An in-depth crash injury investigation was undertaken to provide data to relate the probability of injury to impact severity, in frontal and side impacts. Estimates for the probability of injury for unbelted and belted occupants were established using Probit Analysis This has quantified the reductions in the probability of injury due to seat belt use, and thus provides additional information supporting the effectiveness of seat belts for different impact severities. The results obtained are suitable for matching with experimental test data to establish improved estimates of human tolerance to impact.