While self-driving cars already are being tested on public roads, newly released safety data support the cautionary view that the technology has many miles to go before people can sleep at the wheel.

That doesn’t mean relief is decades away for drivers weary of commuting. It’s possible, even likely, that within a few years mainstream cars will be able to drive themselves reliably — on routes they have mastered, in weather they can handle and on the premise that a driver will be ready to take over when needed. (The White House last week said it will work with auto companies and state governments to speed up the arrival of driverless cars.)

Traditional automakers and technology companies such.. read more