Some days your city is slipping on their responsibilities and you gotta grab crew and do it yourself. An ‘Urban Repair Squad” in São Paulo did just that by painting their own crosswalks on several dangerous intersections. Problem is São Paulo is home to 11 million people, it’s impossible to find a time when the streets are quiet enough to do some guerrilla urban design. So when would all of this mega city be preoccupied indoors? Futebol! Worldcup!
During the different Brazil games, these activists skipped the matches and instead made their city a better place.
Crosswalks are one of the cheapest ways to improve safety in the city. All you need to do is slap some paint down and cars can tell that this painted piece of property is parceled for pedestrians. The more visible and distinct the crosswalk, the more noticeable it will be for drivers, and the safer your grandma will feel crossing the street.
Other folks have taken DIY crosswalks even further. Banksy was the first I remember, using the zebra stripes to get revenge on someone. 
While other groups have used crosswalks to raise awareness of their importance in the public realm.
More crosswalk art can be found at Web Urbanist. I found out about this all from Good.
Hack Your City 