Autumn is here, and that means the risk of hitting deer on rural roads and highways is rising, especially around dusk and during a full moon.
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.
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.
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 breaks and are rear-ended by the vehicle behind them.
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 percent of accidents occur during daylight hours. Deer-vehicle accidents occur eight times more frequently per hour of dusk than daylight, and four times more frequently 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 travel greater distances from covered areas 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.
That high-risk period is also when daylight saving time ends – it happens on Nov. 7, 2021, in the U.S. Shifting the clock one hour back means more commuters are on the road during the high-risk dusk hours. The result is more cars driving at the peak time of day and during the peak time of the year for deer-vehicle accidents.
Overall, given that most U.S. states and more than 70 countries have seasonal "daylight saving" clock shifts, elevated ungulate-vehicle accident rates caused by clock shift may be a widespread problem.