Congestion and Safety on Highways: Towards an Analytical Model

Abstract

Congestion and accidents are important components of the externalities created by road users in metropolitan areas. In the present paper, we investigate the relationship between these two components. Among the factors which influence the number of fatalities on highways are: speed, speed differences and traffic composition. We pay special attention to the impact of congestion on the number of fatalities. The lower speeds which are caused by congestion would lead to lower numbers of fatal accidents. As a result, we expect a parabolic relationship between density and fatal accidents on highways. When densities increase, we would first have a positive relationship due to the increase in the numbers of cars in the system. However, when density becomes so high that speeds are influenced negatively, the number of accidents will decrease. The conclusion would be that in addition to the negative impact of congestion in terms of time losses, we also have a positive impact, since fatalities are reduced. Some supporting evidence is found for a number of countries where relatively low numbers of fatalities are observed during the morning peak.