Police cars are usually passenger car models which are upgraded to the specifications required by the purchasing force. Several vehicle manufacturers, such as Ford and General Motors, provide a "police package" option, which is built to police specifications in the factory. Police forces may add to these modifications by adding their own equipment and making their own modifications after purchasing a vehicle.

Modifications a police car might undergo include adjustments for higher durability, speed, high mileage driving and long periods of idling at a higher temperature. This is usually accomplished by heavy duty suspension, brakes, calibrated speedometer, tires, alternator, transmission and cooling systems, and also sometimes includes slight modifications to the car's stock engine or the installation of a more powerful engine than would be standard in that model. It is also usual to upgrade the capacity of the electrical system of the car to accommodate the use of additional electronic equipment.

Audible and visual warnings

Police vehicles are often fitted with audible and visual warning systems to alert other motorists of their approach or position on the road. In many countries, use of the audible and visual warnings affords the officer a degree of exemption from road traffic laws (such as the right to exceed speed limits, or to treat red stop lights as a yield sign) and may also suggest a duty on other motorists to move out of the direction of passage of the police car or face possible prosecution.

Visual warnings on a police car can be of two types: either passive or active.

Passive visual warnings

Passive visual warnings are the markings on the vehicle. Police vehicle markings usually make use of bright colours or strong contrast with the base colour of the vehicle. Modern police vehicles in some countries have retro reflective markings which reflect light for better visibility at night. Other police vehicles may only have painted on or non-reflective markings. Most marked police vehicles in the United Kingdom and Sweden have reflective Battenburg markings on the sides, which are large blue and yellow rectangles. These markings are designed to have high contrast and be highly visible on the road, to deter crime and improve safety. Another passive visual warning of police vehicles is simply the interceptor's silhouette. This is easily observed in the United States, where the ubiquitous nature of the Ford Crown Victoria in police fleets has made the model synonymous with police vehicles.

Police vehicle marking schemes usually include the word Police or similar phrase (such as State Trooper or Highway Patrol) or the force's crest. Some police forces use unmarked vehicles, which do not have any passive visual warnings at all.

Active visual warnings

The active visual warnings are usually in the form of flashing coloured lights (also known as 'beacons' or 'lightbars'). These flash in order to attract the attention of other road users as the police car approaches, or to provide warning to motorists approaching a stopped vehicle in a dangerous position on the road. Common colours for police warning beacons are blue and red, however this often varies by force. Several types of flashing lights are used, such as rotating beacons, halogen lights, or light emitting diode strobes. Some police forces also use arrow sticks to direct traffic, or message display boards to provide short messages or instructions to motorists. The headlights of some vehicles can be made to flash, or small strobe lights can be fitted in the headlight, tail light and indicator lights of the vehicle.

In addition to visual warnings, most police cars are also fitted with audible warnings, sometimes known as sirens, which can alert people and vehicles to the presence of an emergency vehicle before they can be seen. The first audible warnings were mechanical bells, mounted to either the front or roof of the car. A later development was the rotating air siren, which made noise when air moved past it. Most modern vehicles are now fitted with electronic sirens, which can produce a range of different noises. Police driving training often includes the use of different noises depending on traffic conditions and manoeuvre being performed. In North America for instance, on a clear road, approaching a junction, the 'wail' setting may be used, which gives a long up and down variation, with an unbroken tone, whereas, in heavy slow traffic, a 'yelp' setting may be preferred, which is a sped up version of the 'wail'. Some vehicles may also be fitted with airhorn audible warnings. Also in some European countries, where a hi-lo two tone siren is the only permitted siren for emergency vehicles, a "stadt" siren will be used in cities where it has loud echo that can be heard from blocks away to warn the traffic an emergency vehicle is coming, or a "land" siren will be used on highways to project its noise to the front to produce more penetration into the vehicles ahead to alert the drivers.

A development is the use of the RDS system of car radios, whereby the vehicle can be fitted with a short range FM transmitter, set to RDS code 31, which interrupts the radio of all cars within range, in the manner of a traffic broadcast, but in such a way that the user of the receiving radio is unable to opt out of the message (as with traffic broadcasts). This feature is built into all RDS radios for use in national emergency broadcast systems, but short range units on emergency vehicles can prove an effective means of alerting traffic to their presence, although is not able to alert pedestrians and non-RDS radio users.

A new technology has been developed and is slowly becoming more popular with police. It's called a Rumbler, not only a siren that one hears, but can feel. It uses new technology to not only transmit sound but a vibration feel. The technology used is the siren emits a very low frequency, so one can feel it and hear it. The feeling is that of standing next to a large speaker with pumped bass.

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