Really smart autonomous vehicles that automatically do ridesharing and that intelligently pack vehicles full of 4-6 people on their way in could do it. At that point you're dealing with an experience that's not too different from a bus though (i.e. sharing a space with strangers). There's not enough capacity for everyone to commute in on their own individual autonomous vehicle, nor is there enough parking downtown, so the vehicles would have to be commuting twice daily.