Imagine a slightly longer, single-lane-width version of this running along the middle lanes of Oak and Fell in SF, one every few minutes, almost never late because traffic is irrelevant.

This coming at you from behind is not all that different to a motorbike, is it? You'd have a special traffic light phase at each end to allow it to loop around safely. This has to be much cheaper than digging tunnels anywhere where earthquakes are a concern.