An "accident" is defined as "an unexpected and undesirable event". Accident reconstruction is a branch of causation forensics which involves determining how and why an accident happened - accomplished first by correctly interpreting the clues left by the remaining physical evidence of the accident, then by reconstructing and studying the events preceding, during, and following the accident.
A peculiarity of this profession is that accident reconstructions are methodically worked backwards, time-wise - from the end results back to the beginning sequence of events. All kinds of accidents are investigated through reconstruction methodology plane crashes, crane failures, bridge collapses, vehicular collisions, etc. Reconstructions of traffic collisions, specifically, typically involve determining the factors which contributed to the cause and severity of the collision, such as excessive speed, poor visibility, a defective tire, the failure of an occupant to wear a safety belt, or the disregard of traffic control devices.
Don't the police already do this?
Officers trained in accident reconstruction are rare, and as a result police can, and do, unintentionally overlook fraud and deception and other factual anomalies at an accident scene because they can't reconcile the differences between the physical evidence and the conflicting statements of witnesses. The driver with the most believable story often prevails. Errors aren't uncommon.
Evidence such as tiremarks and furrows in the dirt can quickly disappear. The chances are excellent that, not only will these important clues not have been measured and documented, they won't even be mentioned in the police report.