[dragonwriter]

> Technically it would be no big problem to enforce speed limits widely

More than you suggest.

> The technology for speeding cameras isn't exceptionally complex

But speed cameras are just one piece of enforcement. With those and automatic license plate reading you get a piece of evidence that a car violated the limit at a particular time and place, but even if the law is that a set limit is a hard limit (which is not the case in much of the US, where posted limits are often prima facie but not dispositive limits), that's not all you need for enforcement. You also need legal process to weigh potential counterevidence, to deal with contested identity of actual drivers, etc. This isn't technically complex, but it adds a lot of overhead, which is why even places which legally allow this mechanism deploy it selectively, not comprehensively.