Here's a thought for you: A Prius has just as much of an impact on the environment as a Hummer does, 95% of the time.
That 95% of the time is the time it spends in a parking space.
Mandatory parking requirements are probably the single most destructive regulations on the books today. Developers in most places (and this includes inner cities) must waste phenomenal amounts of space to provide off-street parking for pretty much everything they build- in suburbia this turns into vast seas of baked asphalt surrounding everything in the city.
Public transit users know exactly what I'm talking about if they've ever taken a bus to the mall or a major shopping center. Because the bus stops on the street, it's a 5 or 10 minute walk just to get across the parking lot to the store you want to visit.
Or consider Victoria Gardens: It's a parody of a traditional mixed-use village, a walkable shopping center with pleasant, narrow streets and a lively pedestrian life surrounded by a surface lot so big it can probably be seen from space.
Read this excellent article on minimum parking requirements, and consider just how much it costs to park a car.