Traffic congestion is a severe and growing problem. The immediate consequences of congestion can range from delays to reduced throughput of vehicles. Long-range consequences include reduced safety, environmental pollution, and reduced economic competitiveness. Continued expansion of transportation facilities cannot be the only answer to either immediate or long-range congestion problems. The authors believe that traffic congestion can be relieved by using suitable control measures and strategies. To that end, this paper gives an overview of advanced traffic control for urban road networks and freeway networks, as well as for route guidance and information systems. Fixed-time techniques that use historical data to set up strategies for specific time-of-day control are discussed, but the main focus of the paper is on traffic control strategies that use control inputs in real time to achieve specified goals. Looking at urban road networks, the authors present an overview of approaches to optimal use of traffic lights. The goal is to minimize the time spent by vehicles in the road networks. Control can be exercised by systems that take account of the: • number and constitution of stages at intersections during which a set of traffic streams has the right-of-way; • green light duration for each stage; • time taken for a complete cycle of signals to take place at an intersection; • offset between cycles at successive intersections. The authors define various urban road traffic control strategy types, and give examples of existing strategies. Most industrialized countries now use fixed-time strategies for urban traffic control. The fixed-time strategy SIGCAP is recommended for preventing oversaturation in intersections with high demand variability. Fixed-time strategies that use phase-based approaches to consider different staging combinations are also mentioned. Basic operation is explained for MAXBAND, a fixed-time control strategy designed to provide coordinated control of multiple intersections on a two-way traffic artery. TRANSYT, the most popular fixed-time coordinated control strategy, is described in some detail. This strategy has achieved savings of 16% of average travel time through networks to which it has been applied. Traffic-responsive strategies make use of real-time measurements collected by sensors such as inductive loop detectors. The authors point out that coordinated traffic-responsive strateDigital Object Identifier 10. 1109 /JPROC.2003 gies, when suitably designed, are potentially more efficient than fixed-time systems. They mention the SCOOT system, a traffic-responsive version of TRANSYT that has been used in over 150 cities, pointing out that SCOOT performance deteriorates in saturated traffic conditions. A discussion of several more recently developed coordinated traffic-responsive strategies describes the basic problem they all face: use of solution algorithms that are exponentially complex and that limit real-time solutions to only one intersection. Store-and-forward techniques for modeling urban road traffic control allow use of very efficient optimization methods to coordinate real-time control at multiple intersections. These techniques are able to handle saturated traffic conditions. Store-and-forward strategies are being used in Scotland, Greece, and Israel, with very satisfactory results. The authors believe that, for large traffic networks that include freeways as well as urban roads, store-and-forward techniques offer a feasible approach for real-time integrated control. Looking at freeways, the authors see facilities that are not able to deliver their traffic capacity capabilities during the peak hours when they are most needed because of a lack of suitable traffic control systems. Early in this paper, equations are developed and applied to demonstrate that total time spent by vehicles in a freeway network can be reduced by up to 50% (half the overall journey time) using a strategy based on metering the flow of vehicles through freeway on-ramps. However, the authors note that freeway traffic control is the exception rather than the rule, and the potential of ramp metering for relieving congestion has not yet been realized. The paper points out that ramp metering is a very sensitive technique. Inaccurate, weak metering can fail to prevent congestion, while overly strong metering may prevent full use of freeway capacity. A metering strategy popular in North America is described, as is a strategy used in Scotland. The authors report that at a field trial in Paris, the ALINEA strategy, was the outstanding performer. Furthermore, a multivariable regulation strategy (METALINE) that makes use of measurements of freeway traffic volume as well as on-ramp volumes, appears to offer no advantage over the ALINEA ramp-metering strategy in relieving congestion that recurs from hour to hour and day to day. However, METALINE was able to better handle nonrecurring freeway traffic incidents. The paper details simulation of optimal control for a ring road around Amsterdam, using the AMOC software tool. However, the authors find no implementation, as yet, of advanced control strategies that attempt to minimize the time spent by vehicles in large freeway traffic networks.
Actuated TSCs REF make use of loop detectors, which are electromagnetic sensors mounted within the road pavement.