air traffic control, the system by which airplanes are safely routed into and out of major airports. Air traffic control in the United States is centered in a number of regional control centers that route airplanes along established airways to airport traffic control centers. There Instrument Landing Systems and Microwave Landing Systems enable planes to land safely in almost any weather conditions. Air traffic controllers, who are responsible for maintaining safe distances between planes, are employees of the U.S. Dept. of Transportation. Air traffic control is made possible by special transponders installed in every commercial and many private aircraft, which automatically transmit information on a plane's altitude and speed to the ground controller. The distortion that often affects voice transmissions can be eliminated by the use of cockpit datalinks; collision avoidance systems provide further safety margins. Knowing the course, speed, and altitude of every plane in the sector, the controller can contact each in turn to give landing or course instructions. Modern air traffic control has contributed to making air travel far safer than highway travel, and on a passenger-mile basis safer even than rail travel.