you want to use a 'use background' shader on where you want the shadow to fall... it will only render out where the shadow of your object is (alpha included), which is perfect for compositing.

edit: i would recommend creating a 'faked' scene (objects that stand in where the building, street, and sidewalk would be... looks like you've already got them) and put the 'use background' shader on that. then you can composite your car and shadow over top of your original image.