The National Academies

NCHRP 03-38(4) [Completed]

Traffic Signal Control for Saturated Conditions

  Project Data
Funds: $270,000
Research Agency: KLD Associates, Inc.
Principal Investigator: Edward B. Lieberman
Effective Date: 10/1/1987
Completion Date: 6/4/1992

Medium- and large-sized urban areas throughout the United States experience saturated traffic flow conditions on almost a daily basis. Saturated operating conditions are characterized by the existence of queues that are not able to discharge within a reasonable period of time at a given signalized intersection. These queues may grow to sufficient length to adversely affect upstream intersection operations.

The objectives of this research were to (1) develop procedures and guidelines for applying appropriate signal-timing strategies to minimize the impact of recurring saturated traffic conditions under a wide range of network geometry, traffic flow patterns, and operating conditions; and (2) develop procedures that can be used in computerized signal systems for real-time response to both recurring and nonrecurring saturated conditions.

In response to these objectives, the research has produced a comprehensive two-volume final report. The first volume summarizes the state of the art and current perceptions of the oversaturated traffic environment. It was found that "no coherent approach to the problem of controlling an oversaturated traffic environment exists...."

As a result of the effort to develop a comprehensive understanding and description of the problem, and the finding that no coherent approach exists, the researchers and the project panel decided to develop a new optimal control policy designed expressly for the control of oversaturated signalized networks. Thus the second volume of the final report presents a new approach, designated as the Internal Metering Policy (IMP).

The goal of IMP is to manage the growth of queues to maximize the productivity of the roadway system. That is, the policy is designed to service as many vehicles through the specified roadway system as possible, in a given period of time. This goal is met if the policy influences traffic operations to satisfy the following objectives: (1) control queue formation to prevent spillback into intersections, (2) fully utilize all available service (green indication) time at the highest service rate, (3) effectively use all existing roadway storage capacity, (4) provide equitable service to competing traffic streams, (5) minimize the number of vehicle stops, and (6) minimize delay along the undersaturated elements of the network.

Following comprehensive presentation of the IMP, volume two of the final report concludes with an outline of the next steps in the development that would be required before the policy could be implemented in computerized traffic signal networks.

The final report from this project will not be published in the regular NCHRP report series.

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