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

Woods, Amber N. / Weast, Rebecca A. / Monfort, Samuel S.
Insurance Institute for Highway Safety
April 2024

Introduction: Time changes in the fall and spring result in different light conditions in the morning and afternoon hours, which may impact the safety of U.S. roadways.
Methods: We investigated the number of fatal crashes in the contiguous U.S. from 2010 to 2019 in the 5 weeks before and after biannual time changes with a focus on crashes that occurred at times when light conditions changed.
Results: 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 and bicyclists, increasing after the fall time change (+13%) and decreasing after the springtime change (-24%). Fatal crash counts were strongly associated with ambient light conditions, especially among pedestrians and bicyclists: there were fewer fatal crashes in lighter fall mornings and more in darker spring mornings. In contrast, there were more fatal crashes in darker fall afternoons and fewer in lighter spring afternoons.
Conclusions: Changing light conditions from biannual time changes have differential effects depending on the time of day and the type of road user. 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.
Practical applications: The sensitivity of pedestrians and bicyclists to changing ambient light conditions supports the need for improved pedestrian and bicyclist infrastructure, as well as continued efforts to develop vehicle technology that can help avoid or mitigate crashes with pedestrians and bicyclists.

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