I dunno, I've driven around Chicago, particularly on the Dan Ryan and have not seen a single van parked in a construction area since this has gone into effect. I am looking for them, that's for sure, but they aren't there and the traffic sails along, probably mostly about 65 mph or so. Which is less than the 75-80 mph people usually drive through there if there is not traffic ground to a halt.
If they were REALLY fiendish, they would put them in porta potties, of which there are quite a few, and they would be much harder to pick up visually than a van or van-sized object.
A few years ago, they had a dummy sitting in a car parked on the far side of an overpass that worked very well at making everyone slow down. I don't recall hearing about any accidents involving drivers where those were parked but it worked very well as a deterrent.