Systems that act to control the movement of people, goods, and vehicles in order to ensure their safe, orderly, and expeditious flow throughout the transportation system. Each of the five areas of transportation—roadways, airports and airways, railways, coastal and inland waterways, and pipelines—have unique systems of control.
Roadway traffic-control systems are intended to improve safety, increase the operational efficiency and capacity of the roadway, and contribute to the traveler's comfort and convenience. They range from simple control at isolated intersections using signs and markings, to sophisticated traffic-control centers which have the ability to react to changes in the traffic environment. Traffic-control systems are used at roadway intersections, on highways and freeways, at ramp entrances to freeways, and in monitoring and controlling wider-area transportation networks. Intelligent roadway traffic control, now known as intelligent transportation systems (ITS), is a very sophisticated form of traffic control for roadway and other areas. See also Highway engineering.
The U.S. federal government has designated airspace as either uncontrolled or controlled. In uncontrolled airspace, pilots may conduct flights without specific authorization. In controlled airspace, pilots may be required to maintain communications with the appropriate air-traffic control facility to receive authorization and instruction on traversing, taking off from, or landing, in that controlled area. Air-traffic control systems for controlled areas may be divided loosely into en route and terminal systems. See also Air navigation; Air-traffic control; Air transportation.
Railroads operate high-speed freight and passenger services essentially over an exclusive right of way. Railroads use both semaphore and light signals for traffic control. Semaphores convey visual messages to train operators according to predetermined rules indicating how the train is to be operated in specified areas. Automatic block signaling prevents rear-end and head-on collisions on signal tracks. In this system, track sections are divided into blocks. Only one train is permitted to occupy a block at any time. Blocks are monitored by automatic circuitry that controls traffic signals, indicating the appropriate clear or stop signals to following or approaching trains. Similar block systems are used for subway systems. Centralized traffic-control systems may control hundreds of miles of track signals and switches. A dispatcher at a central location monitors the location of trains by means of visual displays of colored lights on a large track diagram, and can operate the switches and signals at key points from the central control console. See also Railroad control systems; Railroad engineering.
Vessel traffic control consists largely of marine aides that function more for informational, advisory, and guidance purposes than as positive traffic-control devices. Lighted or unlighted buoys indicate navigable areas in coastal waters and within waterways. Lightships and lighthouses with fog signals and radio beacons are placed as markers at prominent points during periods of limited visibility. Radar devices have become common, even on smaller ships. Navigation systems often employ the Differential Global Positioning System. The Vessel Traffic Service (VTS) is available in selected areas. Services may range from the provision of single advisory messages to extensive management of traffic communication and radar services. See also Buoy; Lighthouse; Marine navigation; Navigation.
The 450,000 mi (750,000 km) of pipelines in the United States are a major part of the nation's transportation network, carrying about 25% of all intercity freight-ton mileage. The primary goods moved through pipelines are oil and oil by-products, natural gas, and fertilizers. The movement of goods in pipelines is controlled by systems of valves, pumps, and compressors. See also Pipeline; Transportation engineering.
