THE SEVERITY OF PRIMARY IMPACT WITH METAL OR CONCRETE CENTRAL MEDIAN BARRIERS ON FRENCH MOTORWAYS

Central reserves on French motorways are systematically equipped with median barriers utilizing metal guard rails or concrete designs. In comparison to metal guard rails, experimental data tend to show impacts on concrete barriers as being more severe, and more often involving overturned vehicles. On the other hand, cross-over accidents should be less frequent, as should the number of interventions by motorway workers. Since 1985, accidents have been recorded on the major motorways between Paris and the South of France (about 1000 km). The database allows access to about 50,000 accidents with or without casualties. Information is particularly detailed on motorway design and its role in the frequency and severity of accidents. In order to compare the severity of impacts on metal or concrete barriers, the authors identified sections that had been changed from metal to concrete and, during the same period of time, the the same length and geographical proximity, the same number of "control" sections keeping their metal devices. The severity criteria used compared accidents with or without casualties (study with fatalities is impossible, due to lack of power). Two analyses have been made, one using the section as a statistical unit, the other using the damaged vehicle. Results of the analyses suggest that there is a trend toward an increase of the number of casualties due to the change from metal to concrete barriers. This trend is clear when using the vehicle as the statistical unit. In spite of being non-significant, the trend is the same when analysis is carried out on the overall comparison of the two types of sections. These results take into account the possible change in the number of lanes and certain other confounding factors. Consequently, the value of about 1.9 (which was obtained from the analyses) for the relative risk estimating the risk of being injured in a vehicle colliding with a concrete device, compared to a metal device, seems a reliable basis for conducting an overall cost-benefit analysis.