How does one measure them in a video?
OK, for the hard of thinking:
Speed is the amount of time taken to cover a fixed distance. Got that?
So...if you crash, and Police get your video they will:
1. Review footage, and identify the location.
2. They will look at footage and identify two fixed points (lets say lamp posts, but could be anything like road markings, drain covers, trees, whatever).
3. They go and get in their car and drive to the location.
4. They get out of their car, use their tape measure to measure the distance between the two fixed positions. They write that down.
5. They get back in their car and drive back to their office.
6. They play back the video again, noting the time taken to go between the two fixed positions.
7. They use the distance written on their piece of paper, and the time recorded from the video to calculate the speed.
Simple.
The method is also used for manually verifying speed using footage taken by GATSO type speed cameras, where the time taken for a vehicle to pass a known distance (this time it's marked on the road surface) is used to verify the speed. Same maths.
For more reading, check out
https://en.wikipedia.org/wiki/VASCAR which uses the same maths, and was fitted to almost every Police car in the 80s.
I could go on...