Waymo, the autonomous vehicle business under Alphabet, sent an email to customers of its ride-hailing app that their next trip might not have a human safety driver behind the wheel, according to a copy of the email that was posted on the subreddit r/SelfDrivingCars.

The email, entitled “Completely driverless Waymo cars are on the way,” was sent to customers that use its ride-hailing app in the suburbs of Phoenix.

Both the early rider program and Waymo One service use self-driving Chrysler Pacifica minivans to shuttle Phoenix residents in a geofenced area that covers several suburbs, including Chandler and Tempe. All of these “self-driving rides” have a human safety driver behind the wheel.

A driverless ride is what it sounds like. No safety driver behind the wheel, although a Waymo employee would likely be present in the vehicle initially.

TechCrunch was able to confirm from Waymo that the email went out to members of Waymo’s early rider program. The email is posted below.

Waymo, formerly known as the Google self-driving project, first began testing its technology in 2009 in and around its Mountain View, Calif., headquarters. It’s been a slow and steady roll ever since. The company has expanded its test area to other cities, spun out into its own business and iterated the vehicle design and the sensors around it.

Waymo opened a testing and operations center in Chandler, Ariz. in 2016. Since then, the company has ramped up its testing in Chandler and other Phoenix suburbs, launched an early rider program and slowly crept toward commercial deployment. The early rider program, which required vetted applicants to sign non-disclosure agreements to participate, launched in April 2017.

In December, the company launched Waymo One, a commercial self-driving car service and accompanying app. Waymo One signaled that the company was starting to open up its service. Members of the early rider program were transferred to Waymo One, which allowed them to bring guests and even talk publicly about their rides. More recently, Waymo opened another technical service center in the Phoenix area in preparation to double its capacity and grow its commercial fleet.

While driverless Waymo vehicles have been spotted periodically, they have never been used to shuttle the general public. The introduction of driverless vehicles would be milestone for the company.

And yet, there remains a number of questions. It’s unclear how many of these driverless rides there will be or what constraints Waymo will place on them. It’s likely that these will operate in more simple, controlled environments for months before expanding to more complex situations.