TRAFFIC CONTROL

supervision of the movement of people, goods, or vehicles to ensure efficiency and safety. Concern with controlling traffic has arisen whenever people and their vehicles have become concentrated. Thus, as the volume of wheeled traffic increased in ancient Rome, restrictions had to be put on the number of vehicles and where they could be operated. By the 17th century many European cities had prohibited parking on some streets and had established one-way streets. Traffic on roads and in the air has become of paramount concern because of the tremendous increase in the use of motor vehicles and aircraft since World War II. In both instances government regulationi.e., what kind of vehicles and craft may be operated, who may operate them, and how they are to be operatedprovides a fundamental form of control. In the early days of automobiles there was little concern for regulating traffic, as there were only a few vehicles and speeds were low. The number of vehicles increased dramatically in the early 20th century, however, and their speeds increased as more powerful engines were used and as better roads were built. A system of traffic-control devices was devised that included regulatory devices, such as traffic signals and prohibitory and mandatory signs; warning devices, to alert drivers of hazardous or unusual road conditions; and guidance devices, to give drivers route and other pertinent information. Control of air traffic is considerably more difficult because it must be done for craft moving at much higher speeds within a three-dimensional framework. The most important objective is to ensure that safe distances are maintained between aircraft. The primary area of concern is in and around airports; the sky around them is called controlled airspace. The first systematic attempt to regulate traffic on a large scale took place on railroad lines. As the use of trains became widespread, it became obvious that some method of control was needed to keep trains heading in opposite directions off of the same track and to maintain safe intervals between trains heading in the same direction. The first attempts at interval control, by running trains on timed schedules, was only partially successful. The development of the block system, in which portions of track (blocks) were controlled by signalmen in communication with each other by telegraph, permitted train operators to learn of track conditions as the train advanced from block to block. Colour light signals activated automatically by electric relays in the track greatly reduced potential for human error and allowed traffic to run more smoothly. An important modern innovation is the moving-block system, in which a moving zone, based on speed, is maintained by electronic equipment that automatically adjusts the speed of the train when it enters the zone of another train. Harbours and their approaches and inland canals and rivers are the areas of greatest concern in water-traffic control. The primary methods of control include marking hazardous areas on charts and with lights, sound signals, and buoys; creating shipping lanes in areas of heavy traffic; standardizing forms of communication and of collision avoidance; and establishing agencies to check and to certify vessels for seaworthiness. supervision of the movement of people, goods, or vehicles to ensure efficiency and safety. Traffic is the movement of people and goods from one location to another. The movement typically occurs along a specific facility or pathway that can be called a guideway. It may be a physical guideway, as in the case of a railroad, or it may be an agreed-upon or designated route, marked either electronically (as in aviation) or geographically (as in the maritime industry). Movementexcepting pedestrian movement, which only requires human powerinvolves a vehicle of some type that can serve for people, goods, or both. Vehicle types, often referred to as modes of transportation, can be broadly characterized as road, rail, air, and maritime (i.e., water-based). Traffic evolves because of a need to move people and goods from one location to another. As such, the movement is initiated because of decisions made by people to transport themselves or others from one location to another to participate in activities at that second location or to move goods to a location where they have higher value. Traffic flows thus differ fundamentally from other areas of engineering and the physical sciences (such as the movement of electrons in a wire), because they are primarily governed and determined by laws of human behaviour. While physical attributes are critical in the operation of all modes (e.g., to keep airplanes in the air), the demand or need to travel that gives rise to traffic is derived from the desire to change locations. One of the principal challenges in traffic control is to accommodate the traffic in a safe and efficient way. Efficiency can be thought of as a measure of movement levels relative to the objectives for a particular transportation system and the finances required for its operation. For example, a railroad can be thought of as efficient if it can accommodate the travel requirements of its customers at the least cost. It will be thought of as inefficient if an alternative (e.g., a trucking service) can also meet customer needs but at a lower cost. Safety, the management of traffic to reduce or eliminate accidents, is the other critical reason for traffic control. An airline pilot needs to be warned of high winds at the destination airport just as an automobile driver needs to be warned of a dangerous curve or intersection ahead. Traffic control has as its principal objective to manage the movement of people and goods as efficiently and safely as possible. The dual objectives, however, frequently conflict or, at least, compete. For example, there are frequent cases in which commercial airlines are held on the ground at their originating airport until they receive a clearance to land at a destination. The clearance is given only when the destination airport determines that the number of airplanes expected to arrive at a particular time is small enough that local air traffic controllers can assist the plane in landing without overtaxing their human limitations and compromising safety. In road traffic, intersections with traffic lights (i.e., green, amber, and red indications) will often add a separate lane with a lighted green arrow to allow left turns with no opposing traffic. This frequently results in longer nongreen periods at the intersection, causing an increased delay and a reduction in efficiency and mobility. Traffic control will always be burdened with seeking to satisfy the frequently conflicting goals of safety and mobility. Safety is not the exclusive concern of the traffic control community. Nearly every transportation mode has organizations that regulate operators through a series of licensing procedures, sanctions for inappropriate operating practices, and requirements for continuing training to retain certification to operate. Examples include federal aviation authorities that oversee pilot training (e.g., the U.S. Federal Aviation Administration); road agencies that administer driver's licenses may exist at the provincial level (as in Canada) or at the national level (as is more common in Europe). Transportation safety management is thus accomplished through a complex set of interactions between different agencies at different levels (e.g., national, regional or state, and local) using both formal legal requirements and administrative actions. The following discussion will necessarily focus on safety concerns that evolve from and are a component of the traffic control function. Additional reading Theories and concepts underlying road traffic control are discussed in Adolf D. May, Traffic Flow Fundamentals (1990); and William R. McShane and Roger P. Roess, Traffic Engineering (1990). For air traffic control, see Robert Horonjeff and Francis X. McKelvey, Planning and Design of Airports, 3rd ed. (1983). Coverage of rail traffic control can be found in the series of Transportation Research Record (irregular), published by the Transportation Research Board of the National Research Council in Washington, D.C. Marine transportation control is thoroughly treated in A.N. Cockcroft and J.N.F. Lameijer, A Guide to the Collision Avoidance Rules, 4th ed. (1990); and Charles W. Koburger, Jr., Vessel Traffic Systems (1986). Paul P. Jovanis