injured than male drivers. This conclusion was reached after controlling for area (representing accident severity), and vehicle mass (kerb weight). This big difference with regard to injury severity (injury susceptibility) between male and female drivers justifies further, more in-depth research. The results of the ranking of individual vehicle models, using OS and AI scores as mentioned, proved also to be in line with the theory that smaller and lighter cars offer less protection to their occupants than bigger and heavier cars; while at the same time these smaller and lighter cars are far less aggressive to other vehicles than bigger and heavier cars. With these results we come to the very essence of (in)compatibility: in practice the vehicle models studied appear all but compatible, regarding the big range of OS and AI scores. If cars (and vans) were to be really compatible, these scores should be both far more equal, and preferably also far lower than they are now. It is reasoned that to reach that goal, the current legislation, especially the current EU-Directive concerning frontal collisions, is not appropriate since the barrier test used does not reflect relevant properties of opponent vehicles, and is therefore not able to yield results concerning aggressiveness of the tested car. It is recommended to develop an additional crash test, that will measure aggressiveness of the tested car. It is also recommended to apply such a test within the scope of the current successful Euro NCAP programme, before it becomes part of the Directive. Another recommendation is to study in more detail the developments of the car market in Europe, as far as they relate to changes of models and to changes (increase) of mass. Further analyses based on bigger databases (more years, larger selections of cases per year) are recommended for a more controlled analysis. For the further study of other relevant vehicle factors such as geometrical and structural (stiffness) aspects, and more detail about damage and injury, the use of data from in-depth accident studies is recommended.