I agree on mass transit. Highly recommend Adam Something's youtube video on why self driving cars will increase traffic and waste. Its not a solution for cities large or small. Rural communities may see benefits but they pose weirder problems.
Because at least in the US the airline and car industries hand shake to stop commuter trains.
The west coast regions also have an additional problem where the slopes will need massive amounts of tunnels for high speed rail and are complicated by a lot active geologic zones. So while its the best solution (trains) its expensive but Japan managed to do it. Its not going to be cheap or quick to build the needed infrastructure. Add in most people are heavily invested in car infrastructure when they buy a car. So there's a public will barrier here built out of billions of garages, cars, and driveways sold.
People also pose "flying cars" etc as a solution. Piloting air vehicles requires air traffic controllers and communicating on an extreme level in addition to pilot licenses and security problems. Its not also not a serious answer to transportation.