The Increasing Threat of Highway-Caused Wildlife Mortality and Barrier Impacts on US Public Lands

Sandra L. Jacobson

USDA Forest Service Pacific Southwest Research Station

Highways wind through all the nation’s public lands, including national forests, parks and wildlife refuges, yet these lands are indisputably the best remaining wildlife habitat. US Department of Transportation projects an annual vehicle travel increase of 2.4% through 2013. DOTs are responding to the public’s transportation demands by increasing highway capacity in terms of speed, number of lanes, and improved nighttime safety. National forests are increasingly the location of choice by transportation agencies for new road construction due to environmental justice concerns and free land costs.

Two types of highway-caused impacts to wildlife are vehicle-caused mortality and movement barriers. Highways can cause barrier effects without mortality because some species will refuse to approach as volume increases. A highway will become a complete barrier to movement due to the risk of mortality at a threshold volume, varying by species’ movement type, highway width and configuration, traffic speed, and the proportion of traffic at night. Research on turtles, a representative slow species, has shown a total barrier to movement threshold of 15,000 Annual Average Daily Traffic volume. Many highways crossing National Forest System lands are either at this threshold or will increase to this level in the next 15 years. Sample AADT’s are over 16,000 on I-40 across the Tonto NF and 26,000-30,500 on I-80 across the Tahoe NF.

This paper characterizes the probability of a highway becoming a complete movement barrier for several movement categories of wildlife, and provides a coarse filter map of highways across public lands meeting or projecting threshold traffic volumes posing a barrier threat to wildlife. Recommended refinements in the suggested methodology can yield predictions on high priority sites for mitigation.

corresponding author:

Sandra L. Jacobson
USDA Forest Service
Pacific Southwest Research Station
Redwood Sciences Laboratory
1700 Bayview Dr, Arcata, CA 95521
707-825-2985
sjacobson@fs.fed.us