I want to try to respond in a somewhat "pragmatic" way. First of all, I don't think most cities can afford to build out heavy or light rail to the extent that would be necessary to provide adequate transit for most everyone in an affordable way (both fares and taxes). NYC is an exception and also Washington, DC. Other than that a combination of Bus Rapid Transit combined with advances in self driving cars will ultimately be the solution. I don't even think we are utilizing the full extent of the highways we already have because humans are stupid drivers in the first place compared to what you will see with self driving cars. You can have the cars riding within a foot of each others bumper at high speed and enforce sensible merging and lane changes no matter whether the occupants like it or not. With all the cars traveling very close together and efficiently and smoothly I think you will find that the capacity of every single highway will increase dramatically. Another factor to consider is services like Uber and Lyft once they migrate to self driving vehicles. The fares could be reasonable and you would just order a vehicle on your smart phone where ever you are at to get a ride and the system would automatically send a car to you and you'd even be able to see it arriving on the GPS on your phone. I look forward to this day. Speed limits, congestion, merging, lane changing, stopping, driving conditions can all be managed responsibly and allow a far higher volume of traffic to pass through a given highway. Humans have proven that they don't want to drive properly or safely so I look forward to the day where it is mostly all enforced as self driving and computerized traffic management and flow to go with it.