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Common control techniques

The traffic control devices described in the previous section are summarized in greater detail in the Table. These devices, individually and as a system, assist the driver in making safe, efficient, consistent decisions. There are several additional more specialized cases, which are described below.

Traffic signal controllers are electronic devices located at intersections that control the sequence of the lights. Along with computers, communications equipment, and detectors to count and measure traffic, the controllers are frequently grouped together to control large numbers of traffic signals, either at intersections in a city or on ramps approaching expressways and motorways. While the detailed brand and type of equipment vary greatly, the functions performed by the systems are generally consistent.

There are four basic elements in a computerized traffic control system: computer(s), communications devices, traffic signals and associated equipment, and detectors for sensing vehicles. Traffic flow information is picked up by the detectors from the roadway and transmitted to the computer system for processing. The detectors are normally embedded in or suspended above the roadway. Vehicle counts and speeds are typically measured; vehicle type (e.g., auto or truck) also may be obtained. The computer processes the traffic flow data to determine the proper sequence for the lights at the intersections or ramps. The sequencing information is transmitted from the computer through communications equipment to the signals. In order to assure safe and proper operation, information is also transmitted from the traffic signals to the computer, confirming proper operation. Humans can interact with the system by accessing the computer system in some way.

While these are the general principles, important variations are possible. First, it is common to find some form of computer as part of the traffic signal at the intersection or ramp to be controlled. This allows the local computer to process traffic flow data directly, reducing communications needs and costs. Another variation is that selected vehicles themselves may transmit traffic data directly to the computer system. This is frequently combined with the ability to receive information in the vehicle regarding points of congestion, so the driver can choose to avoid them. If the two-way communication exists between the vehicles and computer system, it may not be necessary to have separate physical detectors.

Another area of application for traffic control devices is their use in traffic restraint (often called traffic "calming"). Rather than use traffic control to increase efficiency of movement, controls are used to create impediments that restrain traffic from sensitive areas. Most commonly applied in older cities whose road network does not match current needs, traffic restraint seeks to funnel traffic onto particular routes by creating impediments to movement on others. These other routes typically have some special value--a historic site or a residential character--that requires protection. Devices typically used include speed bumps, barricades to block streets, turn prohibitions, stop signs, and raised pavement markers.

Traffic restraint also includes programs to foster bicycle and pedestrian travel. Wider sidewalks, sometimes including tables and benches, and bicycle lanes frequently accompany restraint actions. These programs recognize that what is good for vehicular travel may not necessarily be positive for other road users, the environment, or the neighbourhood. An unfortunate aspect of these programs is that their benefits and costs are highly localized. Those living on the "right" side of the restraint device generally experience slow speed and lower traffic volume. Those living along the routes onto which the traffic is funneled must endure increased vehicle volumes and speeds.

Traffic control also can be used to give priority to high-occupancy passenger modes. The objective of such actions is to emphasize people rather than vehicle movement. A variety of techniques are available and are employed in priority treatment approaches. The most common is the dedication of special lanes to the use of priority, or high-occupancy, vehicles. Buses and car pools can use the lanes to move at high speeds along congested expressways and motorways, bypass queues at expressway ramps, and move along congested arterial streets. Because these special lanes are designed to operate uncongested, they provide an incentive, through reduced travel times, for travelers to leave private single-passenger automobiles and travel by multipassenger modes. Buses also may be given priority by allowing only them to turn at intersections and to be provided with extra green time at a traffic signal. The undesirable feature of such systems is that they provide improved service to high-occupancy modes while sustaining or increasing congestion for others. The residual congestion for other road users may result in continued wasteful fuel consumption and high vehicle pollutant emission.