Driverless cars are regulated by a patchwork of state laws. Arizona, like many states, has no restrictions against operating an autonomous vehicle without a person in the driver’s seat. On the other hand, California, where Waymo is headquartered, requires any self-driving car to have a safety driver sitting in the front.

In December, Waymo published a report for California’s Department of Motor Vehicles about how frequently its car “disengaged” — deactivating its autonomous mode because of a system failure or safety risk and forcing a human driver to take over. In the report, Waymo said this happened once every 5,000 miles the cars drove in 2016, compared with once every 1,250 miles in 2015.

Consumer Watchdog, a frequent critic of Alphabet, said that data demonstrated that the cars are not ready to drive without any human intervention and that Waymo was following the Silicon Valley model of “beta testing” a new technology on the public.

“It’s the wrong approach when you’re dealing with self-driving cars,” said John M. Simpson, a director at Consumer Watchdog. “When things go wrong with a robot car, you kill people.”

Researchers believe self-driving cars can be safer than cars operated by human drivers because they are programmed to adhere strictly to traffic laws, they don’t get distracted, and they usually refrain from taking unnecessary risks.

Timothy Tait, a spokesman for the Arizona Department of Transportation, said the state was on pace to exceed 1,000 automobile-related fatalities this year and that its top priority is the public’s safety — particularly by advancing efforts to reduce crashes and deaths on its roads.

“We are closely monitoring emerging technologies like self-driving cars that may ultimately support safer travel and open up opportunities for populations who today are unable to drive for themselves,” he said in a statement.