What are the responsibilities of a skid mark expert

Well a person who does skid marks will normally be an accident reconstuction expert, ( at least in real life) whose job will be to check out an accident scene, or latter by reviewing the autos and the scene and photos form the scene and determine postions of the cars and the such.

But as far as the skid marks, first they can determine what type of tire it was, but a computer tire comparison. Next they can look for and normally find any speical markings on the skid ( tread) so they can match up a specific skid mark to a specific tire.

For braking skids, they get an idea of the speed of a vechile and can compare that to initial point of impact and any secondary impart points.
Since the amount of indetation at the POI should match the estimated speed. ( you can get a determination of speed by POI something's also.

So you can determine direction of travel, estimated speed, type of tire and the such.