Have you ever looked down from a bridge or the upper story of a building and watched a police car drive by below? If so, you may have noticed a number emblazoned across the roof of the car. These numbers are often called aerial roof markings. Their purpose is to make police cruisers instantly recognizable to authorities viewing them from a helicopter or CCTV camera. Police cars are far from the only vehicles with aerial marks, and some of these numbers are not even visible to the naked eye.

Why do police cars have numbers on their roofs?

Police cars have aerial roof markings–numbers and sometimes even letters–so they are recognizable from the air. This helps authorities identify which cars are police cruisers. But it also helps the police in the cars communicate with air or CCTV support.

Imagine an officer is watching traffic through a helicopter or CCTV camera. He or she can see a speeding civilian and several police officers nearby. It is critical that they can quickly communicate which of the police cruisers is closest to the dangerous situation.

The actual numbers on police cars’ roofs vary by department, state, and country. For example, police cars in Australia often have aerial markings that begin with two letters to designate the vehicle’s department. In the United Kingdom, a shape or symbol on the roof of an emergency vehicle communicates its purpose.

What vehicles have aerial roof markings?

You may be able to picture aerial roof markings on the white roof of a police car. But many other emergency vehicles, such as fire trucks or ambulances, also have rooftop markings. In addition, other municipal vehicles such as public buses may have aerial roof marking as well.

Here’s an example: Since 2007, every ambulance in any European Union member country should bear a Red Cross and Crescent symbol or the Star of Life which is the international symbol for medical assistance. Why? So any authorities monitoring traffic can quickly spot an ambulance in a hurry and call ahead to clear the way.

Many cities, such as London, require all public buses to wear aerial roof markings. In London specifically, there are several bus operators. Therefore, every bus must carry a three-letter code representing its operator. After this code, every bus wears a fleet number, also printed on the front and rear of the bus.

What are thermal aerial roof markings?

Some police departments have implemented thermal aerial roof markings to make their cruisers easier to spot after dark. These aerial markers are made of a reflective marking film to reflect infra-red light back to thermal imaging cameras. But they are not visible to the naked eye.

The United Kingdom is one place that uses thermal roof markings on many of its police cars. According to a Home Office memo stored in the UK National Archives, this is because UK authorities found police cars difficult to identify after dark. Not only were simple painted numbers difficult to see, but even the light bar and other emergency equipment was difficult to pick out. Thermal roof markings solved this problem.

Next, learn to identify unmarked police cars or see aerial roof markings on a Ford Interceptor Utility in the video below: