(The Conversation) – Daylight saving time ends in the U.S. and Canada on Nov. 7, 2021, and most of us will be setting our clocks back an hour. There is a long-running debate about the benefit of the time change, given how it disrupts humans’ circadian rhythms, causing short-term stress and fatigue.

Another risk accompanying the time change is on the roads: As more people drive at dusk during an active time of year for deer, the number of deer-vehicle accidents rises.

Deer cause over 1 million motor vehicle accidents in the U.S. each year, resulting in more than $1 billion in property damage, about 200 human deaths and 29,000 serious injuries. Property damage insurance claims average around $2,600 per accident, and the overall average cost, including severe injuries or death, is over $6,000.

While avoiding deer – as well as moose, elk and other hoofed animals, known as ungulates – can seem impossible if you’re driving in rural areas, there are certain times and places that are most hazardous, and so warrant extra caution.

Transportation agencies, working with scientists, have been developing ways to predict where deer and other ungulates enter roads so they can post warning signs or install fencing or wildlife passages under or over the roadway. Just as important is knowing when these accidents occur.

My former students Victor Colino-Rabanal, Nimanthi Abeyrathna and I have analyzed over 86,000 deer-vehicle collisions involving white-tailed deer in New York state using police records over a three-year period. Here’s what our research and other studies show about timing and risk.

Time of day, month and year matters

The risk of hitting a deer varies by time of day, day of the week, the monthly lunar cycle and seasons of the year.

These accident cycles are partly a function of driver behavior – they are highest when traffic is heavy, drivers are least alert and driving conditions are poorest for spotting animals. They are also affected by deer behavior. Not infrequently, deer-vehicle accidents involve multiple vehicles, as startled drivers swerve to miss a deer and collide with a vehicle in another lane, or they slam on the brakes and are rear-ended by the vehicle behind.

In analyzing thousands of deer-vehicle collisions, we found that these accidents occur most frequently at dusk and dawn, when deer are most active and drivers’ ability to spot them is poorest. Only about 20% of accidents occur during daylight hours. Deer-vehicle accidents are eight times more frequent per hour of dusk than daylight, and four times more frequent at dusk than after nightfall.

During the week, accidents occur most frequently on days that have the most drivers on the road at dawn or dusk, so they are associated with work commuter driving patterns and social factors such as Friday “date night” traffic.

Over the span of a month, the most deer-vehicle accidents occur during the full moon, and at the time of night that the moon is brightest. Deer move greater distances from cover and are more likely to enter roadways when there is more illumination at night. The pattern holds for deer and other ungulates in both North America and Europe.

Over a year, by far the highest numbers of deer-vehicle accidents are in autumn, and particularly during the rut, when bucks search and compete to mate with does. In New York state, the peak number of deer-vehicle accidents occurs in the last week of October and first weeks of November. There are over four times as many deer-vehicle accidents during that period as during spring. Moose-vehicle accidents show a similar pattern.

The problem with daylight saving time

We have also found that the daylight saving time clock shift of one hour affects the number of deer-vehicle accidents.

In spring, when deer-vehicle accidents are at an annual low, the start of daylight saving time means a later sunrise and sunset. It results in a small decrease in deer-vehicle accidents. However, in fall, when deer-vehicle accidents are at an annual high because of deer rut, the earlier sunrise and sunset cause a significant increase in deer-vehicle accidents.

The clock shift results in more commuters on the road during the high-risk dusk hours. The consequence is more cars driving at the peak time of day and during the peak time of the year for deer-vehicle accidents. The clock shift results in a 37% reduction in deer-vehicle accidents during morning commuter hours, since fewer commuters are on the road before sunrise, but a 72% increase in accidents during evening commuter hours. Overall, there is a 19% increase in accidents during commuter hours the week after the fall time change in New York.

Deer still cross roads at any time

It’s important to remember that deer-vehicle accidents can occur at any time of day or night, on any day of the year – and that deer can show up in urban areas as well as rural ones.

This is an update to an article previously published on Sept. 21, 2021.