Cars that drive themselves, long a staple of science fiction, could be in auto showrooms in the next few years. Automakers and researchers around the world are testing and refining technologies that allow a car to know where it is going and to communicate with other vehicles. Special sensors and software make the breakthroughs possible. Already, cars are selling with automated features designed to keep them in the correct lane, brake to avoid collisions and park themselves. Technology giant Google, which has tested vehicles with self-driving features on a half-million miles of roads, recently demonstrated a car with no steering wheel, accelerator or brake pedal. It remains unclear, however, how safe super-smart cars would be, how they would affect traffic congestion, how consumers and the nation would pay for the cars and the supporting infrastructure they would need and whether Americans will accept such a radical change in their relationship with automobiles.