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US Highway 93 Wildlife Overpasses
Providing safety for motorists and habitat connectivity for the largest remaining desert bighorn sheep population in the world

Ewe and Lamb BighornNews Update – June 12, 2012

In the past month, there have been 229 bighorn sheep crossings at the three wildlife overpasses along Highway 93 near the Hoover Dam in northwestern Arizona. This makes more than 500 crossings in the past 15 months. Compare this with 32 crossings at the three underpasses along Highway 68 during a two-year study. The photo at right shows adaptation to using the overpasses - mom is teaching the youngster at an early age.

The three overpasses are at MP 3.3, 5.2, and 12.2. The purpose of these overpasses is to provide improved safety for motorists while also minimizing the impacts of habitat fragmentation, which, along with habitat loss, serves as the leading factors for wildlife extinction. The bighorn sheep herd in this area is one of the largest and most important in the country, and efforts such as these are intended to help ensure the sustainability of this herd for future generations of wildlife enthusiasts.


News Update - February 14, 2011

The Arizona Game and Fish Department recently documented the first bighorn sheep to use the newly constructed wildlife overpasses on Highway 93 near the Hoover Dam. Video and still photographs (below) of two rams, one fitted with a telemetry collar, confirmed use of the crossing structure at milepost 5.2. Fencing along the highway, which “funnels” the bighorns to the overpasses, was recently completed and serves as a critical aspect in getting bighorns safely over the highway.

sheep   sheep   sheep


United States Highway 93 is the direct route for most Arizonans to access Las Vegas and has been designated as both the North American Free Trade Agreement (NAFTA) trade route and a leg of the CANAMEX (Canada to Mexico) Trade Corridor. However, increasing traffic levels and accidents, desert bighorn sheep-vehicle collisions, and compromised Hoover Dam security have led both federal and state agencies to act. To address these issues, Arizona Department of Transportation (ADOT) and the Federal Highway Administration (FHWA) constructed the Hoover Dam Bypass. Completed in fall 2010, this new roadway circumvents Hoover Dam by allowing traffic to pass over the Colorado River on a bridge downstream but within sight of Hoover Dam. In Arizona, 15 miles of two-lane roadway from the Hoover Dam Bypass south towards Kingman is now widened to four-lanes. To alleviate the bighorn-vehicle collision component, three wildlife overpasses and adjacent funnel fencing were constructed to allow wildlife to cross the road safely and exclude desert bighorn from the roadway reducing risk for motorists.

The Black Mountain desert bighorn sheep herd is the largest extant population of desert bighorn sheep in Arizona and has acted as a source herd for more than 25 capture efforts of hundreds of transplanted bighorn throughout the Southwest. Desert bighorn sheep generally occur in small fragmented groups or subpopulations that interconnect in what is known as a metapopulation. A “barrier”, like a roadway, a canal, or fencing blocks the movement of animals from others of their kind or essential habitats. Rams have “rut runs” that consist of movement corridors that lead from one ewe band to the next. Infrastructure can prevent rams from travel, thus disrupting the breeding cycle. Likewise, bighorn may be cut off from lambing or feeding areas or interfere with access to adequate water. To mitigate for these effects, Arizona Department of Transportation, Federal Highway Administration, and Arizona Desert Bighorn Sheep Society provided Arizona Game and Fish Department funding to identify locations where bighorn cross the highway to identify the best possible locations for overpasses that would enable bighorn to safely cross the highway.


Collisions with desert bighorn along U. S. 93 in Arizona occur primarily within 20 miles of Hoover Dam. High quality sheep habitat exists on both sides of the highway in this area with ridges running perpendicular to the road acting as travel corridors for the desert bighorn moving back and forth from Wilson ridge to the east and Black Canyon along the Colorado River to the west. Situated in the Mojave Desert, the resident desert bighorn need large expanses of terrain to carry out their life cycles. The sheep appear to use some areas seasonally, such as lambing grounds, but always need access to water and forage. Further, access to breeding sites on both sides of the highway allow genetic interchange and reduce the incidence of inbreeding. Thus maintenance of these travel corridors is important for healthy bighorn populations.


Aerial persuit of a soon to be GPS collared desert bighorn sheep in the vicinity of US 93 - Photo courtesy of George Andrejko  

In 2004, a 2-year study funded through the ADOT Transportation Research Center began with the capture of 36 desert bighorn sheep; fitting them with GPS radio collars allowing AGFD biologists to analyze bighorn movements and pinpoint locations where the sheep preferred to approach and cross the highway. Information from the GPS also provided researchers a baseline level of permeability, or the ability of sheep to cross the highway. 

The desert bighorn sheep focused their movement along ridges that were intercepted by the highway to approach and cross U. S. 93. 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. Data suggests that crossing structures built at these locations would be the most effective locations for crossing structures. Biologists familiar with bighorn sheep behavior predicted that sheep may readily exploit overpasses as they readily climb cliffs to avoid danger. Given the terrain, the cost of building the overpasses was less than the cost of alternatives (underpasses) and in support of the recommendations in AGFD, the ADOT and FHWA committed to designing and building 3 overpasses; the first ever in Arizona and the first anywhere in the world for desert bighorn sheep! 

Aerial & approach view of a wildlife overpass along US highway 93- Photos courtesy of Scott Sprague

Post Construction

The post-construction phase of the U. S. 93 bighorn sheep movement project began in 2011 with the completion of the capture of 35 more desert bighorn sheep. This will allow AGFD to evaluate the effectiveness of the completed wildlife overpasses and fencing. Permeability of the highway corridor to desert bighorn sheep will be compared to that measured in the earlier phases of the project. Cameras are installed in the vicinity of crossing structures to further enhance our understanding of how bighorn approach and cross the highway and identify other wildlife that may utilize these structures.   


The upgrades to U.S. Highway 93 will benefit the traveling public through efficiency and safety, furthermore, the wildlife overpasses will reduce the incidence of wildlife-vehicle collisions while maintaining habitat connectivity. Although it may take several years for a large segment of the population to readily utilize these new structures, we have already documented their use by sheep even as the concrete seems to barely dry. The partnership between natural resource and transportation agencies represents the first time in Arizona that wildlife research was conducted and results incorporated into project design before construction began and also represents the success in partnerships between agencies of differing disciplines but similar goals.


Our research was funded through the ADOT Transportation Research Center, Arizona Desert Bighorn Sheep Society, Federal Aid Wildlife Restoration program and Arizona Game and Fish Department.


adot federal hgwy admin natl park service bighorn2 bighorn


Final Report from AGFD to ADOT and FHWA:

Other Wildlife Highway Studies in Arizona

Video – Wildlife-Highway Projects in Arizona

For More Information

Arizona Game and Fish Department, 5000 W. Carefree Highway, Phoenix, AZ 85086-5000
Research Branch

Jeff Gagnon
Chad Loberger
Scott Sprague
Ray Schweinsburg


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Related AZGFD Info
- Desert Bighorn Sheep
- Watchable Wildlife
- Sign up for AZGFD eNews
Links to US 93 YouTube Videos
- 1st Bighorn Sheep Overpass Crossing
- Highway 93 Bighorn Sheep Project

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