In many places, the freeway is already there. Self-driving cars/vans/etc are road vehicles and road vehicles come with the inherent advantages of being able to

In many places, the freeway is already there. Self-driving cars/vans/etc are road vehicles and road vehicles come with the inherent advantages of being able to (potentially) traverse a much higher portion of the world than fixed-guideway vehicles. Furthermore, additional coverage area can be retrofitted out-of-band (ie. with map updates) instead of having to build it additional tracks.