Why did the deer cross the road?

“Saw a deer
What a thrill
‘Til it came
Through the grill”

Many of us have been there.

You’re driving along, minding your own business, maybe enjoying the scenery, when all of sudden there’s a deer on the road, right in your path.

Your heart pounds. You swerve or hit the brakes. And there’s a full body hit, or a thump as a hoof catches your fender, or a lucky miss.

Whatever the outcome, (other than a serious wreck), as your heart rate returns to normal you likely wonder “Why did that deer cross the road right there that minute?”

Washington Department of Fish and Wildlife (WDFW) and Washington Department of Transportation (DOT) staff wonder, too. With DOT crews picking up deer and elk carcasses off roadways across the state nearly every day, and recording those collections by mileposts, WDFW wildlife researchers recognized a ready-made data set to analyze and come up with some answers to that question.

With some 200 human deaths and insurance costs of nearly $2 billion annually across the nation due to deer/elk-vehicle collisions, the answers are not just a curiosity. They might indicate ways to reduce the impact, at least on Washington’s 7,046 miles of highways that receive 31.6 billion miles of travel each year.

For “Analysis of Ungulate-Vehicle Collision Sites Along State Highways in Washington State,” WDFW research biologist Woody Myers led a study of 14,969 deer and 415 elk carcass collection records between 2000 and 2004.

The data confirmed some intuitive notions -- deer/vehicle collisions are greatest where deer populations are most dense and where traffic is greatest. Because of those factors and others, high collision sites are clustered, not random, and DOT posts deer crossing signs accordingly.

Although collision counts need to be measured at a finer or more precise scale than simply within mileposts to draw any definite conclusions, Myers also observed a number of other patterns, including:

• Highest number of deer removals occurred during the fall and highest number of elk removals occurred during the winter
• Majority of sites with unusually high deer/vehicle collisions were associated with posted speed limits of greater than 50 miles per hour
• Somewhat lower collision counts were recorded on roads with more curves and vice-versa (straighter roads were associated with higher collision counts)
• Lower collision counts were observed on eastern Washington rural interstate highways, eastern Washington urban principal arterials, and western Washington urban minor arterials –at least compared to other roadway classifications
• Comparing the DOT data with established landscape databases revealed a few more patterns, including:
• Cover, forage, and water available near roads were generally associated with higher collision counts in eastern Washington mule and white-tailed deer models
• Southern aspects (south-facing roadside slopes) were generally associated with higher collision counts in western Washington rural models
• Lower collision counts were associated more with steep roadside slopes (more than 35 degrees) than with moderate slopes (less than 35 degrees)

The source of the data – carcass collections – is limiting in a number of ways, Myers notes. Many deer or elk may be hit on roads but don’t die, or die from injuries far from the road, so those collisions are never counted. DOT collection records can’t determine time of day of collisions and day-of-the-week records may only reflect worker schedules.

For example, more than twice as many carcasses are collected on Mondays than any other day of the week, and Saturdays and Sundays have the lowest counts; that may only indicate that staffing is minimal on weekends, so Monday collections include animals hit on Saturdays and Sundays.

Despite the limitations, the information is shedding some light on the basic question, and more.

The answer to why did the deer cross the road depends on local deer densities and seasonal movement patterns, near-road vegetation and whether it provides forage or cover, the level of development in the area, the terrain in general, and the time of year, Myers says.

“For example, deer generally move lower, near human development including roads, in the fall,” he said. “That’s also the rut or breeding season for deer, when bucks are less wary and all deer are moving around more.”

The data probably says as much if not more, though, about why the vehicle strikes the deer. Myers says increasing speed limit and traffic volume showed the greatest association with high numbers of collisions. Road sinuosity, or the number of curves or straight stretches, roadway type, and even the season of the year also influence the number of deer/vehicle collisions.

“For example, in the fall we shift to daylight savings time so suddenly more job-commuting drivers are out on the roads during dimly lit hours when deer on the road are harder to see,” he said.

So what can be done to avoid crossing paths with deer in such a traumatic way?

For the driver, slowing down is the first step, especially from dusk to dawn and during the fall and winter. Paying attention to DOT’s deer crossing signs, and particularly “high kill area” signs, can be critical. Speed limit reductions at night along stretches of highway with a high rate of collisions might be in order.

In general, new highway development in high deer or elk use areas should be avoided.

Deer or elk could be protected from collisions with construction of fences along roadways with high collision rates, especially if combined with existing bridges or development of over or under passes to allow natural movement. Such efforts have proved effective in other parts of the country.

Washington DOT is already exploring wildlife crossing projects, such as the Snoqualmie Pass East section of Interstate 90 (for more information see
www.wsdot.wa.gov/Projects/I90/SnoqualmiePassEast/HyaktoKeechelusDam/).

Myers’ study concludes that while the analysis of the DOT data provided some insights and confirmed conventional wisdom about deer/elk vehicle collisions, additional research would be helpful, including review of existing telemetry data of radio-collared deer and elk to assess movement near and across highways, field inspection and mapping of high collision sites to further document habitat associations, and driver surveys about deer/elk collisions.