Deer herd using wildlife underpass.

Do They Work?

If we want to know whether wildlife-crossing structures work, it’s important to define what success looks like. For the coalition of engineers, scientists, and government officials involved in the renovation of US 93 North, success was centered around two things:

  • Reducing wildlife-vehicle collisions
  • Maintaining habitat connectivity for wildlife

After 14 years of on-the-ground monitoring and research before, during, and after construction of wildlife crossing structures, we can definitively state that US 93 North crossing measures have reduced wildlife-vehicle collisions and maintained or improved habitat connectivity for deer and black bear.

Our research was concentrated in three main study areas as well as three adjacent road sections that had no wildlife crossing structures (the “control” sections).

Reducing human and wildlife traffic accidents

The number of deer using wildlife crossing structures in the three main study areas versus walking across the pavement averaged 6,293 per year; for black bear that number was 305 crossings per year. Diverting animals away from the road surface means fewer potential wildlife-related car accidents, injuries, or deaths.

Wildlife fences are effective in reducing collisions with large mammals, but their effectiveness depends on the length of the fencing and associated measures.

In the three main study areas where wildlife fencing was implemented, there was a 70-80% reduction in wildlife-vehicle collisions. Interestingly, the number of wildlife-vehicle collisions significantly increased in the nearby unfenced (control) areas.

Although wider lanes and shoulders, longer sight distances, and more gentle curves make rural highways like US 93 North safer in general, our research suggests that without the implementation of wildlife mitigation measures like fencing and crossing structures, the number of wildlife-vehicle collisions will be greater than before highway improvements were made.

Maintaining habitat connectivity for wildlife

The wildlife cameras recorded 22,648 successful animal crossings per year in the 29 crossing structures that were monitored.

Twenty different species of medium-sized or large-sized terrestrial wild mammals used the crossing structures successfully. Most of the crossings were by white-tailed deer (69%). Mule deer and domestic dogs and cats each represented about 5% of the successful crossings. Black bear represented 1.6%.

While the number of black bear crossings was similar both before and after the structures were implemented, more deer use the crossing structures than had previously crossed the highway at grade.

Interested in more statistics, charts, and analyses? You’ll find them in the report (PDF).

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