DST and Road Safety: A Double-Edged Sword
Daylight saving time's effect on road safety is nuanced. The extra evening daylight during DST is often cited as a traffic safety benefit — and for evening commutes, this is largely true. But the spring transition itself introduces a period of elevated accident risk due to sleep deprivation. The overall picture is complex.
Spring Forward: Elevated Crash Risk
In the days following the spring DST transition, studies consistently show an increase in traffic accidents due to:
- Sleep deprivation: One hour of lost sleep impairs reaction time, judgment, and attention to the same degree as mild alcohol intoxication.
- Drowsy driving: Research from the AAA Foundation for Traffic Safety found that missing even 1–2 hours of sleep nearly doubles crash risk.
- Darker morning commutes: During the first weeks after spring forward, commuters face darker mornings, increasing pedestrian and bicycle collision risks.
A study published in Current Biology (2020) found a 6% increase in fatal traffic crashes in the U.S. in the week following the spring DST transition, attributing the increase to circadian misalignment and sleep loss.
The Evening Daylight Benefit
During the main DST period (spring and summer), more daylight in the evening genuinely reduces certain types of accidents:
- Pedestrian fatalities are higher in darkness. More people commute home in daylight during DST months.
- A 1996 study by researchers at the University of Michigan estimated that DST prevented approximately 901 pedestrian deaths and 727 passenger deaths annually in the U.S.
- Children walking home from school benefit from more afternoon daylight.
Autumn Transition: A Brief Safety Improvement
The autumn clock change, when people gain an hour of sleep, is associated with a brief decrease in accident rates, consistent with the improved alertness that comes from extra sleep. However, the autumn transition also introduces darker evening commutes sooner, which can increase pedestrian risks over the following weeks.
The Net Effect
When researchers attempt to calculate the net effect of DST on road safety across the entire year, results are mixed. Some studies find a modest net benefit (due to safer evening commutes outweighing the spring transition spike); others find no significant net effect. The consensus is that permanent standard time, with consistent schedules and no abrupt disruptions, would likely produce the most stable safety outcomes.