If I drove every day, my car would probably consume roughly ten gallons of gasoline a week. Commuting by bike for 20 weeks, that's 200 gallons of gas I don't consume. I generally manage to commute uninterrupted for about 26 weeks before short daylight and the need to carry bulkier equipment forces me into the car.
By cycling instead of driving, I make available 150 to 180 parking spaces in the course of a season.
When I lived where I could commute by bicycle all year, the rest of the motoring public got to enjoy that much more of the gasoline and parking I wasn't using up. But now that area has turned into a megalopolitan meat grinder of hustling traffic honking past "Share the Road" signs.