NHTSA has developed and conducted a vehicleto- vehicle crash test program to evaluate the statistical correlation between vehicle performance measures and the probability of driver fatality in a crash partner vehicle. The test program uncovered some concerns regarding NHTSA’s rigid barrier data collection and review methods. The vehicle-tovehicle tests did not provide clear insight into the mechanism behind the fleet correlation, but did emphasize the complexity of vehicle compatibility and the changing safety priorities related to improved occupant restraints.