The solution seems to be to put of more of them so people constantly expect their presence. More modern designs are also capable of calculating average speeds over longer stretches of the road.
Yeah - here in the UK most long roadworks segments on motorways have a speed limit of 50mph, with average speed checks. It works remarkably well - I barely ever see anyone speed significantly through these segments, as opposed to segments with periodic cameras where people will speed and then slow down for the cameras.