When it comes to self-driving cars, what’s safe enough?

Self-driving vehicles passed a major milestone in November when Waymo’s minivans hit the streets of Phoenix without backup human drivers — reportedly making them the first fleet of fully autonomous cars on public roadways. Over the next few months, people will get a chance to take these streetwise vehicles for a free spin as the company tries to drum up excitement — and a customer base — for its launch of a driverless taxi service.

But even as these cars are ditching human supervisors, many people doubt the safety of machine motorists. A whopping 85 percent of baby boomers and even 73 percent of millennials confess to being afraid to ride in self-driving cars, according to a recent AAA survey. And while Waymo claims its vehicles are designed to be the world’s most experienced drivers — based on road tests as well as clocking millions of virtual miles — there’s still no consensus among experts about how safe is “safe enough” when it comes to street-smart cars.

It’s especially difficult to tell whether self-driving cars have earned their licenses when scientists are still writing the driver’s test.

Besides the sheer convenience of being able to take your hands off the wheel, the major appeal of self-driving cars is safer roadways. After all, mechanical chauffeurs can’t get drunk or distracted — factors involved in 29 and 10 percent of fatal accidents, respectively. But the only surefire way to evaluate autonomous cars’ reliability is test-driving them in real traffic, explains Nidhi Kalra, an information scientist at the RAND Corporation in San Francisco. “I think a lot of people were thinking, ‘Oh, we’ll just wait until the companies do enough test-driving,’” she says. “You could wait until the next millennium until that happens.”

In a 2016 study, Kalra and a colleague showed that self-driving cars would have to trek hundreds of millions or perhaps billions of miles to demonstrate with comfortable certainty that they caused fewer fatalities than the average person (about 1.1 per 100 million miles driven). Based on the current number of self-driving cars, that task could take decades or centuries to complete.

Tech developers hardly have that kind of time, so companies like Waymo assess their vehicles’ safety by pairing real driving time with practice on a private track and millions of miles a day in computer simulations.

Still, simulations can’t replace the value of actual road experience, says Philip Koopman, an electrical and computer engineer at Carnegie Mellon University in Pittsburgh. “What about the scenarios they didn’t know [to simulate]?” he says. “Weird, weird, weird stuff happens out on the roadways.”