France, Germany, and Japan aren’t terribly flat. Their HSR also goes through populated areas between major cities. If anything, the problem is too little population in CA, not too much.

Robo-cars can’t travel at 150+ mph and pick up power while they’re moving. Not to mention that a fixed guideway is safer at high speeds and metal/metal friction is lower than rubber on road.

The best solution would be HSR and rail with short-distance rental cars at the endpoints.