LOS ANGELES — When cars exit the tunnel of the next 15 years, they’ll be like giant smart phones.

Their sensors will capture sight, sound and motion and transmit the information to the Internet quickly and affordably. The $100 billion app economy built on data from smart phones would look small compared with the $750 billion in revenue produced around cars.

The forecast has automakers buzzing. As they accelerate spending on developing self-driving cars, they’re devoting enormous attention on what to do with data that those high-tech devices generate — beyond making the drive automated. Among the possibilities: selling details about driving patterns to real estate developers or using it in personalized insurance calculations.

But the road to such a future could be more treacherous than traditional giants of the auto industry expect. They’re assuming the costs of sensors will fall enough to justify data collection efforts. They’re betting they can easily transfer the data, though it could be thousands of times the volume discharged by smart phones. And they’re hoping their role — and moneymaking potential — in the new realm isn’t marginalized.

The challenge comes from Silicon Valley breathing down the neck of the old guard. Younger auto rivals Tesla Motors, NextEV, Faraday Future and even Apple could have a head start on reinventing the technical guts of cars to pair with software.

Automakers tell NextEV U.S. Chief Executive Padmasree Warrior that they’re bulking up software development teams. The former chief technology officer at Cisco Systems and Motorola tells them that’s good, but not sufficient.

“Because of the heritage they have, it’s hard to say, ‘How can I build this from ground zero?’” Warrior said in an interview at the L.A. Auto Show. “They are trying to look at it as, ‘Can I just get the software and put it on an existing hardware platform,’ and I’m (making) the point it never works that way.”

The full package will come together for some, technologists say. But their concern is that the likes of Ford, Toyota and others are underestimating obstacles. What seems five years away could be 10 years out, and business and strategy executives are jumping the gun on technology development teams.

Genivi, an alliance of auto parts companies, is working with online technology groups on standards that would enable app makers to write one program and have it function across many vehicles. Without that simplicity, potential buyers for car data may be scared away.

Tech experts think they can get over the smaller hurdles. Sensor costs and sizes need to fall, so cars don’t end up expensive, ugly robots.

The concerns are “definitely in the category of things you can get over in the next couple of years or next five years,” said Sanjay Ravi of Microsof.