The phrase "One Less Car" is indicative of the alternative transportation movement in North America. As the phrase would indicate, the movement advocates the use of non-car (and less polluting) transportation. Bike, skate, blade, walk, bus, just don't (single occupancy) car. The movement isn't just about less pollution; it's also about safer and calmer streets and parks, and about sensible and sustainable transportation.
I was introduced to the phrase "One Less Car" from a sticker on a friend's bicycle. It is an indication that one is doing one's part for alternative transportation by using a bike. I've also seen them on skateboards, on the helmets of rollerbladers, and even on a baby stroller. Why don't you have one?
http://www.transalt.org/ is a good example of a Web site dedicated to alternative transportation.
My personal anecdote: I was going to visit the node on E2 to relate my personal anecdote, but there was no node, hence the above. I was driving down the road in Vancouver's Chinatown and saw a "One Less Car" sticker on the gas guzzling land yacht ahead of me. That's right, a one less car sticker on a car. I've come up with two explanations. Either the driver has a dull sense of humour, or else he uses the sticker as a hunter would the head of an animal he has shot or otherwise killed, and is there as a trophy to signify the kill.