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.