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California on Wednesday unveiled proposed regulations for autonomous cars, a long-awaited step for car companies seeking guidance before eventually selling them to consumers.

The proposed rules hold motorists responsible for obeying traffic laws, regardless of whether they are controlling a car's movements. Deciding how much to hold computers responsible for accidents and other missteps has been at the center of burgeoning debates over how to regulate driverless cars.

Autonomous vehicles are far from being offered for sale alongside other cars, but are increasingly under development among both traditional auto makers and Silicon Valley companies such as Alphabet Inc.'s Google. Regulators in California are taking the unusual step of developing regulations for the vehicles ahead of time as harbingers of them pop up in traditional cars through features including automatic brakes.

California's proposed regulations would require consumers to get a special state-issued driver's certificate after receiving training from a car company on how to use a driverless vehicle. Autonomous cars would also have to pass a test administered by a third party before being sold. Auto makers would only be allowed to lease driverless cars, as opposed to selling them outright.

The regulations also won't permit a car designed to move without a driver, which means Alphabet's driverless vehicle prototype initially wouldn't be allowed. An Alphabet spokesman had no immediate comment.

The state's motor-vehicle department is planning two public feedback sessions on the regulation, the first to be held in late January.

Alphabet has been testing autonomous vehicles in California since 2009. It has a special permit for testing, but that requires a specially-qualified driver sit behind the wheel ready to take over controls.

The technology company has a fleet of vehicles running around the state and earlier this year started testing in Texas, a state that has no regulations. Other states, including Nevada, Michigan and Florida, have similar testing regulations. Other states have decided not to regulate autonomous cars for now.

"Our concern is safety," Brian Soublet, the California Department of Motor Vehicles' chief legal counsel, said in response to a question about whether the regulations might hurt Alphabet's efforts.

"DMV believes that manufacturers need to obtain more experience in testing driverless vehicles on public roads prior to making this technology available to the general public," the agency said in a statement. The agency said it would develop regulations later for driverless cars.

Alphabet officials in the past have questioned why extra regulations are necessary if the vehicles are following existing traffic regulations, and can be demonstrated to do so better than human drivers.

The proposed regulations came after months of study by California's Department of Motor Vehicles. The state's legislature required that the DMV develop regulations for both testing and the deployment of autonomous vehicles. Executives from auto companies have openly said they are concerned about releasing vehicles to consumers before governments give the green light.

Tesla Motors Inc., which offers perhaps the most advanced semiautonomous features available to consumers through its electric vehicles, has said it would only be a few years before fully driverless cars are ready to hit the roads. But Tesla has said regulations—or a lack of them—could hinder widespread use of the technology.

Regulators on other fronts have proposed rules after technology became more prevalent. The Federal Aviation Administration only recently regulated the use of flying drones, years after their widespread use.

Among other proposed rules, California suggests requiring auto makers to submit monthly reports regarding the performance and safety of their autonomous vehicles. The regulations would also require auto makers to disclose information vehicles are collecting and take steps to prevent cyberattacks.

Write to Mike Ramsey at michael.ramsey@wsj.com

(END) Dow Jones Newswires

December 16, 2015 14:35 ET (19:35 GMT)

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