Waymo’s self-driving Chrysler Pacifica minivans spent a day with Chandler, Ariz Fire & Police Department, testing the car’s detection system for emergency vehicles.

The minivans are outfitted with Waymo’s latest suite of sensors, which can hear from twice the distance and can also see flashing lights from much farther away.

See Also: Google narrows case against Uber as court date nears

The software running the self-driving car is able to decide what to do in different situations, for example, it pulls over if an ambulance is behind, but will yield at an intersection if the vehicle is ahead.

Working for a full day on a range of practice tests allowed Waymo to collect significant amounts of information on the slight alterations in light, sound, and size of emergency vehicles on the road.

This allows Waymo cars to detect an emergency vehicle even if the vehicle’s sound or size doesn’t match up to specifics stored on the car. If the system does spot differences, it will file the sounds and look into the system, for future encounters.

“This training is key to reliably detecting and responding to emergency vehicles in Arizona and beyond. By teaching our cars this advanced capability, we’re moving closer to bringing truly self-driving technology into the world,” said Waymo on Medium.

In the past few months, Waymo has moved from primarily California to Arizona for the latest batch of self-driving tests. It chose Chandler and Phoenix as its launch locations for the on-demand pickup service, and has yet to launch in California, due to the state’s self-driving laws.