Daylight saving time and fatal crashes: The impact of changing light conditions

Time changes in the fall and spring result in different light conditions in the morning and afternoon, which may impact U.S. roadway safety.

We investigated the number of fatal crashes in the contiguous United States from 2010 to 2019 in the 5-weeks before and after biannual time changes focusing on crashes that occurred at times when light conditions changed.

Fatal crashes among motor-vehicle occupants decreased in the 5 weeks after the fall time change (-7.1%) and increased in the 5 weeks after the springtime change (+12%). The reverse was true for fatal crashes among pedestrians/bicyclists, increasing after the fall time change (+13%) and decreasing after the springtime change (-24%). Overall, time changes resulted in a net decrease of 26 fatal pedestrian/bicyclist crashes and an increase of 29 motor-vehicle occupant crashes. Pedestrian and bicyclist fatal crash counts were strongly associated with ambient light conditions: the net decrease of all 26 pedestrian/bicyclist crashes could be attributed to the changing light conditions, but only 2 of the additional motor-vehicle occupant crashes were associated with changing light conditions.

Changing light conditions from biannual time changes have differential effects depending on time of day and road user type. The switch to daylight saving time in the spring results in an increase in motor-vehicle occupant fatal crashes and a reduction in pedestrian/bicyclist fatal crashes, while the opposite occurs following the switch to standard time in the fall. These effects are largely dependent on the ambient light conditions at the time of the crash, especially for pedestrians and bicyclists.

The sensitivity of pedestrians and bicyclists to changing ambient light conditions supports the need for improved pedestrian and bicyclist infrastructure and continued efforts to develop vehicle technology to help avoid or mitigate crashes with pedestrians and bicyclists.