I think what you might be missing is that self driving cars will...

I think what you might be missing is that self driving cars will primarily be a productivity enhacing device...i.e the roads will be used much more efficiently, hence freeing up the effective capacity of the road, and there will likely be many more trips on the roads...the use of cars as taxis is an interesting one but probably a fairly small part of the overall mix.....there will likely be other things they will be able to do under the deed to change effective toll pricing anyway. In short I think self driving cars will boost road productivity a lot which will be good for road owners....