Definitely not upside down, I was thinking of short inclines, possible as a sort of tunnel bypass. Assuming all the cars are networked for mechanical failures and the batteries can handle it. But that's only after the 5 minutes I spent considering the idea and mostly stolen from I Robot movie like most of Tesla's ideas.
The biggest hurdle might be if the public would be tolerant of such extreme angles. It would feel like a slow roller coaster.
You've got to be kidding. That's mighty uncomfortable when the angle is only a bit steep, the inclined lanes would be exclusive to expensive cars with this kind of capability (which no tax payer will want to subsidise), and when there's a traffic jam that takes a minute longer than your battery capacity, your car will just fall on top of another?