> Surely this isn't something we need to guess at, there are roads with such systems and similar roads without and those can be compared.
It's mainly done for political reasons or money raising reasons rather than safety. These cameras are usually on motorways which are the safest roads to drive on. It's all just for show, it doesn't actually make us any safer.
At least in Scotland, they're predominantly used during road works (and they do vanish afterwards). I can imagine they're worried about incidents especially when there's at times only a line of cones separating cars from workers.
Also I would think the majority of cars have cruise control (the last three cars I owned were all less than £2000 second hand and all had it) - driving at a specific speed is a button press away.