yes pretty simple, say there are 71 yards between the 2 points, and you covered that distance in XX.x seconds in the video, add a little math converting yards / second to miles / hour and you have your speed.
So if you are really worried about your speed, the last thing you want to do i videotape yourself driving, or at least let "the man" get his fingers on that video
Good thing is dashcams are still a fairly unknown thing in the west, so with a half stealthy install and no one will ever notice your camera anyway.
Im okay with speed now, and even on the occasions where i slip a little over the limit ( just about every drive i take ) if i get caught out just pay the price and move on.
On the motorways here the default speed are 130 km/h with 110 zone at connecting roads, but i never do 130, don't feel the need for it 110 are fine for me.
Where as in the 90ties it was 150 - 200 km/h all the time, and back then the limit was 110 on the motorways.
so some good do come of getting older.
In the old days when cops still worked here, they some times used helicopters to time cars in between 2 T signs painted on the road, and then radio to a cop up ahead to pull the car over.
But that was in the old days and cops working with military as police here don't have helicopters of their own, but i don't think they use that format any more now that they got laser guns and the ATK cars.
Denmark only have 10 fixed speed traps, but around 100 of the ATK cars, and god know how many laser guns,,,,,,,, and a cronic shortage of police, we are down to 50% makpower due to many cops going on pesnion and this come as a surprise to people in charge.