Sites like Mcity in Ann Arbor, Mich., are like miniature cities, allowing companies such as General Motors, Honda, Ford, Nissan and Toyota to test driverless cars. The real question they face: How good do autonomous vehicles have to be to replace cars with drivers? “When you no longer have a human doing the sensing and decision-making, the car has to be flawless,” said John Maddox from the University of Michigan Mobility Transformation Center.