Lower extremity injuries are a frequent, costly, and debilitating result of automobile crashes. Accident surveys attribute most below-knee injuries to intrusion of the footwell structure. This paper correlates the level of footwell intrusion with the frequency and severity of lower limb injury. The 1988-1993 NASS files reveal that the severity and likelihood of below-knee injuries are highly dependent on the vehicle change in velocity during the crash and the level of intrusion, but not on the size of the vehicle. Nevertheless, 71% of below-knee injuries sustained by front seat occupants in head-on crashes occur with less than 3 cm of intrusion. A trauma center study of lower extremity injuries sustained in severe crashes showed trends similar to the NASS results. NHTSA crash test data exhibited no correlation between axial leg load and the magnitude of footwell intrusion. A parametric computer simulation of offset frontal crashes also showed little sensitivity of the axial load to the level of intrusion. The accident investigations, crash test data, and simulation results suggest that factors such as the vehicle's change in velocity and the rate and timing of intrusion must be considered when examining injury mechanisms of the lower extremities.