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Highways and railroads have come under increasing scrutiny as potential agents of population and habitat fragmentation for many mammalian species, including grizzly bears (Ursus arctos). Using Global Positioning System (GPS) technology and aerial Very High Frequency (VHF) telemetry we evaluated the nature and extent of trans‐highway movements of 42 grizzly bears along the U.S. Highway 2 (US‐2) corridor in northwest Montana, USA, 1998–2001, and we related them to highway and railroad traffic volumes and other corridor attributes. We employed highway and railroad traffic counters to continuously monitor traffic volumes. We found that 52% of the sampled population crossed highways at least once during the study but that crossing frequency was negatively exponentially related to highway traffic volume. We found that grizzly bears strongly avoided areas within 500 m of the highway and that highway crossing locations were clustered at a spatial scale of 1.5 km. Most highway crossings occurred at night when highway traffic volume was lowest but when railroad traffic was highest. Highway crossing locations were flatter, closer to cover in open habitat types, and within grassland or deciduous forest vegetation types. Nighttime traffic volumes were low, averaging about 10 vehicles/hr, allowing bears to cross. However, we project that US‐2 may become a significant barrier to bear movement in ∼30 years if the observed trend of increasing traffic volume continues.

We present a method for evaluating the cumulative effects of human activity on grizzly bear (  Ursus arctos) habitat in the Northern Continental Divide Ecosystem of western Montana. Using logistic regression, we modeled the relative probabilities of female grizzly bear resource selection from telemetry data, TM satellite imagery (greenness), elevation, human activity points, roads, and trails. During spring, adult female grizzly bears were positively associated with low‐ and mid‐elevation habitats. Logistic regression coefficients were negative for all road and human activity variables. Summer and fall coefficients were also negative for road, human activity, and trail variables. During summer and fall, females were positively associated with mid to high elevations. Coefficients were positive for greenness during all seasons. Extrapolations of seasonal potential and realized habitat models were made to other areas on the western side of the region where no telemetry data existed. During spring, much of the Bob Marshall Wilderness exhibited a relatively low probability of use by female grizzly bears, but the converse was observed during summer and fall. The mapping and extrapolation process highlighted areas where habitat restoration would have the greatest benefit. These areas were typically low‐elevation spring habitats with high road densities and private lands where urbanization occurred. We recommend that habitat management agencies implement reductions in road densities in seasonal habitat and implement methods to maintain habitat function on private lands.

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