sticks to direct traffic, or message display boards to provide short messages or instructions to motorists. The headlights and tail lights of some vehicles can be made to flash, or small strobe lights can be fitted in the vehicle lights. Audible warnings Audible warnings are the sirens on the vehicle. These sirens alert road users to the presence of an emergency vehicle before they can be seen, to warn of their approach. 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 makes noise when air moves past it. Most modern police vehicles use electronic sirens, which can produce a range of different noises. Different models and manufacturers have distinct siren noises; one siren model, the Rumbler, emits a low frequency sound that can be felt through vibrations, allowing those who would not otherwise hear the siren or see the emergency vehicle to still know it is approaching. Different siren noises may be used depending on traffic conditions and the context. For example, on a clear road, "wail" (a long up-and-down unbroken tone) is often used, whereas in heavy slow traffic or at intersections, "yelp" (essentially a sped-up wail) may be preferred. Other noises are used in certain countries and jurisdictions, such as "phaser" (a series of brief sped-up beeps) and "hi-lo" (a two-tone up-down sound). Some vehicles may also be fitted with electronic airhorns. Police-specific equipment A wide range of equipment is carried in police cars, used to make police work easier or safer. The installation of this equipment in a police car partially transforms it into a desk. Police officers use their car to fill out different forms, print documents, type on a computer or a console, and examine different screens, all while driving. Ergonomics in layout and installation of these items in the police car plays an important role in the comfort and safety of the police officers at work and preventing injuries such as back pain and musculoskeletal disorders. Communication devices Police radio systems are generally standard equipment in police cars, used to communicate between the officers assigned to the car and the dispatcher. Mobile data terminals are also common as alternative ways to communicate with the dispatcher or receive important information, and are typically a tablet or a dashboard-mounted laptop installed in the car. Suspect transport enclosure Suspect transport