Highways put wildlife at risk of negative effects from habitat fragmentation, altered survival and reproduction, disrupted gene flow and more. Interstate 5 (I-5) is a high-traffic highway that runs an unbroken 1,381 miles, from Canada to Mexico, potentially acting as a continental barrier to wildlife. We propose the definition that “a continental barrier is any divide across a continent that prevents the movement of wildlife, population gene flow, and/or access to other populations that results in habitat fragmentation, connectivity issues, and/or population structuring across all ecoregions along its length”.

To develop a method to classify continental highway barriers, we conducted a meta-analysis of scientific and technical literature for I-5. We used an observed (in literature) to expected (from habitat suitability models) ratio approach to characterize impacts. We split I-5 into seven regions based on the US Environmental Protection Agency’s level-III ecotypes and major cities. Using the US Geological Survey’s predicted habitat maps, we created a list of all native mammalian species that are ‘ecologically expected’ (EE) in each of the seven regions. For each of the 115 expected species, we used Google Scholar to search for phrases using combinations of *region*, “interstate 5”, and *Genus species*. Results from over 2,000 searches were collected if they fell into one of two categories: 1) effects of I-5 on any species of interest in a region (OI-5), or 2) any species interactions with highways (not I-5 specific) in a region (OA). Literature attributes included species, data collection methods, and analysis methods. 

 Across all regions, we found that ungulates had the highest OI-5 and OA to EE ratio (0.8 and 0.9). Lagomorphs had the lowest OI-5 and OA to EE ratio (0.32 and 0.37). For all species, Southern CA had the highest OI-5 and OA to EE ratio (0.63 and 0.82), while Northwestern WA had the lowest (0.12 and 0.26). Ungulates are the only order that had a significant amount of OA literature found across all regions. While there was literature for multiple species across multiple regions, and some regions had literature on a majority of species, no single region had a significant amount of species with OI-5 and/or OA literature. 

There are large knowledge gaps for how I-5 affects species across its length. This suggests that while we know how highways affect some species within regions, there is not enough knowledge on how transportation systems affect species to a larger extent. Of the literature analyzed, 82% support that I-5 could potentially be a continental barrier, causing negative effects to species across multiple regions, but there is insufficient research to fully support this claim.