I'm reposting this from another thread:
If you want to get the cars out of your city, even if for starters it's just in a small area for one day, September 22 is Car Free Day all over the world. Participate or organize an event in your city. Portland has organized a street fair during which we're going to reclaim some public space for a day with workshops on how to be car-free, art, music and bike fun.
Somewhere in the UK: http://www.lbwf.gov.uk/index/news/car-free-day-04.htm
Int'l index: http://www.ecoplan.org/carfreeday/cf_index.htm
History of Car Free Day: http://ecoplan.org/carfreeday/general/origins.htm