> If I would have to rate difficulty of Traffic enforcement, I would have put it quite low... What makes it so difficult?

The problem with traffic enforcement is that the police want it to remain available as a pretext to stop anyone when they want to.

For example, the MUTCD recommends that speed limits typically be set to the 85th percentile traffic speed. One of the reasons for this is that speed differentials are very dangerous. Another is that you typically want laws to have at least a little bit of leeway between "thing law abiding people are expected to actually do" and "thing with government-imposed penalties", rather than having them separated by 0.

But US speed limits are regularly set 10-15 MPH below that speed, so that the majority of traffic is exceeding them. Because then the police have an excuse to pull over anyone they want to. This is also often used a source of fine revenue because as long as they don't do so much enforcement that people actually comply with the law, they can periodically park a patrol car on the side of the road and "efficiently" hand out one ticket after another.

Which is the problem with automated enforcement. If people were consistently fined then you would get compliance rather than widespread violations. At which point people might object more strongly to the speed limits not being set by the book. But regardless of which way that comes out, it causes you to lose the fine revenue and the pretext to stop anyone, since you stop having both of those if you do either of the things that results in most people being in compliance with the law.