Hey romullus,
the problem with the shadowcatcher material is, that the cars are reflecting the tunnel materials. If i change the material of the tunnel, here to shadowcatcher, the cars are not reflecting anything. The only workaround here, is to use the rendered image of the tunnel as backgroundimage for the shadowcatcher. This would be fine for the time the cars are in the tunnel, but later in the animation the cars are outside where many tress, houses etc. are narrow to the street. A solution line "render selected for compositing" would be realy nice. ;)