In What is Accident Reconstruction? Dr. Alexander Zhukov, M. Eng., Ph.D., and accident reconstruction expert witness writes:
Accident Reconstruction is a scientific attempt to determine how a traffic accident occurred. It is a procedure by which the circumstances of a traffic collision are proven or estimated by working backward from the resulting damage and evidence. For example, the length of skidmarks, final resting positions of vehicles involved in the crash, and the amount of crush damage to the automobiles can be used to determine, or at least approximate, the speeds and/or directions of travel of the vehicles before the collision.
An Accident Reconstructionist is a person specially trained in using these procedures to determine the collision circumstances. The Reconstructionist is an expert witness usually retained by either the plaintiff or defendant in a lawsuit, who can be of great benefit in cases where fault for a collision is in dispute. Some police agencies have their own personnel trained in accident reconstruction who will be assigned to prepare independent reconstruction reports about particularly severe traffic collisions. Do not assume, however, that the police officer who prepared your accident report is a qualified expert. Most of the time, he is not.