Google's self-propelled vehicles will of course have to follow road traffic rules and, in accordance with road traffic rules, a driver stops the car immediately when a police vehicle approaches his car and stops him from driving.
In addition, the police sometimes participate in some pursuit operations. During these operations, every police vehicle does not follow the traffic rules, i.e. it can violate traffic signals and run without any restrictions speeds. 
The other cars that happen to be at the point of tracking should do aside to disturb it.
So Google developed one system detection of police vehicles that will adapt to its self-driving cars.
The detection system can automatically detect police vehicles from the flashing siren lights.
Police vehicles (US) have a lighting system with four different light sources in a horizontal configuration. 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 vehicles extraordinary emergency, such as ambulances.
