Basically, vehicle collision speeds are computed from crush damage and rest position. Knowledge of how a vehicle came to a stop can also play a part in the calculations if the information is good enough. Engineers use two principles of physics to determine impact speeds - conservation of momentum and conservation of energy. The calculations can be complicated and today are generally done on a computer, but the underlying ideas are simple.
Common sense suggests that higher speeds result in more damage to the vehicles, more crush damage or sheet metal deformation. This idea is essentially correct, and if we know the structural properties of the vehicles, i.e., how stiff they are, we can calculate the crush energy represented by the damage and the associated kinetic energy (speed energy) lost by the striking vehicle. Fortunately, information about vehicle stiffness is frequently available from government or industry crash tests. (Like the ones we see on TV., the barrier impacts with the crash dummies.) Calculations from crush measurements and stiffness parameters are generally used to calculate "delta V" , the severity index, and while part of a speed estimate, they are usually not all that can be done -if additional information is available.
Did you ever try to push your car down the street by hand? It can be done, of course, if the brakes are off and the transmission is in neutral. Vehicles will roll fairly easily if the wheels are free to turn. But suppose you tried to push it sideways. Here the effort required would be very high, something like the force required to move it forward against locked wheel braking, about 80% of the weight of the car. Now picture a car spinning out following an intersection accident - most vehicles don't leave a collision headed in the same direction they had before the impact. The vehicle will eventually come to a stop, but its motion will be a combination of free wheeling and sideways skidding, and generally, a very complicated combination of the two. Speed will be lost in this spinout, the speed the vehicle had at the moment of separation following the impact. The speed after separation can be used to estimate the speeds prior to impact, using the principle of the conservation of momentum. The calculation can be difficult, and the results are only as accurate as the data used to produce them.
See: "Post Impact Trajectory Analysis..", The Accident Investigation Quarterly, Fall, 1994. Or call us for more information.
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