Driverless cars are not perfect, but just how safe must they be before being set loose?

DETROIT – As autonomous car technology rapidly progresses, makers of the cars face the difficult question of how safe they must be before they're ready to move people on highways and city streets. Right now, companies such as Google, Audi and Mercedes-Benz are testing the cars in a small number of cities to demonstrate they can be safer than human drivers. They also must figure out what level of risk is acceptable to both government regulators and a potentially skeptical public. Government statistics show that human mistakes are responsible for 94 percent of the 33,000 U.S. traffic fatalities each year. Autonomous cars won't get drowsy, distracted or drunk, so in theory they could eliminate those mistakes and save an estimated 31,000 lives a year.