Waymo’s self-driving Pacifica minivans recently got a dedicated testing day with the Chandler, Ariz. Fire & Police Department, wherein the autonomous vehicles spent time in focused demo runs where firetrucks, police cars, motorcycles, ambulances and more navigated streets in and around the Waymo vehicles.

This test was the “first emergency vehicle testing day” Waymo has run with its new vehicles (they’re exposed to them in the course of the ordinary vehicle road trials, of course, but this is the first dedicated day around this testing). The goal was to collect as much sensor data as possible regarding the emergency vehicles, and how they sound and look as they pass, lead and trail behind the autonomous cars

The resulting data helps form a library from which Waymo can pull when training its vehicles on the proper behavior and set of responses to use when encountering ambulances and the like on real roads – obviously a key ingredient that needs to be addressed before public road tests can become commercial service deployments.

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The new hardware on the Pacificas, which Waymo developed entirely in-house, includes audio sensors that can detect sirens up to twice as far away as the original set, Google says. The new vision, radar and LiDAR equipment can also see emergency vehicles further out than they were able to before, which helps explain why Waymo wanted to do a first day of dedicated testing with the new equipment: It’s going to be better able to observe emergency vehicle behavior in real-time and model appropriate paths and responses with this new kit.

Waymo’s currently using Chandler and Phoenix as test beds for its initial autonomous on-demand passenger pick-up service, but the data gathered from these local tests will help its self-driving software more generally be able to recognize even emergency vehicles it hasn’t encountered before, and react appropriately.