Recently the streets of Lyon were taken over by faux traffic signs that matched real ones in color scheme and dimension, but had an entirely different message:
One hundred signs ranging in subject matter from fish to hearts to literal messages were placed all over the city in a manner that would likely make you do a double take if you actually came across one. (Hmmm, does a rabbit / fish mean a two-way street? Is that net thing-y a sign for a geodesic dome ahead?). Made by 52 artists from around the world, the street sign project is meant to make you stop and think, and in many ways to stop and smile.
The signs were erected in a bid for the title of Capitale Europeanne de la Culture en 2013, which they unfortunately lost. Marseilles was named instead (they certainly didn't turn their streets into a gallery).
See all of the signs on the BoPano Website.