Traffic and parking are the biggest issue i see with cars and space efficiency. Both can be significantly improved on with self driving. Especially if most people opt for public ownership of cars and not private. Something think will become more popular as self driving takes over and lowers the cost of taking the self driving equivalent of a taxi or Uber.
By the way i think self driving cars will make trains more popular. As trains suck at first and last mile transportation. Self driving solves the first and last mile issues.