The principle of the conservation of linear momentum assumes that the only forces that act on vehicles during a collision are the collision forces between the vehicles. From Newton's Third Law the collision forces that act on the vehicle are equal in magnitude but opposite in direction. The direction of the force can be reported as a global angle or as a local angle. When measured as a local angle that is relative to the vehicle it is called the Principal Direction of Force or PDOF.

Principal Direction of Force (PDOF)