You should watch some of the TV shows where they show the LE and insurance investigator doing accident reconstruction. They check the tires, measure the skid marks, talk to witnesses, look at the damage to both cars and the surrounding bushes, trees, buildings, etc, look at video footage if there is any, etc etc. They can even tell if your headlights and taillights were on and working at the time of the accident by the condition of the filament inside the bulb. Accident reconstruction is a well known science and a standard procedure in aircraft, train, space craft, truck and automobile accidents. Today, they can simple pull the car computer and read it and it will tell them the vehicle speed, how hard the accelerator pedal was being pushed, weather the brakes were being applied, steering angle of the steering wheel and even things like how loud the radio was on and if the windows or doors where open. The computer will also tell them what lights were on and if the lights were working or where burnt out. Erratic driving would be easy to detect by how rapidly the driver changed direction and changed their speed and whether they had to slam on brakes when they turned or stopped or if they coasted to a stop.
Around here, if there's a serious accident and if there is a death involved, the police will keep the road closed for 8 to 12 hours after the accident so that they can photograph and record everything and collect all of the evidence, etc.