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Animal crossings over and under highways can save big dollars — not just lives — says new study

This landscaped overpass allows wildlife to safely cross newly widened Interstate 90 near Keechelus Lake in the Washington Cascades.
Courtesy of WSDOT
This landscaped overpass allows wildlife to safely cross newly widened Interstate 90 near Keechelus Lake in the Washington Cascades.

Collisions between vehicles and large animals, like deer, are not only scary. The medical, car repair and cleanup costs really add up. That is according to a new study out of Washington State University that supports the case for building more wildlife crossings on highways.

The Oregon and Washington transportation departments are adding wildlife overpasses, underpasses and related fencing bit by bit, as scarce funding allows. The new study published in the journal Transportation Research Record by a WSU doctoral student looked at the efficacy and economics of those road safety improvements.

Wisnu Sugiarto of the WSU School of Economic Sciences tallied the number of serious animal-vehicle collisions in the vicinity of 13 crossing structures in Washington. He compared those numbers to a control group of highway stretches that didn’t have the wildlife safety features. The raw data came from crash reports filed to WSDOT from 2011-2020.

Sugiarto found a favorable cost-benefit analysis. The study estimated each crossing structure could save society between $235,000 and $443,000 annually through collision reductions. The savings varied based on structure size, design and location.

“If you plan on a structure having a 30-year lifespan, then there’s a lot of benefits,” Sugiarto said in an interview. “I hope that with this amount of benefits – in terms of quantified benefits – can give some information for transportation planners to think about.”

The study said the construction cost for a wildlife underpass can range from $500,000 to $2.7 million. The cost for an overpass can range from $2.7 million to $6.2 million. Often, the cost represents an incremental add-on to a large highway improvement project.

By Sugiarto’s analysis, the more expensive wildlife bridges were more effective in reducing collisions than cheaper culvert designs. This finding derives partly from which species prefer which structure design. Deer are by far the most commonly found carcasses along state highways in the Pacific Northwest. Previous studies using wildlife cameras showed deer prefer airy, open crossings and shy away from enclosed culvert underpasses. On the other hand, tunnel-like passages appeared popular with bears and amphibians.

WSDOT published a report of its own earlier this year on the benefits of wildlife crossings for road safety. Aside from matters of life and death, WSDOT estimated the average vehicle-deer collision resulted in economic costs of $9,175. Hitting an elk ballooned the average cost per collision to $24,242 and a moose even more, $42,652 per collision.

Notably, the bipartisan infrastructure package passed by Congress in 2021 created a new $350 million competitive grant program for states, local governments and tribes specifically to build more wildlife crossings over the next five years.

“A survey of close to 500 state and federal transportation agency representatives identified funding as the #1 barrier to making a national investment in wildlife crossing structures,” ARC Solutions executive director Renee Callahan said in a statement celebrating the bipartisan infrastructure package as “a crucial tipping point in improving human safety, reducing wildlife mortality, restoring connectivity and creating jobs.” ARC (Animal Road Crossing) is a partnership of groups seeking to promote road safety through wildlife passages.

The decade-long project to widen I-90 from Hyak to Easton, Washington, on the east slope of the Cascade Range includes a number of new wildlife underpasses. In addition, one prominent overcrossing was completed in 2018. A second overhead bridge solely for animals is planned and funded near Easton.

On a different stretch of highway, SR 522 near Monroe, the state rebuilt a bridge with an increased span to accommodate wildlife crossings underneath. Highway 97 in Washington’s Okanogan Valley is another deer collision hotspot getting attention.

Driving under the striking, arched and landscaped animal crossing over the six lanes of I-90 is what originally got Sugiarto interested in the topic.

“I was really curious because, first, it didn’t seem like there were any cars driving on the bridge,” Sugiarto said. “So, after I did some reading I learned that wildlife-vehicle collisions are an issue in many locations.”

During the 2022 Session of the Oregon Legislature, lawmakers increased funding for wildlife crossing projects. ODOT has focused on Highway 97 south of Bend, which had a very high rate of mule deer versus vehicle collisions. ODOT has already lined miles of this highway with 8-foot-tall fencing to guide deer and other wildlife to underpasses for safe crossing. Those sections then experienced major drops in collisions. Meantime, Oregon's Driver & Motor Vehicles Services agency started selling specialized license plates earlier this year to raise money for projects intended to prevent collisions with wildlife.

The WSU study did not attempt to quantify the benefit of improved habitat connectivity, but acknowledged that this is another valid reason for policymakers to justify spending on wildlife crossings. Major interstates such as I-90 and I-5 in Washington and I-405 in California sever the ranges of animals, such as cougars, and can consequently contribute to inbreeding within isolated populations.

This story has been updated.

Now semi-retired, Tom Banse covered national news, business, science, public policy, Olympic sports and human interest stories from across the Northwest. He reported from well known and out–of–the–way places in the region where important, amusing, touching, or outrageous events unfolded. Tom's stories can be found online and were heard on-air during "Morning Edition" and "All Things Considered" on NPR stations in Washington, Oregon, and Idaho.