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Reconnecting Nature



In collisions between motor vehicles and wild animals, the animal almost always loses. That doesn’t mean the cars win though. Collisions with large animals can damage cars and injure or kill the people inside them. It’s particularly bad in the case of large, high-traffic roads where an animal must dodge high-speed vehicles across multiple lanes. Wildlife continue to try to cross the road, however, because of biological imperatives: to feed or to mate. The animals can’t solve the problem we imposed on them.

The Robert L. B. Tobin Land Bridge, in Phil Hardberger Park in San Antonio, Texas, is the largest dedicated wildlife overcrossing in the United States. PHOTO COURTESY OF SPAWGLASS

Humans have begun to address the issue by building dedicated wildlife crossing routes. These crossings have been constructed in numerous states, some bridging over existing roads, others tunneling under.

A large example of the undercrossing variety is currently being constructed in Southern California’s Riverside County. California State Route 60 (the Pomona Freeway) runs east out of downtown Los Angeles, a major route to the desert cities around Palm Springs. About 70 miles out, it gets to the far edge of the suburban sprawl and climbs through a rugged mountainous landscape of mini canyons known locally as The Badlands. That 4.5 miles of road cuts through a small swath of wilderness that is a connector between two larger wild areas to the north and south. The freeway snaking through it is a notoriously twisty four-lane slab with narrow lanes and narrower shoulders, where semis and four-wheelers jostle each other for the right of way. The serpentine roadway is wedged in between the mountainous outcroppings, with no other access route on either side through most of its length.

To give wildlife living north of the highway safe access to the wilderness on the south, a series of undercrossings were designed as part of a larger project to widen the freeway. Adding a truck lane in each direction, plus new shoulders, the construction plan requires demolishing the existing road (in stages) and rebuilding it from the ground up, with the rebuilt road having numerous underpasses dedicated for wildlife.

The project is being constructed by Skanska, a member of multiple AGC chapters, with major funding from the Riverside County Transportation Commission (RCTC), who is leading the project with oversight by the California Department of Transportation (Caltrans). It includes two large-animal crossings, 20’ x 20’ tunnels under both sides of the freeway. There are also numerous culverts for smaller animals. In addition, wildlife fencing is being installed along much of the freeway, to divert animals to the tunnels.

Rafael Gutierrez, senior project manager for Skanska, describes the challenging nature of the site by saying that the “badlands” name is appropriate. “It’s not only steep and tall hills, but it’s really bad material. There are a lot of landslides in the area.”

That difficult landscape is considered a sensitive environment, and it provides habitats for all sorts of wildlife that had to be protected against im-pacts from construction. The project was required to have an authorized biologist on site before and during construction, who found nesting bats feed-ing their newly born young nesting in the existing 6-foot arch. When the bats were done nursing and left the area, work could begin.

There are two endangered bird species on the site, with 500-foot radius legal protection around them. The environmentalist team maps nesting birds and issues a weekly report to help maintain adequate buffers around them.

Protecting wildlife was not the biggest construction challenge, however. It was the limited access to the site. The freeway they were going to demol-ish and rebuild is the only way in and out.

Bryce Johnston, RCTC senior capital projects manager, recalls, “The state was not going to allow any lane closures. I had to negotiate, so we were able to get a six-month closure of one lane.”

From that lane, they were able to excavate and build the first new truck lane and shoulder. The new pavement could then take traffic and allow the closure and demolition of the next lane over.

Working conditions were unusually cramped. “When we were working in the middle section,” relates Gutierrez, “we had 30-foot shoring on both sides of the excavation to support the old road, and then support the new road while we built the next box. There’s not a lot of room for work, and it was a deep excavation. We were digging with one excavator tossing behind it, and another excavator was taking it from there and tossing material behind it.”

When completed, the 20’ x 20’ tunnels are expected to be used by deer, coyotes, bobcats and mountain lions. The project is scheduled for comple-tion in summer 2022.


Overpass-style wildlife crossings have gained wider public awareness recently, in part due to one video that went viral, posted by the Utah Wildlife Migration Initiative. Compilation footage from the overcrossing on Inter-state 80 near Parley’s Summit shows animal traffic day and night. Users of the bridge included deer, elk, porcupine, bear, mountain lion, bobcat, moose, coyote, raccoon and numerous small species.

