Not sure if you CAN kill those. I never bothered trying because after about 6 rounds from the Tiger's 88, I figured it would be more effective to just go after the convoy themselves.
EDIT: completly irrelevent, but I hear tell you used to (still are?) be able to destroy the runway of an airfield. Wouldn't affect anything, it would just look bombed out and crattered. But it required an insane amount of ordnance (somethine like 72k?) to destroy.