In perhaps 10 to 20 years, self-driving cars might double the capacity of the existing freeways (without adding any lanes).   Small self-driving buses might circulate through neighborhoods more or less continuously, connecting people to the main bus lines.  "Ride-sharing" of self-driving cars (uber, lyft, etc.) may increase transportation services (lower cost, better service) without increasing the number of vehicles.  So, there will be alternatives on about the same time scale as building a giant system.  The giant system is definitely risky.