Sheep overpasses could prevent wildlife-vehicle collisions

Thirty minutes from Boulder City to Willow Beach.

That’s all the time it took for Hyrum, my youngest son, and me to reach the water’s edge near the popular marina located about 12 miles downriver from Hoover Dam. Anglers and boaters who have frequented Willow Beach will recognize this as a significant improvement from when heavy traffic, a two-lane highway and security checkpoints in recent years made this trip last more than an hour.

The abbreviated travel time to Willow Beach can be attributed to the newly completed Mike O’Callaghan–Pat Tillman Memorial Bridge — the centerpiece of the Hoover Dam Bypass Project — and the widening of U.S. Highway 93 south of Hoover Dam.

In addition to the improved travel time, no doubt a boon to Las Vegas anglers, we also noticed during our trips to and from Willow Beach three new overpasses along Highway 93. While overpasses along America’s highways are not new, these overpasses are different. They are not accessible from the highway, nor do they funnel traffic onto the highway. In fact, these overpasses aren’t designed for vehicular traffic but for foot traffic. Or, to be more accurate, four-footed traffic — primarily desert bighorn sheep.

Given that Arizona’s largest population of desert bighorn sheep is found in this part of the state, and because the sheep tend to move back and forth on both sides of Highway 93, there was concern that more vehicle traffic could lead to increased sheep-vehicle collisions. There was concern that the widened roadway might create a barrier to the sheep herd’s natural movement between habitats and sheep subgroups.

To lessen those worries, state and federal transportation agencies worked with the Arizona Game & Fish Department to identify areas where sheep cross the highway in significant numbers. The wildlife agency outfitted 36 sheep with radio collars that transmitted the animals’ locations every five hours. During the two-year study, biologists collected more than 75,000 waypoints, data that allowed them to pinpoint four ridges the sheep used when approaching and crossing the highway.

"The ridges offered the bighorn good visibility to avoid predators, higher quality forage compared to surrounding landscapes and terrain that offered the best possible link to larger areas of suitable sheep habitat," the AZGFD said. "Data suggests that crossing structures built at these locations would be most effective in familiarizing bighorn associated with the highway to crossing structures.

"Biologists familiar with bighorn sheep behavior predict that sheep may readily exploit overpasses as they readily climb cliffs to avoid danger."

Since the sheep overpasses are the first of their kind, it will be interesting to see how well the animals adopt them as a means of safely crossing what is sure to be a busy highway.

Between now and 2012, the AZGFD will monitor sheep movement so it can learn how effective wildlife overpasses are as a mitigation measure. If the sheep use them, the overpasses could become a valuable tool for preventing wildlife-vehicle collisions in areas where sheep and other wildlife cross highways in significant numbers.

Freelance writer Doug Nielsen is a conservation educator for the Nevada Department of Wildlife. His "In the Outdoors" column, published Thursday in the Las Vegas Review-Journal, is not affiliated with or endorsed by the NDOW. Any opinions he states in his column are his own. He can be reached at

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