When Google’s self-driving car project began testing in the Bay Area back in 2009, its engineers focused on highways by sending its sensor-laden vehicles cruising down Interstate 280, which runs the length of Silicon Valley’s peninsula.

More than 15 years later, the cars are back on the freeway—this time without drivers. On Tuesday, the project, now an Alphabet subsidiary we all know as Waymo, announced that its robotaxi service would now drive on freeways in the San Francisco Bay Area, Los Angeles, and Phoenix.

The new service marks another technical leap for Waymo, whose robotaxis currently serve five US metros: Atlanta, Austin, Los Angeles, Phoenix, and the San Francisco Bay Area. The company says it will launch in several other US and international cities next year, including Dallas, Miami, Nashville, Las Vegas, Detroit, and London.

Waymo also announced Wednesday that it would begin curbside pickup and drop-off service at San Jose Mineta International Airport, allowing passengers to, theoretically, travel autonomously all the way from San Francisco to San Jose—a service area of some 260 square miles. Waymo has been offering its autonomous taxi service on area service roads since the summer of 2023, but the new freeway service could cut in half the time it takes for a robotaxi to travel from San Francisco to Mountain View, Waymo user experience researcher Naomi Guthrie says.

“Freeway driving is one of those things that’s very easy to learn, but very hard to master,” Waymo co-CEO Dmitri Dolgov told reporters last week. Highways are predictable, with (mostly) clear signs and lane lines, and a limited set of vehicles and players (trucks, cars, motorcycles, trailers) that a vehicle’s software must learn to recognize and predict. But Waymo executives said that, despite a year of employee- and guest-only highway testing, safety emergencies on highways are relatively rare, so the team was unable to collect as much real-world data as it needed to train its vehicles to operate safely there. Complicating the project was the fact that highway crashes, at high speeds, are subject to the laws of phsyics—and so more likely to maim or kill.

To get ready for highways, Waymo executives say, engineers supplemented real-world driving data and training with data collected on private, closed courses, and data created in simulations. Two onboard computers help create system “redundancies,” meaning the vehicles will have computer backup if something goes wrong. The vehicles have been trained to exit highways in the case of emergencies, but will be able to pull over as well. Waymo execs also say they have and will work with law enforcement and first responders, including highway patrols, to create procedures for vehicles and riders stranded on highway shoulders, where hundreds of Americans are killed every year.