After decades of focus on car designs that improve the crash protection of occupants in their own cars, some theorists have refocused their attention on vehicle aggressivity, or more generally, the compatibility of vehicles when they crash with each other. Real-world fatal crash data reveal important issues of compatibility related to the broad mix of types and sizes of vehicles in the fleet. However, these data also show that incompatibility among passenger vehicles has accounted for only a small proportion of crash fatalities on U.S. roads and that modifications of the more aggressive vehicles, though appropriate and necessary, will have relatively small effects. Interventions to curtail the development and sale of the largest and heaviest passenger vehicles would be ineffective. The greatest benefit can be expected from continued improvement in the crash protection afforded by each vehicle to its own occupants, particularly in side impacts and particularly among the smallest vehicles.