Vehicle-to-vehicle crash compatibility is a complex subject that has been extensively researched during the last 40 years. For the purposes of this paper, compatibility is defined as the optimisation of vehicle design to help minimise the number of injuries and fatalities that occur in collisions between passenger vehicles. For the evaluation of compatibility in these collisions, the criteria of selfprotection and partner-protection are considered together in a measure of ‘total safety’. It is also shown that separate evaluations of self-protection and partner-protection should not be used to guide regulatory policy on passenger vehicle to passenger vehicle compatibility because they are less effective at bringing about reductions in the total number of injuries and fatalities in passenger vehicle to passenger vehicle collisions.

Front-to-front passenger vehicle collisions from the German In-Depth Accident Survey (GIDAS) relational database are evaluated, and it is shown that, in a collision between two vehicles with unequal masses, the driver of the lighter vehicle typically experiences a higher risk of injury than the driver of the heavier vehicle. However, by analysing these accidents at the collision level, it is shown that the ‘total safety’ of front-to-front collisions between passenger vehicles in the German fleet is independent of the mass ratio of the involved vehicles. In other words, the ‘total safety’ of a collision between a heavier passenger vehicle and a lighter passenger vehicle is equivalent to the ‘total safety’ of a collision between two equally massive vehicles. It is therefore concluded that mass-dependent criteria cannot be justified as the principal evaluative measure in future regulations that aim to address compatibility in front-to-front collisions between passenger vehicles.

Structural homogeneity is analysed using collision simulations between a mid-sized passenger car and a larger Sports Utility Vehicle (SUV). Vertical and horizontal structural homogeneity are analysed separately by using homogeneous ‘shields’ as substitutes for the bumper crossbeam structure. The simulations show that the vertical alignment of primary structures and improved vertical homogeneity result in improved compatibility. If vertical homogeneity is achieved, horizontal homogeneity between the main load paths does not provide additional benefit and hence this should not be prioritised in a compatibility assessment. The assessment of horizontal homogeneity is only relevant for small overlap collisions outboard of the vehicles’ longitudinals.

Finally, the ability of various barriers and test procedures to evaluate compatibility is discussed. It is concluded that vertical alignment may be evaluated by measuring load cell wall forces and that low speed tests may be used to improve homogeneity.