It made sense in The Caves of Steel, written several years before by Asimov, at the much smaller scale of an indoor city. On an interstate scale it honestly seems goofy and inefficient.
Sure, as you're moving a lot more material per person (and even if the engine is more efficient, upright humans aren't optimally aerodynamic -- or even as good of an approximation as typical cards -- so end to end efficiency is still going to be a problem.)