Excellent picture from Muenster, Germany, demonstrating the amount of space needed to transport the same number of people by car, bus or bike. 

The original image can be found here, with discussion here.
Climate change and peak oil are primary reasons to move to more mass transit and cycling, but this image makes it pretty obvious how much extra space we'd have if we did it, too!

Kind of puts a lot of things into perspective - how much of our planet do we really want to dedicate to our obsession with private cars?