Optical illusions have no place on the road. You might think it wouldn't enter anyone's mind, but apparently that's not the case. Some time ago there were trials conducted with painted speed bump illusions, and now a company called Preventable is running trials with a decal made to look like a child playing with a ball. These are bad ideas for two reasons. One, when drivers encounter them first, they draw undue attention from other driving tasks, potentially causing more accidents in the process. Second, once drivers get used to the illusions, they are trained to ignore them, which means they are less likely to pay attention to real road hazards, causing more accidents in the process.
In short, the upshot of optical illusions on the road is more accidents and more complacent drivers, which is the exact opposite of the intended effects. Ironically, it might be safer to put real people on the road in order to get drivers to slow down. At least they won't learn to ignore them.
Mokalus of Borg
PS - Perhaps what we need is a dynamic speed-responsive hazard.
PPS - That is, one which reacts more strongly the faster you're going.