Waymo, the driverless car company owned by Google parent Alphabet, has begun offering lifts to passengers with no safety driver for the first time.

Until now, the company's self-driving cars have had a Waymo engineer sat behind the wheel, observing the car's behavior and ready to take control if necessary. But now Waymo believes its technology is intelligent enough to drive on public roads all by itself. The cars without safety drivers are only operating in Phoenix, Arizona for now.

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The progress report was given by Waymo chief executive John Krafcik this week. He explained that passengers driven by these rider-only cars are part of Waymo's early-access program, and they have signed non-disclosure agreements to prevent them talking about the service, at least for now.

Waymo hasn't commented on exactly how many rider-only journeys it has completed, but indications are that the number is very low for now, reports Reuters.

The company also announced this week that it is making progress with its luxury autonomous taxi service, which uses a fleet of Jaguar I-Pace electric cars. The company tweeted a photo, embedded below, showing some of these cars at a new factory in Detroit, where they have been fitted with Waymo's new fifth-generation self-driving technology.

"We've opened the world's first factory 100% dedicated to L4 autonomous driver integration," Waymo tweeted, adding: "Our factory recently completed 30 I-Paces that are Now in CA for development and testing."

By L4, the company is referring to Level Four automation, which is where a car is capable of driving itself on public roads and in almost any circumstances. Level Five is the only higher level, which remains theoretical for now and is where the vehicle can drive absolutely anywhere, in any weather conditions, with zero human interaction.

Krafcik also spoke about Waymo's plans for fitting its self-driving technology to trucks as well as cars. The autonomous haulage system, known internally as 'Husky', is currently being tested on trucks in Michigan, Arizona and Georgia.

On the technology, the Waymo boss said: "We think trucking is a really interesting application of the Waymo driver. However, for now at least, US regulation does not allow the use of automation with the heaviest class of haulage trucks.