The National Academies

NCHRP 03-90 [Final]

Operation of Traffic Signal Systems in Oversaturated Conditions

  Project Data
Funds: $600,000
Research Agency: Kimley-Horn & Associates
Principal Investigator: Douglas Gettman
Effective Date: 3/14/2008
Completion Date: 7/14/2012

The objective of this research is to develop a guide for operating coordinated traffic signals in oversaturated conditions on networks, including arterials.  Some of the strategies and methods that should be addressed in the guide include metering, gating, turn restrictions, dynamic lane control or assignment, offset weighting, signal control priorities, and negative/backward progression. Congestion pricing and other travel demand management strategies should not be addressed. The guide should incorporate the results of previous research on single intersections to provide a comprehensive document.


NCHRP Web-Only Document 202: Operation of Traffic Signal Systems in Oversaturated Conditions, includes two volumes.
Urban and suburban intersections operating in oversaturated conditions are now very common and are expected to become even more prevalent. Oversaturation in a signal network can be complex due to the interactions between the intersections. Understanding the underlying cause(s) of a network’s oversaturation is the critical first step in mitigating it, but there is no coherent method of determining this. As a result, guidance on how to mitigate traffic signal oversaturation, both in space and time, is becoming increasingly important. 
Previous research has largely focused on operating a single oversaturated intersection. Additional research is needed that focuses on systemic approaches to dealing with oversaturation in a signal network of four or more intersections. 

Task 1.  Review relevant domestic and international literature and summarize the state of the art in operating coordinated traffic signals in oversaturated conditions.  

Task 2Summarize the various definitions of oversaturation for a signalized network. Develop a diagnostic framework for identifying oversaturation in a signal network and the underlying causes based on observable conditions. Techniques for measuring the inputs to the framework, including the physical extent and time duration, should be described. Approximately halfway through this task, conduct a web conference with the NCHRP project panel to present the work that has been done on this task and to discuss the potential interviewees for Task 4. 

Task 3.  Describe multimodal objectives and performance measures for managing a signal network. These objectives and performance measures may vary as a function of the degree of saturation. Catalogue operational strategies and methods to manage oversaturated signal networks that correspond to these operational objectives and performance measures. Summarize documented benefits and maintenance requirements of these strategies and methods. 

Task 4.  Conduct interviews with at least ten experienced practitioners with varied backgrounds to get feedback on the Task 2 diagnostic framework and to determine how they select strategies and methods. Expand the catalog of strategies and methods to include those identified in the interviews. Refine the diagnostic framework and, if possible, develop a concise definition of oversaturation in a signal network.  

Task 5.  Within 6 months of the effective date of the contract, submit an interim report that includes a summary of the results on Tasks 1 through 4, a detailed outline and mockup of the guide that will be produced in Task 6, criteria for evaluating the effectiveness of the guide, and the plan for the pilot testing of the guide in Task 7. The research agency will be required to meet with the project panel approximately 1 month later to obtain NCHRP approval before beginning subsequent tasks. 

Task 6.  Develop the guide. The guide will present a rational approach that includes (1) diagnosis of the type and causes of oversaturation in a traffic signal network, (2) setting of the operational objectives and performance measures for the system, and (3) selection of appropriate operational strategies and methods for mitigating the oversaturation. The guide must foster the best use of the existing infrastructure and be designed for both inexperienced and experienced engineers and, as appropriate, signal technicians. Operational strategies and methods must be fully described, including the equipment and software requirements. Tools that are available for carrying out the steps of the rational approach should be identified, including real-time system monitoring. The guide should be primarily focused on alleviating recurring congestion, but should be applicable to planning for special events and other types of incidents. 

Task 7.  Work with five transportation agencies in applying the guidance to specific, diverse networks. Following implementation of the selected strategies and methods, assess their effectiveness in mitigating congestion. Document the obstacles and objections raised by the agency personnel and revise the guidance as applicable to increase its utility.   

Task 8. 
Submit a final report, including the revised guide as a stand-alone appendix.

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