Calls for Police and Emergency Service generally fall into a category of 3 tiers which are known as Codes.

The severity of the call is what determines the type of police or emergency response.

Code 1 Response: These are usually calls to the police which are "Non-Emergency" in nature. Someone may call the police because their neighbor's stereo is too loud or something similar. Dispatch will assign a unit to respond Code 1. A Code 1 response involves no police lights or siren and the officer or officers will drive normally to the complaint location. If while on the way to a Code 1 a more serious call comes in those officers may be redirected to more serious emergencies.

Code 2 Response: Calls typically involving the potential for violence or medical attention but still haven't escalated to an extreme emergency provoke a Code 2 response. An argument outside of a bar which turned into a minor fist fight might call for a Code 2 response. During a Code 2 response officers use less intrusive light bar patterns and occasional taps of the manual siren activation button. The officers have a much more rapid pace but are not inclined to seriously disturb traffic flow.

Code 3 Response: Code 3 response calls are those of the most serious nature and involve people in extreme distress. Calls of this nature include people with dangerous weapons such as Guns and Knives. Domestic Violence calls, Motor Vehicle Accidents and other extreme often life threatening emergencies. Officers responding Code 3 have lights and sirens activated and demand right of way on all roadways they must use to arrive at the complaint location. Code 3 response has to potential to cause accidents because some motorists and pedestrians may not hear or see the police car responding. Officers are trained extensively on Code 3 response because of this. When approaching intersections officers are trained to switch the siren function from "Wail" (long drawn out sounds to capture public attention from far distances) to "Yelp" (Short bursts of sound to capture attention close by). Some departments teach officers to cut the siren off when surrounded by large amounts of traffic as some drivers panic when hearing sirens close behind them.

"Blues and Twos": Is the term for Code 3 response in the United Kingdom involving activating the blue lights and sirens (which in the past only had two tones). British Code 3 response is often much more calculated as pedestrian traffic is much heavier in most British cities. British Police Vehicles often feature "Battenburg" markings which make them easier to see and feature "Zebra" caution chevrons which are sometimes found on fire engines and construction vehicles as well.

Code 3 Response is the most visible aspect of police work the public is familiar with but its intent is sometimes often misunderstood. Each police department has different guidelines for Code 3 response but it is mostly uniform around the world.