FALL is the season of apples, frost, turning leaves and roadkill. A 2008 congressional study found that one in 20 reported motor vehicle collisions is animal-related, and the numbers peak in autumn. Annually, these incidents result in about 26,000 injuries and 200 human deaths.
Across the country, collisions with deer — the most common type of animal-related incident — cost more than $8.3 billion per year, including vehicle repair, medical services, towing, law enforcement time and carcass disposal. The damages increase when larger animals like moose or elk are hit.
Spring and autumn, when animals are migrating, searching for mates or evading hunters, are the riskiest periods. Many animals, including deer, are active at dusk and dawn, when twilight reduces their visibility to predators — and to drivers, resulting in more collisions. The transitions to and from daylight saving time are especially hazardous because the timing of our commutes shifts overnight before animals can adjust to avoid the rush hour. This also contributes to the seasonal spike in animals killed.
It's hard to quantify the full impact of vehicle collisions on wildlife populations because most reported incidents involve larger animals like deer (and even those are underreported). The toll on smaller creatures like squirrels, salamanders and birds goes largely uncounted, but a recent study estimated that as many as 340 million birds are killed by vehicles annually. For 21 species listed by federal authorities as threatened or endangered — including the Canada lynx, the red wolf, the Florida panther, the crested caracara and Florida scrub-jay — road death is a major threat to survival.
How can we reduce this carnage? Most collisions with animals occur on two-lane highways that have relatively low traffic volumes (fewer than 5,000 vehicles per day). With greater awareness, motorists can adapt their driving. Research shows that drivers who anticipate danger can halve their reaction time and cut the risk of collision.
Roadside signs may warn us of wildlife crossing these stretches of road, but if drivers rarely encounter animals, they can become habituated to the warnings. Flashing lights augmenting these signs can slow drivers, but they're most effective if used only during the riskiest periods. Drivers are less likely to ignore animal crossing warnings that are activated by systems that detect moving animals in real time, but these dynamic signs are relatively rare. Ultimately, any system that relies on altering driver behavior will only have limited success.
Surprisingly, changing animal behavior is more promising. The most effective tactic uses fencing to channel animals toward structures that safely cross roads. Panthers and alligators in Florida travel through culverts under a section of Interstate 75 known as Alligator Alley. Grizzly bears in Canada's Banff National Park use overpasses designed for wildlife to cross the Trans-Canada Highway. Texas is planning underpasses along Highway 100 that, when constructed, could protect our 50 remaining ocelots.
In Wyoming, pronghorn following the 6,000-year-old "Path of the Pronghorn" (our only federally protected migration corridor) are now guided by fences to overpasses and underpasses that cross a highway that lies between their summer and winter ranges. When work on the safe passages and fencing was completed in 2012, we watched the initial crossing attempts of the pronghorn in suspense.
When they first encountered the guide fences, they chaotically tried to get through, unaware of the nearby overpass. In time, though, every group of pronghorn found its way over the busy highway. Their instinct to reach their winter range exceeded their fear of the unfamiliar structures; now, these are simply part of their migration path.
The value of conserving this magnificent phenomenon — one of the last intact long-distance terrestrial migrations — is, in one sense, immeasurable. But it works in monetary terms, too. Before the project, drivers risked colliding with the 140-pound horned animals that crossed the highway by the thousands during their twice-yearly migrations. Over time, the cost savings from avoided collisions will offset the initiative's $9.7 million price tag. Based on the accident reduction rate so far, the investment will pay off in about 12 years.
Later this year, Congress must reauthorize a transportation bill that includes provisions designed to reduce animal-vehicle collisions and protect both drivers and wildlife. Congressional representatives must back clauses in the bill that would empower transportation agencies to use these engineering solutions where needed.
Drivers must do their part to avoid wildlife collisions by slowing down and paying attention when there's a high probability of encountering wildlife. But the best way of avoiding collisions is to build highways that reduce the hazards for drivers and animals alike where the risks are highest.
Amanda Hardy is the assistant director of the North America program for the Wildlife Conservation Society, where Renee Seidler is an associate conservation scientist.