Very nice vehicles, though I thought I'd quickly mention something; the wheel model you used on the semi tractor and other heavy vehicles is off centre. Can you fix that, perhaps? Also, have you looked into other ways of making the animations work on certain vehicles (bus, dump truck). Some prefer having their brakes intact. The third thing is probably just nit-picking, but can you try smooth shading the curvy bits on your vehicles?
Otherwise, keep up the good work!