While a massive wildlife bridge is being built across the busy 101 Freeway at a cost of $92 million, some scientists say there is a better — and cheaper — way to stop animal roadway carnage.
“Wildlife crossings don’t solve roadkill. Sure, they provide a safe way across the road but fencing is the only way we know that stops roadkill,” said Fraser Shilling, director of the UC Davis Road Ecology Center, a program of the Institute of Transportation Studies.
“Fencing remains the only way to reduce wildlife vehicle collisions on the statewide scale,” Shilling said in an interview June 19, shortly after his 11th report was released about animals killed on roadways, “Roadkill: A Preventable Natural Disaster.”
Shilling, who studies the intersection of transportation with wildlife sustainability, supports building more wildlife crossings but also more fencing along freeways and roadways adjacent to rural habitat — to prevent wild animals from trying to cross busy highways.
He pointed to a 6 1/2-mile fencing project on both sides of the 241 Toll Road in Orange County, stretching from the 261 Toll Road north to the 91 Freeway near Yorba Linda/Anaheim Hills as a model for fencing that saves the lives of mountain lions in the Santa Ana Mountains.
The Foothill/Eastern Transportation Corridor Agency built the 10-12 feet high fence which is also buried 2 feet deep to deter animals from digging and crawling under it. It was built between 2014 and 2016 at a reported cost of about $10 million. The project won an international award.
“They went from one to two mountain lions dying to zero. Give credit to them,” Shilling said. The agency has not had a single road kill of lions, deer or bobcats along this section of the 241 since the project was completed in January 2016, said Doug Feremenga, environmental planning manager with the agency.
The Wallis Annenberg Wildlife Crossing in Agoura Hills, under construction and set to open in early 2026, will span 10 lanes of the 101 Ventura Freeway at Liberty Canyon Road. “The bridge will allow for wildlife to cross freely over the 101 freeway without the threat of death or accidents, and will ensure the survival of many isolated species,” states the official website.
Shilling said the main attribute of the crossing is to allow mountain lions in the Santa Monica Mountains to safely cross into a vast rural territory of open space to the north, where they can mate with lions containing varying genetic material.
Inbreeding is causing genetic mutations and could lead to their extinction in 15 years, wildlife biologists say. Eventually, the loss of genetic diversity can lead to reduced birth and survival rates, a phenomenon dubbed “inbreeding depression.”
But he’s not so sure the lions will use the crossing, because they typically avoid areas with vehicle lights and noise. The 101 Freeway at the crossing sees 300,000 vehicles a day. “The crossing itself is a hypothetical solution until it is proven. You have to block all that traffic noise and lights,” he said.
The crossing developers will build tree-and-shrub entryways leading to the bridge entrances. And the bridge itself will be covered in soil, grasses, trees and greenery to lure cats into thinking it is an extension of the rural hillsides on each side of the freeway.
It will also have the first-ever vegetative sound wall, plus four feet of soil on the bridge for plantings to further absorb sound from cars traveling below, explained Beth Pratt, California regional executive director for the National Wildlife Federation and founder of The Wildlife Crossing Fund on Tuesday, July 2.
To cut down on light pollution, the crossing will be painted with a dark stain to absorb rather than reflect light, she said. Recently, students of Travis Longcore, an environmental scientist at UCLA and an expert on animals and light pollution, witnessed a barn owl fly across the crossing, which now has its main girders in place above the freeway lanes.
“We have the top experts in the world working on this,” Pratt said. “I don’t think there is any doubt these cats and other wildlife will use this crossing.”
While Shilling supports more wildlife crossings and underpasses, he said the better solution for reducing roadkill is to erect fences near roadways. But in the case of the Santa Monica Mountains, that would keep the isolated mountain lions from reaching the other side, negating the purpose of enhancing healthy breeding.
“If we just did fencing, we wouldn’t fix the problem,” Pratt explained.
On June 15, a mountain lion was killed after being struck by a car on the 101 Freeway near Agoura Hills and the wildlife crossing. “That lion at Liberty Canyon is dead because there is no fencing there,” he said.
Pratt said the Annenberg crossing will include up to two miles of exclusionary fencing that will lead critters to the freeway crossing. “Yes, fencing is absolutely a tool for connectivity,” Pratt said.
Winston Vickers, a veterinarian and epidemiologist with the UC Davis Wildlife Health Center, studies mountain lions living in the Santa Ana Mountains. The area is bounded on the north by the 91 Freeway, the east by the 15 Freeway and includes the counties of Orange, Riverside and San Diego.
He said wildlife crossings and fencing can work in tandem. Often fencing can lead mountain lions to the wildlife crossings, whether under or over a freeway. He’s advocating for more fencing along Ortega Highway and on other parts of the 241 toll road, where mountain lions cross and get killed.
He’s been keeping track of mountain lion deaths in this region for 20 years and said the fenced portion of the 241 has cut way down on mountain lion roadkill. “Yes, it has been incredibly effective in reducing vehicle strikes of many species,” he said, namely deer, bobcats and mountain lions.
Vickers and Shilling estimate the cost of fencing at about $100,000 a mile, much less than the cost of a concrete and steel wildlife crossing over a freeway.
In the broad region Vickers surveys, he’s counted 13 mountain lions killed from vehicle strikes from 2020 through the first six months of 2024, or about two per year.
“There’s still plenty of fencing that needs to be built in the region,” he said on Tuesday, July 2.
He believes the Agoura Hills wildlife crossing, which will be the largest of its kind in the world, will have enough greenery and sound walls to block out light and noise.
The state is far behind other states in supplying funding for fencing and for wildlife crossings to protect mountain lions and other wildlife habitat dying at alarming rates from vehicle strikes, Shilling said.
His study from the UC Davis research team estimated that 613 mountain lions were dead from being hit by cars in California during an eight-year span, about 70 big cats a year.
The number of mountain lions killed by vehicle strikes in California is rising each year, he said. He estimated 5% of their population is disappearing just from vehicle collisions.
“We love mountain lions but we keep driving on highways very quickly, which is why we have fewer of them,” he said. Other factors killing them off are eating prey that have ingested rodenticides left outside homes, schools and buildings to kill rats and mice that weaken big cats’ immune systems and invite deadly diseases.
The number of mule deer killed on roadways in the state every year is estimated at 48,442, or about 10% of the 475,000 deer in the state, the UC Davis report said. Deer killed on roads may explain why their numbers are declining in California, said the report.
Keeping track of where deer and cougars are killed tells scientists and others where they are coming from. The hotspots may make the best locations for fencing or wildlife crossings, Shilling said.
“If you track where animals die on roads, you also track where they are alive,” Shilling said.
Some Southern California hotspots for animals killed by cars include the following, according to Shilling:
• 15 Freeway south of Temecula
• 15 Freeway in the Cajon Pass, between the San Gabriel and San Bernardino mountains
• 5 Freeway at Tejon Pass, also known as the Grapevine
• 101, 405 freeways in Los Angeles. Although these are so busy that mountain lions hesitate to cross them, Shilling said.
• 10 Freeway, west of Palm Springs (bear roadkill)