I imagine if dashcams were compulsory they would also want to record speed and other data on the video too.
Much of the dashcam footage I've seen on youtube leads me to believe the person posting the video was speeding themselves!
It cuts both ways.
I had that scenario once - fortunately, there were plenty of road markings which I could use. Looking on google maps, I measured the distance between two known points as observed on my cam & then timed the footage. Worked out I was doing 5mph under the posted limit.
I was also accused of tailgating - which was hilarious since I pointed out that for most of the vid the two-second rule could easily be applied. They responded by asking me to look at a particular time slot - where we were setting off from traffic lights! Strange how one person sees it as tailgating when, as we were both moving off, the gap was clearly getting larger.
Maybe the commentor felt I should have waited for the car in front to move two seconds ahead before I even set off?
Actually, looking at the way some people set off from lights, I can only imagine that this is what does happen.
I do get annoyed, though, when you have a nice, straight, country road with a posted 60 limit. There is a serious prang & plod has to close the road for an eternity to take pics, measurements etc. Surely compulsory cams would mean that roads could be cleared up immediately & investigators d/l all footage at the side of the road.
Or am I being too simplistic?
I used to have a work colleague who was a bit of a funny one. He drove like a loony, reckoned I drove too slow & yet one time I picked him up he reckoned I was driving towards him waaaay too fast. I gave him my footage & told him to work out my speed - he had to admit I was only doing 27mph over the 1/4 mile he could see me. And yet you often get pedestrian witnesses say "the car came round the corner way too fast"
