Urmson called it "a perplexing move" in which "California seemed to shrink back from its leadership."
With more than 1.3 million miles logged on the road with their self-driving cars, Urmson said the technology is safer than human-driven cars.
"People are telling us daily that fully self-driving cars are worth a shot," he wrote. "The status quo on our roads is simply not problem-free -- it has a real cost, not only in productivity and stress, but in lives damaged and destroyed by the mistakes of human drivers. Around the world, 1.2 million people die on the roads each year."
The Department of Motor Vehicles' proposal will now undergo a review -- including public input in January and February -- before final regulations are likely published sometime next year.
Google does have the support of some government leaders. On Thursday, California's Lieutenant Gov. Gavin Newsom issued a statement saying the proposal could "create road blocks to innovation" and ultimately drive the industry to another state.
"We must guard against unreasonably holding back California from doing what it does best -- inventing the future," he said.