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Environmental Review Toolkit


Exemplary Ecosystem Initiatives — 2011

Arizona: US 93, Hoover Dam to Milepost 17 (MP 17)

Photograph of eight black mountain sheep underneath and alongside the edge of a highway underpass

Figure 1. Black Mountain desert bighorn sheep
Source: Arizona Department of Transportation (ADOT)

The US 93, Hoover Dam to milepost 17 (MP 17) project constructed three wildlife overpasses and four bridges along US 93 in Arizona as a way to promote safe animal crossings. The project integrates wildlife connectivity strategies into the transition of US 93 from a two-lane highway to a four-lane divided highway between Hoover Dam and MP 17 and is recognized for being the first of its kind in Arizona.

In widening US 93, the Arizona Department of Transportation (ADOT) recognized that animal-vehicle collisions could increase and that migratory patterns of the Black Mountain desert bighorn sheep could be adversely affected. The Black Mountain Ecosystem supports the largest herd of this species, residing in northwestern Arizona. By developing a wildlife-friendly infrastructure system, ADOT reduced habitat fragmentation for these and other animals.

To address these issues, ADOT partnered with several agencies, including the Arizona Game and Fish Department (AGFD), Bureau of Land Management, Federal Highway Administration, and National Park Service. Solutions developed through ADOT's partnerships with these agencies created overpasses that allow animal movement but limit interactions with passing traffic.

Photograph of a wildlife overpass across a divided highway, connecting a large valley to a hill

Figure 2. Wildlife overpass over US 93
Source: Arizona Department of Transportation (ADOT)

AGFD conducted a global positioning system (GPS) study to examine migratory patterns of desert bighorn sheep in the project area and identify the most appropriate locations for the overpasses. During construction, AGFD continued to monitor the herd to evaluate its use of the structures. Still cameras and video surveillance provided additional details. This tracking process continues today to analyze habitat connectivity along the corridor.

The project's final design includes three strategically-placed wildlife overpasses, four bridges, and numerous pipe and box culverts. Seven-foot-high game fences frame the overpasses to promote the use of these crossings and limit highway access. The bridges and culverts assist the movements of smaller animals like desert tortoises and protect and restore hydrological features. These structures collectively support access for all wildlife species across US 93 and maintain ecological integrity by providing a connected habitat network.

For more information, contact Michael J. Kondelis, P.E., Arizona Department of Transportation, at