Waymo, the acknowledged leader in self-driving cars, has been making serious progress in its mission to bring products and services to the world.

But none of it might have happened if the acknowledged godfather of the industry, Sebastian Thrun, had listened to his instincts.

When Google co-founder Larry Page first approached Thrun about building a self-driving car that people could use on the real roads, Thrun scoffed and told him it couldn't be done.

What happened next forever taught Thrun the difference between being an expert and being a visionary.

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The self-driving car company, Waymo, took another big step forward this week in its ambitions to usher in a new era of robot taxis.

The company received permission in its home state of California to carry passengers in its cars, as long as they don't pay for the ride and there's a human safety driver behind the wheel.

Waymo launched the first commercial autonomous ride-hailing service in the US, Waymo One, in parts of Arizona in 2018.

But, Sebastian Thrun, the man who birthed Waymo into the world back when self-driving cars was just a crazy idea inside of Google, says the whole thing might not have happened if Google founder and then CEO, Larry Page, didn't have a stubborn streak.

Back in the mid-2000's, Thrun was having a spectacular career in robotic vehicle academics first at Carnegie Mellon (still famous for its research which is why its home to rival Uber's program) and then at Stanford.

In 2005, his Stanford team won the $1 milion prize DARPA challenge with a robot vehicle called Stanley that drove through the desert at high speeds. In 2007, his Stanford team built a robot car called Junior that won second place in the DARPA challenge, which awards a $1 million prize, by completing 60 miles of tasks on a simulated cityscape track that involved weaving around traffic from stunt drivers and other robot competitors.

Two years later, Page called him, wanting to turn these academic wins into a real thing for use by real people on real roads.

In what he describes as the most "embarrassing" story of his career, Thrun scoffed.

"Back in 2009, when I had become officially the godfather of self-driving cars, and the world expert (maybe there were four or five experts, but I was certainly at the top of my field), Larry Page comes to me and says, 'Hey, Sebastian, I want you to do a car in California that can drive anywhere in California, any road. Up to that point, the best we had ever seen was an empty dessert track, or an empty city that could go at 10-15 miles per hour. Now, he wanted me to drive cars in the commuter hour at 45 mph or 75 mph on [Interstate] 280," Thrun recently told Business Insider.

"My gut reaction was, 'No, this can't be done.'"

He turned Page down flat and thought that was the end of that.

Of course, it wasn't. "I'm embarrassed about it when I talk about it. Larry came back the next day and said, hey Sebastian, let's talk about this. Why can't it be done? And I said, 'it can't be done.'"

A short time later, Page came back and said, "Ok, you say it can't be done. You're the expert. I trust you. So I can explain to Sergey why it can't be done, can you just give me a technical reason, why it can't be done?," Thrun recalled.

He had been thinking of the risks of taking the tech as it was and putting it on the roads with real people (and he still thinks about those risks today).

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But when he was pushed on the idea, he had to admit to Page and himself, "Oh, I can't find a technical reason. And that was the start of the Waymo team, that specific moment. Because then he said, Ok, Sebastian, you may be right. But if there's a 10% chance that you are wrong, let's give it a try because you may be able to save 1 million lives per year on the streets.'"

That argument swayed him to leave the confines of academic research and try and build self-driving cars that could one day be turned into a real product that real people use. Page "kind of persuaded me to do it," Thrun says.

And only "18 months or so later, we had this car driving on pretty much every street in California. We did the 1,000-mile off-road challenge, called the Google 1,000, where we had to drive completely hands off," he recalls.

Today, of course, he proudly cites that the cars have logged more than 10 million miles. Waymo has spun out of Google in 2016 as its own company. Earlier this year, Waymo announced it was opening a factory in Detroit to mass produce its vehicles. And Thrun has turned into quite the prolific entrepreneur himself. He went on to found online school Udacity and runs it as CEO, and he's CEO of Page's next robotic vehicle company, Kitty Hawk, working on flying cars.

But that moment when Page challenged his knee-jerk no was a lesson the professor never forgot: The best ideas are the ones that everyone thinks are crazy, especially the experts.

"It was embarrassing to me because I was the world expert and I was supposed to be the person who knows what's the case and I was the worst person to ask about this," Thrun said. "But experts tend to be experts of the past, not the future."