This summer, Audi held an elaborate media event in Spain to celebrate a new sedan that would allow drivers to let go of the wheel and pedal while in traffic jams and pay attention to something else. The driverless technology heralded a new era for mainstream auto makers in their pursuit to create fully autonomous vehicles. But the rollout by Volkswagen AG's luxury unit--with production for the U.S. originally planned to begin this year--is now uncertain, as the car maker continues to wait for Congress to pass legislation paving the way for autonomous vehicles. "It's not a matter of needing permission, it's more a matter of wanting to introduce the car when we know what the ground rules are," said Brad Stertz, Audi's director of government affairs, who acknowledged a shift in timing. Audi's conundrum reflects the challenge for auto makers as they plunge into an awkward phase of development years before fully driverless cars are ready: how exactly to pass control back and forth between driver and machine.