My experience working on roads was always that speed limits, generally 40 km/h, were routinely ignored by the vast majority of motorists.
Best way to stop them seems to be to deliberately create a situation that will make them slow down. Just throw some gravel on the open part of the road, leave a hose lying across it etc. Mud works really well - those with the "hot" cars don't seem to like getting them dirty and hence slow down.
Alternatively, a call to Police requesting a speed camera be set up is effective. Likewise a road worker standing beside the road armed with an ordinary camera and a notebook - most don't realise that Police generally will act on reports of law breaking drivers from roadworks crews and the official proceedure, as per the law, is to take note of all offending vehicles.
The law is the law and there's a damn good reason why those working on the roads don't want trucks especially (yes truck drivers certainly are massive offenders on this one and they're the ones that scare the **** out of the workers for obvious reasons) hurtling through worksites at such speed as to have no chance of stopping safely.
Personally, I see no reason not to put a speed camera in every worksite set to 1 km/h above the posted speed limit. It's clearly a situation where it might actually save a life.