Waymo was able to compile a library of sights and sounds from the event thanks to its minivans' upgraded sensors. The new suite of sensors include an audio detection system designed in-house and an upgraded LiDAR and vision system, which are capable of seeing emergency vehicles and their flashing lights. They also allow the technology to recognize other types of emergency vehicles it hasn't seen yet.

Waymo is already using the data it collected to teach its self-driving system how to detect where sirens are coming from. By being able to point out the direction where emergency vehicles are located, its autonomous cars can move to the side if they're passing from behind or yield at an intersection to let them pass first.

"This training is key to reliably detecting and responding to emergency vehicles in Arizona and beyond," the company said on its blog. "By teaching our cars this advanced capability, we're moving closer to bringing truly self-driving technology into the world."