In basic traffic accident investigation, you report what happened. In the reconstruction phase of a collision, you not only report what happened, but you find out why it happened. You find out what caused it, what could have prevented it, and what will prevent it from happening again. In many cases, you find that there are multiple causes that all worked together to cause a collision. It's like a giant puzzle, but it's real.
Some puzzles are harder than others, some much harder.
You have to try to find all of the pieces of the puzzle and put them together. Then once you have found the right pieces, the true cause of the collision can almost always be found. All of these are professional organizations which promote testing and research, education, knowledge and safety in the field of traffic accident reconstruction. These organizations have a broad range of members from many different fields, including law enforcement, private investigators, attorneys and engineers. My experience goes beyond my police background to include investigations for insurance companies, attorneys and private investigators.