Optical Illusions Used to Slow Traffic

Philadelphia is reportedly using some interesting new road decals that are made to look like a road obstruction (but are otherwise flat) in an attempt to slow speeding drivers:

City, state, and federal officials will roll out a program aimed at getting drivers in Philadelphia to heed the speed limit. One part of this involves 3-D images that will look like bumps in the road.
...
[Chief traffic engineer] Denny says this effort includes the deployment at about 100 intersections of high tech decals that create a 3-D image, to make drivers think there's something in the intersection:

"Plastic material that is laid down, and it gives the illusion of being a hump in the roadway. And therefore people react to it as though it were a hump, and slow down. The driver sees this in the roadway, and they think that its some protrusion up out of the roadway, and not a perfectly flat surface. So they slow down before they drive over it."

Interesting. Hopefully the decals don't cause drivers who are speeding to slam on their brakes and cause the other drivers following too closely behind to rear-end them.