The self‑driving vehicles of Google must of course follow traffic rules and, according to traffic rules, a driver stops the car immediately when a police vehicle approaches his car and signals him to stop.
Additionally, the police sometimes take part in some pursuit operations. During these operations, each police vehicle does not follow traffic rules, meaning it can violate traffic signals and run without any speed limit. 
The other cars that happen to be at the chase location should pull over to disrupt it.
Thus Google developed a police vehicle detection system that will integrate into its self-driving cars.
The detection system can automatically identify police vehicles from the flashing siren lights.
The police vehicles (of the USA) have a lighting system with four different light sources in a horizontal arrangement. The four light sources emit a specific red and a blue light alternately.
As soon as Google's self-driving car detects a police vehicle, it turns right and stops. The system can also be used to detect other types of emergency vehicles, such as ambulances.
