Another way to look at the question of the semis crashing into each other (Season 8) is to consider the case of one semi travelling at 50 mph and crashing into a second semi that is stationary, with the brakes locked. Obviously, the second semi's brakes are not strong enough to instantaneously stop the moving semi. (Its own brakes could not do it!) Thus, at collision, some energy is dissipated in skidding the tires and brake pads of the stationary semi. This energy is not available for deforming the moving semi, which takes deformation equivalent to a crash into a wall at some speed *less* than 50 mph.

Suppose, instead, that the second semi is in motion. How fast does it have to be going in order to be equivalent to a solid wall? As the masses are equal, it must be going the same speed, 50 mph. That is, each semi perceives that it is hitting a solid wall at a speed of 50 mph.