The paper presents a generalised ‘Triangle Method’ to evaluate the energy loss in road accidents. The method allows one to evaluate the energy lost by both the colliding vehicles in car to car or motorcycle to car impacts, considering all the main possible configurations of accident. The advantage over the usual methods is that the method does not require a knowledge of the stiffnesses of the vehicles, and only two parameters are needed to define the damage geometry. The latter can be easily evaluated by visual inspection on a suitable photographic documentation of the damage, without the need to perform any direct measurements on the vehicles. Furthermore, the method can be used also in the very frequent cases in which some of the data about one of the vehicles are missing or in accidents involving side parts of the vehicle, such as areas near the wheels or the front, that have different behaviour from that tested in the classical crash tests. For accidents involving powered two wheelers, the energy loss can be evaluated by motorcycle wheelbase shortening, and a method to evaluate the energy loss by the car is also derived.