The United States is a federal state,under which states,counties and cities have relatively large power.Things like the color of police car lights belong to local power,and the federal goverment does not interfere.
Generally speaking,most parts of the United States use red and blue warning lights.It's also the most popular collocation in the world.Most federal police cars take this form.
Although the usual red and blue color combination is normal,in order to improve the warning effect,some states will add a yellow warning color.Denver police car,for example,have yellow warning lights.
Genery,the state and federal that use red and blue together with the police lights are the red lights on the driver's side and the blue lights on the copilot's side.This has almost become a common practice in red and blue areas around the world.When the driver sees the red warning light on the rearview mirror,it's time to pull over.
It's only less common than the traditional red and blue combination.It's all blue alarm light.All blue warning lights are mainly used in some southern and northeast states in the United States,plus Hawaii and Puero Rico,about a dozen states.This is also a feature of identifying southern states.
In addition to some southern states,there are also some place in the north with all blue lights.Well known are some big cities,such as Chicago and Bosto.In fact,Boston is located in the whole state of Massachusetts with all blue lights.
