Why momentum and not energy? I'd say the latter. It is energy that deforms your body. So, reduce the speed by 15%, reduce energy by 28% (2*15% to first order, o

Why momentum and not energy? I'd say the latter. It is energy that deforms your body. So, reduce the speed by 15%, reduce energy by 28% (2*15% to first order, or 1-(1-15%)^2 exactly). Or, to put it differently, if the vehicle has twice the mass, speed should be reduced by 30%.