Many traffic collisions are the result of the driver’s failure to notice the other vehicle. It is often cited in police reports that the driver “looked but did not see”. The purpose of Daytime Running Lights (DRLs) is to increase the visual contrast of DRL-equipped vehicles. Visual contrast, which is the difference in brightness between two areas, is an important characteristic enabling a driver to detect objects. This paper begins with a brief regulatory history of DRLs in the U.S. and how General Motors Corporation (GM) introduced DRL-equipped vehicles. It also describes a DRL effectiveness study conducted by Exponent Failure Analysis Associates of San Francisco for GM. The study compared the collision rates of specific GM, Saab, Volvo and Volkswagen vehicles before and immediately after the introduction of DRLs. Since DRLs are not visible from behind a vehicle, rear-end collisions were not included in the study. Information from police accident reports and registration data shows that GM customers have avoided more than 25,000 vehicle collisions since GM began equipping vehicles with DRLs in 1995.