Accident analyses - speed estimation

[Ken]

I have dash-cam footage of a car accident and realized I should be able to calculate a nearby car's speed.
Count video frames from tire to tire against the white-line road markings.
Given the cam's 30 FPS rate, car wheelbase, white-line size, I figure you can do this.

Has anyone used this?

 

[jokiin]

speed is just time taken to travel a distance, if you have reference markings you can measure then it can be calculated

 

[Nigel]

If you have a GPS log for your camera then you can see how many white lines per second you are passing at known speed, count the white lines per second the other car is passing and multiply your GPS speed by the difference in the white line per second count - that gives you the other cars speed without having to go and measure the location of all the reference points with surveying equipment. If you were at constant speed with a constant GPS speed reading then the result can be quite accurate, if you were changing speed at the time then you need to consider delays in the GPS data etc.