In a near future, Humans are reluctant as ever to release the grasp on outdated infrastructure. They end up building a highway that consists of a giant conveyor belt, with 'entry gates' that speed up vehicles enough to enter. The idea behind this is that the belt travels at 70mph while the cars on it also travel 70mph, decreasing travel times while keeping it relatively safe to travel at such high speeds due to the baseline of the belt.
Ignoring cost and energy requirements for such a structure, what challenges would one face using this method? How would a vehicle reliably and safely dismount the belt other than an exit ramp that slows you in stages?
EDIT: I was thinking perhaps something similar to the idea about the train that never stops; a secondary platform that you merge on to which then slows down and deposits you back on to the normal streets -- but how would this work with high traffic density?