California regulators puzzle over how to know that (driverless) cars of the future are safe

California's Department of Motor Vehicles will miss a year-end deadline to adopt rules governing the self-driving cars of the future.

The reason: Regulators first must figure out how they'll know whether the cars are safe.

Safety is a chief selling point of self-driving cars, which thanks to an array of sensors promise to have superior road awareness and reaction time.

But with the technology still in prototype testing, there are plenty of unanswered questions: What happens if a car's computers freeze?

DMV officials say they won't let the public get the cars until someone — whether the manufacturer or an independent tester — certifies that they're safe. But for now there are no agreed-upon standards, so rating safety would be like grading a test without an answer key.