United States Department of Transportation
i
Evaluation of Desert Bighorn Sheep Overpass Effectiveness : U.S. Route 93 Long-Term Monitoring
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2017-05-01
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[PDF-5.52 MB]
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Abstract:During the planning stage of upgrading U.S. Route (U.S.) 93 from a two-lane highway to a four-lane divided highway, there were concerns for the local desert bighorn sheep (DBS) population. Vehicle collisions with DBS were estimated at 11 per year prior to reconstruction and would likely increase after reconstruction. To overcome collision concerns, the Arizona Department of Transportation (ADOT) integrated three overpass (OP) structures—two 50 feet wide and one 100 feet wide. They are Arizona’s first wildlife OPs and North America’s first OPs for DBS. These OPs, along with three culverts and the dry washes under two bridges, were linked with fencing to limit DBS access to U.S. 93 and guide DBS to the safe crossings. From March 2011 through March 2015, researchers evaluated the success of these mitigation measures with video and still cameras, Global Positioning System collars on DBS, and DBS-vehicle collision monitoring. DBS used the OPs (5862 crossings) more than the dry washes under the bridges (474 crossings) and more than the culverts (195 crossings). Passage rate (crossings/approaches) at the OPs increased by 210 percent from years one to four of the study. Collared DBS crossings and passage rates increased by 100 percent and 1367 percent, respectively, from years one to four. DBS shifted their distribution of crossings more evenly across the study area and crossings became focused at crossing structures. Vehicular collisions with DBS were reduced by 68 percent in the first two years of monitoring until ADOT maintenance personnel addressed DBS breach points, after which only one vehicle-killed DBS was documented during 2011-2015, accounting for an 86 percent reduction overall and a 97 percent reduction in the years following repair of the fence breaches. These findings indicate that appropriately located 50-foot-wide overpasses connected with 7- to 8-foot ungulate-proof fencing, along with animal escape ramps, can reduce DBS-vehicle collisions and promote habitat connectivity. Post-construction monitoring can document effectiveness of mitigation measures and quickly identify areas of DBS access for modification or maintenance, to ensure long-term success of the measures.
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