The boss of a Google-funded self-driving car project has issued an autonomy reality check, saying the technology might never actually work.

That's the word from Waymo CEO John Krafcik, who told the Wall Street Journal that the sci-fi vision of autonomous cars (vehicles that can navigate any road or area in any weather conditions without any driver input) will never become a reality, instead saying self-driving cars will always "have restraints".

Driverless technology is measured in five distinct levels. Level one means the car can control a single function automatically, like cruise control, while level two allows for a driver to temporarily remove their hands and feet from the controls. Level three reduces the driver to an in-case-of-emergency proposition on some roads, with human intervention only required if an accident is imminent. Level four is "eyes off, hands off" motoring, with a car able to complete most trips by itself, while level five is complete autonomy in all situations, from highways to gravel tracks.

But while many companies are approaching level four autonomy, Krafcik says that true level five cars might never actually happen.

"Autonomy always will have some constraints," Krafcik said.

"It's really, really hard. You don't know what you don't know until you're actually in there and trying to do things."

Waymo is launching its first paid driverless-taxi network in Phoenix, Arizona, as part of a small-scale commercial trial, using its fleet of Chrysler Pacifica people-movers to ferry customers on dedicated routes between their homes and local shops.

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