I don't know if they have these in other parts of the country, but here in the Bay Area the air quality control board declares days where the pollution levels are higher than normal as Spare the Air days. In fact today (13JUN2000) is a spare the air day. The idea behind this is to correct the pollution level by asking people not to drive their cars, or if they are going to drive, not to drive more than necessary. It isn't enforced and is completely voluntary, but you are strongly encouraged to participate. Ironically traffic is usually a lot worse than normal on Spare the Air days. There are very few people who seem to care.