They measure skid marks, study angles of impact, analyze paint transfers... in short, they try to reconstruct the accident using reverse engineering. This helps them assess who is at fault and what, if anything, could have been done to prevent it. Their findings also help make roadways safer through improved engineering on substandard stretches of highways, etc.
A highway accident investigator conducts scene-based investigations to record and gather the available physical evidence and examines vehicles and related components. He also obtains details regarding the background and circumstances surrounding the incident.