The Seattle area already has decided bus lanes (with non - bus right turns). Basically, at-grade transportation that interacts with traffic fails during the peak periods of travel, particularly at turning points, because of other traffic. During the off peak it would work well, but then you've failed to solve the commute problem

If you operate dedicated routes that have limited interaction with traffic you found achieve higher efficienct, but now people are being dropped off at what is functionally a bus stop and have to walk to work. This could actually work really well, except choke points like highway ramps will wreak havoc.

Building an entire extra set of lanes everywhere is prohibitively expensive, and even then is only feasible in the outlying areas

It's a very challenging problem in terms of space utilization and management, even if the technology magically worked in the best possible way. AV only solves some of it