Deer-Car Collisions Rise When Daylight Saving Time Ends

Forgoing the “spring forward, fall back” pattern could save 33 human lives, 37,000 deer and more than $1 billion per year, study suggests

Deer standing in a field of grass
Nearly ten percent of all deer-vehicle collisions occur in the two weeks surrounding the time change in the fall.

Every autumn, when the clocks fall back an hour in several parts of the world, more drivers end up on the road after sunset. While many people find commuting in the dark to be unpleasant, scientists say this shift can have dire consequences: It can actually be deadly.

New research, published this week in the journal Current Biology, finds the November switch from daylight saving to standard time corresponds with a 16 percent increase in deer-vehicle crashes in the week after the clocks change. Scientists estimate that maintaining daylight saving time year-round would prevent 33 human deaths, 2,054 human injuries and 36,550 deer deaths, while also saving $1.19 billion in collision costs per year.

“A seemingly simple change—not changing the clock back in the fall, not falling back—would lead to such a marked reduction in collisions throughout the country,” study co-author Laura Prugh, a quantitative wildlife scientist at the University of Washington, tells Evan Bush of NBC News.

Lawmakers, citizens, scientists and other experts have fiercely debated the twice-a-year shift between daylight saving and standard time, which this year takes place on November 6. In the United states, the practice officially started in some capacity in 1918, with the rationale that aligning darkness with a later clock time would save electricity to go toward the war effort. Now, some have argued for doing away with the time change altogether and replacing it with either permanent standard or daylight time.

The new paper suggests, at least for some drivers and deer, that adopting daylight time year-round is the way to go: Scientists estimate this would lead to an overall 2.3 percent drop in deer-vehicle collisions per year.

In contrast, they say that permanent standard time would correspond with 5.1 percent more deer-vehicle collisions. Those 73,660 extra crashes would cause an estimated 66 additional human deaths and 4,140 injuries, as well an extra $2.39 billion in collision costs each year, per the paper.

To reach these conclusions, the researchers analyzed more than one million deer-vehicle crashes across 23 states from 1994 to 2021. They compared that data to traffic volume numbers for each state between 2013 and 2019, paying close attention to the weeks right before and after the spring and fall time changes.

They found that most car accidents involving deer occurred between sunset and sunrise. But these crashes were most common at dusk: They occurred 14 times more frequently in the two hours after sunset than in the two hours before sunset.

Close-up of a clock face
The time change debate rages on. Pexels

The autumn spike in vehicle collisions with deer stuck out. As it stands now, nearly ten percent of all deer-vehicle crashes occur in the two weeks surrounding the time change in the fall, when some deer are up to 50 percent more active as a result of their mating season.

“We believe that this fall spike really happens due to the overlap of these two factors: the breeding season and the change from daylight saving time back to standard time,” Prugh says in a statement.

White-tailed deer (Odocoileus virginianus) go into “rut,” or mating season, right around when Americans change their clocks. In the fall, crashes on the East Coast increased more dramatically than those on the West Coast, likely because there are more white-tailed deer in the eastern states.

“The timing could not be worse,” Prugh tells Science News’ Bethany Brookshire.

Meanwhile, mule deer (Odocoileus hemionus), which are more common on the West Coast, go into rut later in the fall or early winter.

As such, switching to permanent daylight saving time would affect regions differently—the scientists estimate deer-vehicle crashes would drop 8.3 percent in Maine, but might actually increase by 2.5 percent in Kansas, for instance. Individuals living on the far eastern side of a time zone would likely experience a bigger reduction in deer-related car accidents than those in the far western part of the time zone.

In March, the U.S. Senate approved a bill—the Sunshine Protection Act—that would make daylight saving time permanent starting in 2023. As yet, the bill hasn’t gotten too far; it has stalled in the House of Representatives.

But some medical professionals argue the opposite due to the time change’s effects on the body. As humans function better with more morning sunlight, some doctors, such as several with the American Academy of Sleep Medicine, prefer permanent standard time, per NBC News.  

Now, the new findings give policymakers and public health experts one more factor to contemplate as they debate the time change moving forward.

“It’s a big number,” says Tom Langen, a biologist at Clarkson University who was not involved in the study, to NBC News. “It’s likely correct that the time shift, and particularly the shift from daylight saving time to standard time in the fall, results in some human deaths and a lot of accidents that would not happen.”

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