Driving & Driving Test Tips: approx speed of vehicle, momentum analysis, departure angles

Question
A collision involving an suv and a car.  I would like to estimate the speed of the car.  I was in the SUV, which was hit head on at the rear wheel (the SUV was making a left turn and the car was going straight (through a red light), and spun it 180 degrees within (approx) a second.  The car stopped 100ft past the collision (according to the officer) and the debris was 50ft beyond the collision (again according to the attending officer). My question is basically how fast do you think the car may have been going in order for it to take 100ft of road following a collision with an SUV to stop?
I appreciate yor time and expertise (if you need more details please let me know)

Elizabeth

Answer
There is a lot of info that I would need to complete an analysis of this sort.  First you must remember that a vehicle at speed is carrying kinetic energy (KE=1/2 Mass * Velocity (in feet per second) squared).  Part of this energy would be dissapated by the amount of damage done to the vehicles.   Then the rest would be dissapated by coming to a stop 100" from the point of impact, and the amount of energy that was imparted to the SUV to get it to rotate.  But I would need to know if the driver of the car braked that distance (which is not usually the case) or the car came to a stop by rolling or some type of braking (such as a locked wheel or wheels from the damage it sustained), and the speed of the SUV prior to the impact.  This would then have to be put together with the angles of approach of the two vehicles, and then their respective departure angles from the point of impact.  These are the ingredients needed to accurately complete a momentum analysis to estimate the initial speed of the car before the impact.

If the car made a panic stop, then it would have been going roughly 45 miles per hour (after the impact), but drivers don't usually have the cognition to brake after being involed in a collision.

If you have more information that you can provide even though you may not have all of the info I listed, please send it on to me.  I definitely would have to know how the car came to a stop post impact and could probably then be able to make a better estimate of the speed.  Photos would also be a plus to assess the speed needed to cause the damage sustained by the respective vehicles.