Comment by JimboOmega

It allows the entire system to be controlled by one system - e.g., no need for each car to have its own LIDAR system. ATO is way easier than self-driving cars. You get all the benefits of being able to group vehicles together, have all stop simultaneously in response to hazards, etc.

Plus, like with most rail systems, you get the big benefits of having a custom right-of-way. No need to contend with pedestrians, people parking in the traffic lane, vehicles that aren't as fast as others, etc etc.

Of course... if you want to build a new underground right of way, what's the best way to use it? Is a big open question in my mind. You could go for electric BRT, for instance, or big-dig style freeway.