Accident Externality and Vehicle Size - Evidence from Swedish Collision Accidents

Accident Externality and Vehicle Size - Evidence from Swedish Collision Accidents


L Jonsson, G Lindberg, VTI, SE



It has been a trend that the weight of new vehicles has increased over time and this has fostered a debate on the accident risk of so called SUVs on other road users. It is a well-known fact that the weight of a vehicle significantly influences the consequences of a crash. A higher weight increases the vehicle?s crashworthiness and thereby decreases the injuries or fatality risk of the occupants in the vehicle. But at the same time a higher vehicle weight also increases the aggressivity which leads to larger injuries and a higher fatality risk for occupants sitting in the other involved vehicle in a two vehicle crash. A vehicle?s crashworthiness will affect the internal cost in a collision while the aggressivity will influence the external cost imposed on the other involved vehicle. To reach economic efficiency this external effect of vehicle weight on the injuries and thereby the accident cost imposed on the other involved vehicle should be internalized. Without internalisation the driver will choose vehicle weight based only on the own beneficial effect of weight and ignore the disadvantageous effect on the injuries of those sitting in the other vehicle.

To explore this externality dimension we analyse the accident cost divided into internal and external accident cost on collision accidents in Sweden. The study is based on a database including all collision accidents in Sweden involving two passenger cars during five years. The database contains information on road infrastructure, vehicle characteristics including vehicle weight and characteristics of the occupants in the vehicles including their injuries. To get a measure of the accident cost the Swedish official economic valuation of slight injuries, severe injuries and fatalities are applied.

In each accident the two involved vehicles are divided into the lighter one and the heavier one and the effect of weight is examined separately for the two groups. The accident cost that falls on the lighter vehicle increases with the weight of the heavier vehicle and decreases with own weight. Given that a vehicle is the heavier one in the crash neither the own weight nor the weight of the lighter vehicle significantly affect the accident cost. These results imply that a car fleet with less variance in vehicle weight would be beneficial for traffic safety.

The expected external accident cost is also calculated and is shown to increase rapidly with vehicle weight. The paper discusses different solutions to internalize this external accident cost such as a multiplicative or additive tax on the insurance premium in a no-fault insurance system.


Association for European Transport