From ArborWiki

Annual August event racing decorated shopping carts. Generally starts at the Fleetwood Diner. If you don't already know when it's happening, you're not the intended audience.

News stories:

• Ann Arbor News on the 2006 races: http://www.mlive.com/columns/aanews/talk_about_town/index.ssf?/base/news-0/1156776095296300.xml&coll=2