MEASURING THE SAFETY EFFECT OF ROAD MEASURES AT JUNCTIONS

The effects of road measures implemented at main road junctions in 198 4-1986 were studied on the basis of police-reported accidents that had occurred at the junctions in 1983-1987. The data were obtained from a n extensive junction inventory performed by the District Offices of th e Finnish National Road Administration in 1988. A total of 325 three-a rm and 298 four-arm junctions were included in the study. The number a nd type of accidents were studied at each junction both before and aft er the implementation of the measure. The effects of the measure were determined by comparing the observed number of accidents after the mea sure with the number that would have been expected to occur at the jun ction if the measure had not been implemented. The expected number of accidents was calculated on the basis of the observed number of accide nts in the before period and the expected number of accidents as predi cted by an accident model, utilizing at the same time the information on the variation of the number of accidents at similar junctions revea led by the model. Likelihood functions determined for the effects of t he measures were used for studying the accuracy of the estimates of th e effects. The likelihood functions can also be utilized later when ne w before-and-after studies are performed. Road lighting, stop signs, s ignal control, and lowering of the speed limit value were found to dec rease the number of accidents. Through-flow junction widenings, additi onal lanes for turning vehicles, and road widenings, however, did not seem to affect the safety at junctions to any marked extent. If we had not accounted for the regression-to-the-mean effect, these measures w ould have seemed to have a positive effect on safety. The study method also enabled us to quantify the regression-to-the-mean effect. The ma gnitude of the regression effect was on average 20%, i.e. the number o f accidents would have decreased by 20% at the junctions studied even if the measures had not been implemented. The magnitude of the regress ion-to-the-mean varied greatly between the different measures.