Personal property springs to mind.
You would have to make the argument that your book bag is subject to search anywhere
until and
unless you enter your own vehicle. It's saying that the method for your transportation dictates the reasonableness of a search. Nothing about your book bag changed having moved from a bus to your vehicle.
But I admit this is more rhetorical than a inquisitive. "Reasonableness" of TSA checkpoints at airports has been justified because of the method of transportation. My point is there is NO legal threshold to cross when moving the searches into ANY form of public transportation or private transportation on public roadways.
Expect suspicion-less sidewalk, car, bus, and train searches in your immediate future.