Daytime running lights : costs or benefits?

The present study deals with the possibility that road users in the vicinity of a vehicle with daytime running lights (DRL) would suffer from a decreased conspicuity because of (he presence of that vehicle. In an experiment the primary effects of DRL on the conspicuity of other road users were investigated, with particular attention to other factors that might moderate the magnitude of any DRL-effect (type of background, type of other road user, level of expectation with regard to DRL, etc.). Subjects viewed colour slides depicting natural daylight scenes of traffic intersections, and were instructed to determine as fast as possible whether other road users were present or not (present-absent search task). Their reaction times were the main dependent variable. The results showed that when the car had its headlights on, the subjects reacted faster than when the headlights were off. None of the interactions with DRL as a factor showed a negative effect of DRL. Based on the results of the present experiment it is concluded that no evidence was found that the conspicuity of road users in the vicinity of a DRL-vehicle suffered from a car having its daytime running lights on.