This May 13, 2014 file photo shows a row of Google self-driving Lexus cars at a Google event outside the Computer History Museum in Mountain View, Calif.

Google’s futuristic self-driving cars needed some old-fashioned human intervention to avoid 11 crashes during testing on California roads, the company revealed on Tuesday, results it says are encouraging but show the technology has yet to reach the goal of not needing someone behind the wheel.

With Google’s fleet logging tens of thousands of miles each month, the 11 instances would be the equivalent of a car having one event every three years, based on how much the average vehicle is driven in the U.S.

There were another 272 cases in which failures of the cars’ software or onboard sensors forced the person who must be in the front seat – just in case – to grab the wheel during roughly a year of testing.

Though Google did not release detailed scenarios, the problems included issues with the self-driving cars seeing traffic lights, yielding to pedestrians or committing traffic violations.

There were also cases where intervention was needed because other drivers were reckless, and several dozen instances of an “unwanted maneuver” by Google’s car.

“There’s none where it was like, ‘Holy cow, we just avoided a big wreck,’“ said Chris Urmson, who heads Google’s self-driving car project. During this phase of testing, Google cars usually stay below 35 mph, although they also drive on highways.

“We’re seeing lots of improvement. But it’s not quite ready yet,” Urmson said. “That’s exactly why we test our vehicles with a steering wheel and pedals.”

Bryant Walker Smith, a professor at the University of South Carolina who closely follows self-driving car developments, said the rate of potential collisions was “not terribly high, but certainly not trivial.”

He said it remains difficult to gauge how Google’s cars compare to accident rates among human drivers, since even the best data underreport minor collisions that are never reported to authorities.

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