Evaluation of Bighorn Sheep Overpass Effectiveness: U.S. 93 Long Term Monitoring

Bighorn-vehicle collisions are a safety concern to motorists. Furthermore, highways block animal movements between seasonal ranges or other vital habitats, fragment habitats and populations, reduce genetic interchange, and disrupt viable population processes. Recent research on desert bighorn sheep movements and crossing patterns associated with US 93 point to the barrier created by this highway through the largest bighorn sheep population in Arizona (AZ), as well as the impact of bighorn-vehicle collisions (McKinney and Smith 2007). US 93 (MP 2-17) was reconstructed from a 2-lane to a 4-lane divided highway beginning in early 2009 and was completed in late 2010. Concerns exist that this reconstruction could further fragment bighorn habitat and contribute to increased bighorn-vehicle collisions.The Arizona Department of Transportation ( ADOT) constructed 3 wildlife overpasses (the first in AZ) and 2 bridges, with fencing linking the structures to promote bighorn passage across the US 93 corridor while reducing motorist collisions with sheep. This represents a tremendous commitment on the part of ADOT and warrants thorough evaluation to assess the efficacy of overpasses in promoting permeability, particularly since few overpasses exist in North America. To assess the efficacy of the planned US 93 wildlife overpasses in promoting permeability, it is necessary to compute bighorn passage rates as an objective metric, comparing passage rates before and after the passage structures are completed using GPS telemetry (Dodd et al. 2007). Wildlife use of the overpasses will be monitored by video surveillance. Research here will complement ongoing studies along SR 260, I-17, I-40, US 89, and SR 64 and together they provide valuable insights into the impact of traffic volume on a range of wildlife species and approaches to mitigate highway and traffic impact.