I generally agree that self-driving cars are going to take this niche, but not with tunnels. Tunnels add the same dedicated infrastructure problems as mass public transit.

I'd suspect most car trips today are 1 or 2 passengers with the back seat and trunk empty; we'll eventually see new form factors of on-demand vehicle that trim off unneeded space. If you need to get from A to B alone, no cargo to speak of, you order a ride that covers that class and it's small. If you're taking a shuttle from the airport with your whole family and luggage, you order a ride with those specs.