Prior to using broken glass for speed calculations, a few other items should be addressed:
1) Broken glass upon impact with the ground does not stop on contact, it continues at speed until acted upon by an external forces...i.e. you'll may need to consider and approximate some nominal friction coefficient for glass/ground and use slide to stop equations to determine the 'range' of distances the glass may travel upon impact with the ground.
2) The speed of the vehicle when the glass broke...It does not instantly break and some change in initial impact/travle speed may occur prior to breakage.
3) Any other objects struck prior to hitting the ground...i.e., the other vehicle. If the other vehicle is travelling in the opposite direction or at a speed different than the vehicle with broken glass than the glass may not continue in its initial speed and direction.