The I-80 crossing was designed by WSP, a member of multiple AGC chapters, and built in 2018. Like the Route 60 project, it was piggybacked onto a much larger widening project, adding a climbing lane up the 7,120-foot mountain pass that is the highest point in the state. Joshua Sletten, assistant vice president and project manager, recalls that UDOT was willing to carve $5 million out of the project for a crossing, and they knew they couldn’t do a tunnel for that price. WSP designed a two-span bridge, 45-feet wide and 320-feet long near the top of the pass, that could be constructed with just three night-closures of I-80.

Video footage from the Utah Wildlife Migration Initiative shows animal traffic day and night on the overcrossing on Interstate 80 near Parley’s Summit. Users of the bridge included deer, elk, porcupine, bear, mountain lion, bobcat, moose, coyote, raccoon and numerous small species.

Sletten details several aspects that made this different than a conventional vehicle overpass. They spent considerable time imagining the right sur-face that would attract wildlife, eventually rejecting a vegetated bridge. “We ended up putting in small river rock, similar to a rocky area in the Wa-satch.”

Drainage was another important consideration, since the 8 inches of river rock could hold many tons of water. They also had to consider the possibility of 10 feet of snow load, since there would be no snow-clearing of the overpass. It ultimately was designed for heavier loading than a conventional vehicle bridge. They also built 10-foot wildlife fence for five miles in either direction along the highway.

The Utah Department of Fish and Wildlife was closely involved in the project, although Sletten relates that they were skeptical it would succeed. They feared that small animals would avoid being trapped out on the bridge by larger predators. They expected it to take about three years for animals to start venturing across it. It took just six months.


In Texas, they built a land bridge for a slightly different audience, for animals and people. Phil Hardberger Park is a 311-acre natural enclave within the city of San Antonio, created in 2010 on property formerly operated as a dairy farm. From its inception, the park has faced a challenge: A major highway, Wurzbach Parkway, cuts it in half. There was no pedestrian route from one side to the other, although one was indicated on the original mas-ter plan.

This deficiency was finally remedied by the construction of the Robert L. B. Tobin Land Bridge, believed to be now the largest wildlife overcrossing in the United States. Built by SpawGlass Construction, a member of multiple AGC chapters, through its Civil Division, it is 150-feet wide at the top (165 feet at the base), and spans over a 106-foot wide six-lane roadway.

I-80 was closed to set the girders for the Parley’s Summit overcrossing. PHOTO COURTESY OF JOSHUA SLETTEN, WSP

To span Wurzbach Parkway without a center support, the bridge is supported on 16 arched steel girders spanning 165 feet.

“The unique part about those,” Keithley points out, “is that the outer two girders on either side are actually arched and curved in, in what we call a Pringle-shape.” The girders are set in massive concrete counterports on each side.

Girders were set during five weekend-long road closures. The 8-inch cast-in-place concrete deck was formed and poured through a series of over-night road closures, working up either side of the arch to balance loading, and pouring the top one-third of the deck last, in a single night. The deck was then fully waterproofed because the bridge is vegetated and irrigated.

The landscaping completely conceals the arch profile. The steep dips at the ends of the arches were filled with geofoam, up to 8-feet thick, and then it was loaded with dirt — about 2 to 2.5 feet thick at the very top — and contoured.

“To look natural, it’s got little berms,” explains Keithley. “It’s a little wavy; it doesn’t match at all the structure of the bridge.” It has low plants, na-tive grasses expected to grow fairly tall, and even a few trees. There’s a meandering 8-foot dirt path for human visitors.

The bridge is paired with the Skywalk, an elevated pedestrian walkway that rises 18 feet off the ground (gently — it is wheelchair accessible) and lets visitors walk through the tree canopy. A steel structure, it is 6-feet wide, 1,000-feet long, and connects from a point near the north entrance of the park to the foot of the land bridge. It is a popular attraction.

If there were any doubts about animals’ willingness to use the land bridge, they were quickly dispelled.

“Before the bridge was opened,” recalls Keithley, “the wildlife was using it. While we were working, shutting down at the end of the day, we had deer and coyotes, before the bridge was open to the public.” Since opening, bobcats and foxes have been seen as well, mostly at night. It is believed that, when the plantings grow in and provide better cover, animals will start to cross the bridge during the daytime, too.