Safe Passage for Wildlife in State College

In 2008, the final stretch of I-99 replaced U.S. 220 between Bedford County and State College in central Pennsylvania. For decades, U.S. 220 had cut through the forests of Bald Eagle Mountain, hugging the Juniata River and Bald Eagle Creek in the valley between two mountain ridgelines. The area is home to black bear, bobcat, coyote and deer, and to a wide variety smaller animals; muskrat, beaver, porcupine, turkey, a variety of reptiles and amphibians, and the threatened Allegheny woodrat all make their home in the Nittany mountains.

The construction of I-99, which would be wider and carry higher-volume, faster-moving traffic than U.S. 220, represented an impermeable barrier to wildlife moving between the ridge habitat and the valley floor[1]. This cut off animals from migrating throughout their range and divided species populations, increasing the danger
for animals that attempted to cross the busy highway. Beginning in the early planning stages, the U. S. Fish and Wildlife Service (USFWS) advocated for the new highway to incorporate wildlife crossings to mitigate the impact that the new interstate would have on local wildlife.

Location, location, location

Underpasses were a lower-cost option for wildlife crossings than overpasses. An added benefit of installing underpasses was that many of them could be created by modifying culverts that already existed under the highway. Culverts were also necessary to maintain the hydrology of the area – allowing water to flow under the highway to prevent flooding – but could simultaneously be designed to facilitate passage of animals as well.

Photo by Stephanie Wein, © Google 2025. | Used by permission

The crossings needed to be located where they would be most utilized by animals. To narrow down the most useful crossing locations, several factors were considered. First, observation of which mile markers had the most animal-vehicle collisions on the existing road was used. This data showed that most wildlife deaths were concentrated around a small number of mile markers. A second factor for choosing potential wildlife crossing underpasses was the intersection between the road and waterways. This is because wildlife often use waterways as travel corridors, with terrestrial animals often using stream valleys to move throughout their range[2]. For this reason, places where the highway intersected streams would be prime candidates for locating wildlife underpasses. In addition, PennDOT staff completed a variety of on-the-ground survey work, including looking at where animal tracks and scat indicated paths frequented by wildlife. State officials also spoke directly with hunters and property owners who were very familiar with the area, and who frequently observed animal movements[3]. A final consideration for underpass siting was identifying areas where the newly constructed I-99 highway would cut off landowners from accessing parts of their property[4].

Designing with wildlife in mind

To make sure that wildlife used the crossings, PennDOT incorporated a set of established best practices. These included: ensuring there was a natural soil and vegetation bottom to the underpass with “dry footing,” meaning that even if a stream was present, there needed to be a dry travel strip for terrestrial animals to use. The crossings had a minimum height of 14 feet to accommodate large animals like bear and deer. Each underpass also needed a minimum of 10 acres of transition zone on either side to connect it to the surrounding habitat, and the crossings needed to include native plantings so they had continuation of habitat up to and through the underpass.

Wildlife Crossing #5 with a transition zone, a dry and level pathway, fencing, and daylighting between the two travel lanes.Photo by PennDOT | Public Domain

Some other design considerations included using fencing to keep animals off of the roadway and direct them towards the underpasses, as well as installing skylights in the median between the two travel lanes to allow daylight to penetrate the underpass. Daylighting a wildlife crossing underpass helps maintain its climate and improves visibility for prey species that are hesitant to enter dark spaces. The result is a safer, more appealing environment for animals to enter, and it allows vegetation to grow.

Photo by Stephanie Wein | TPIN

Successful crossings

After the underpasses were completed in 2010, there was a noticeable drop in wildlife-automobile collisions at the mile-markers where crossings were installed. In 2009, there was an average of 9 animal deaths across each of the 5 sites in 2009. By 2010, wildlife mortalities fell to an average of only one[5].

PennDOT also monitored these crossings with cameras and added sand to the bottom of the tunnels (sand traps), which allowed researchers to clearly identify animal tracks. These monitoring efforts revealed that a wide diversity of species were using the underpasses, from mammals like bear, woodchuck, raccoon and deer to large birds like geese and great blue heron[6].

Possum (left) and bobcat (right) tracks at I-99 wildlife crossing #5Photo by PennDOT | Public Domain

References

1.  Skelly & Loy, I-99 Presentation for PennDOT, 2012.

2. USDA Natural Resources Conservation Service,
Conservation Programs & Practices for: Helping People Help the Land Fish and Wildlife Habitat (fact sheet), accessed 5 February 2025.

3. Jennifer Kagel, PennDOT Liaison, US Fish & Wildlife Service, personal communication, 6 December 2024.

4. Ibid.

5. Skelly & Loy, I-99 Presentation for PennDOT, 2012.

6. Ibid.