Some highways were built in the Basque country before environmental impact assessment processes required the implementation of wildlife crossing structures. Some highway sectors were selected because they intersected with sites of high landscape connectivity between protected natural areas. These were identified using a Geographic Information System-based connectivity model that represented the spatial ecology of large and medium-sized mammal species. Permeability of road sectors was analysed by calculating the density of crossing structures considered adequate for target species and comparing it with the recommended density in technical prescriptions for new highways. Additionally, factors that could limit movement across these structures were also analysed, such as the shortage of forest cover in the surroundings or the presence of obstacles. An approximation of permeability deficits has been obtained, and a set of corrective measures that must be implemented into landscape defragmentation frameworks has been identified.