Google's cars have done about 300,000 miles of road testing, but not without incident, Brin said. The most the cars have achieved without "safety-critical intervention" -- or a driver needing to take control -- is about 50,000 miles, he said.
"That's not good enough, and we're continuing to work to go beyond that," he said.
"Safety is a huge challenge for us. That's one of the most difficult things that we undertake from a technology point of view, because there are never enough 'nines' in terms of getting things right," Brin said, using a reliability term from the computer industry.
"What happens when a computer breaks down, or when the tire blows out or something unexpected happens? We spend night and day fretting about all sorts of rare possibilities, and I'm optimistic that we're going to be able to solve [these issues]," he said.
Vehicle safety isn't the only thing on people's minds, however. Consumer Watchdog, a nonprofit group, tried to get the California bill blocked unless it included language that prevented companies from collecting data for marketing or any other non-safety purposes.
Brin thinks the benefits will outweigh other concerns. Self-driving cars will be more fuel-efficient, lead to less accidents, and open doors to blind people and others who are "under-served by the current transportation system," he said.
"Some people have disabilities, others are too young, some people are too old, sometimes we're too intoxicated," Brin said.
Self-driving cars will also relieve congestion, according to Brin, because they will be able to drive closer together on highways
But toward the end of the press conference, he came back to how much work remains to be done.
"It's dealing with every possible eventuality," he said, "and we're dealing with a long list of eventualities."
But humans have overcome those challenges before, he said. "For instance, for airplane flight."
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