The National Academies

NCHRP 03-97 [Completed]

Traffic Signal Analysis with Varying Demands and Capacities

  Project Data
Funds: $850,000
Research Agency: Dowling Associates
Principal Investigator: Richard Dowling
Effective Date: 3/27/2009
Completion Date: 9/26/2011

The objective of this research is to revise the HCM 2010 signal analysis methodologies to allow determination of the effectiveness of traffic signal operations and timing plans given variability in demands and capacities.


The revised report has been received and publication is expected in Spring 2015. The project results were incorporated into SHRP2 research efforts and are being incorporated into a major update of the HCM in NCHRP Project 03-115.


The signalized intersection analysis methodology described in the Highway Capacity Manual (HCM) is one of the most widely used tools for evaluating the operation of signalized intersections. It has traditionally been used to quantify the delay and level of service associated with an existing intersection and with any proposed changes to the intersection’s geometry, timing, or traffic demands.  The HCM also includes methodologies for analyzing signalized interchanges and urban streets that are based on the signalized intersection methodology.
Recently, two new national priorities have been identified: providing motorists with reliable travel times and maintaining signal timing. Determining whether travel times are becoming more or less reliable requires knowledge of the variation associated with the travel time, rather than just the estimate of the mean value currently produced by the HCM.  This variation could be due to variations in either the traffic demand or the capacity of the traffic signal but the HCM methodologies assume that these are fixed values. In addition to those caused by the random events commonly associated with unreliability (e.g., rain, crashes, special events), demand variations occur naturally during the day and may become larger as timing plans age. 
Transportation agencies have a broad range of options of operational approaches (e.g., actuated, pretimed, traffic adaptive) and timing plans for traffic signals. Most agencies, though, have limited resources to analyze and implement these operational approaches and timing plans. It is fairly easy to assess how well an operational approach or timing plan will work for specific traffic demands but much harder to determine the range of demands over which the operational approach or timing plan will operate satisfactorily.   A more robust operational approach or timing plan could operate for more hours of a day, handle nonrecurrent conditions more effectively, and not require retiming as often. 
The HCM signal analysis methodologies (including the interchange and urban streets methodologies) can be an important tool for agencies to use in developing more robust operational approaches and timing plans and in reducing travel time variability.  Research is needed to improve the capabilities of these methodologies in these areas.
Through NCHRP Project 3-92, the TRB Committee on Highway Capacity and Quality of Service is developing a new edition of the HCM, scheduled for publication in 2010. It is expected that the results of NCHRP Project 3-97 will be considered for inclusion in a subsequent edition of the HCM. 
Task 1.  Review draft material for the HCM 2010, project descriptions for relevant SHRP 2 efforts, and related literature to identify gaps in the abilities of the HCM 2010 methodologies to assess the reliability of traffic signal system performance. Describe these gaps and the steps needed to address them. In collaboration with the relevant SHRP 2 contractors, develop a plan for coordinating those efforts, particularly with regard to sharing data. This coordination plan should be kept current throughout the project.

Task 2. 
Develop an approach to setting operational goals for traffic signals at the intersection, corridor, grid, and network levels. This approach should consider, at a minimum, the area type, roadway function, facility users (including pedestrians, bicycles, and transit), proximity to special event locations, and potential for diverted traffic. List measures of effectiveness that support those goals and describe techniques for estimating these measures. Define intersection-level measures of effectiveness that should be generated by HCM methodologies and identify key measures that should be considered as a distribution rather than a point estimate so that the reliability of system performance can be assessed. 

Task 3
Develop a framework and conceptual basis for the methodologies that will be developed in Tasks 6 and 7. Identify the data that will be needed to develop these methodologies. Develop a data collection and analysis plan to generate this data that will be carried out in Task 5. 

Task 4. 
Within 6 months of the contract start, submit an interim report documenting Tasks 1 through 3 and presenting an updated work plan for the remaining tasks. An interim meeting of the project panel to discuss the report with the research agency will be required. The research agency shall not begin work on the remaining tasks without NCHRP approval. 

Task 5.  Carry out the data collection and analysis plan, as approved at the interim meeting with the project panel. 

Task 6.  Develop a method of estimating or characterizing the distribution of lane group capacities for a given signal timing operation or plan. In addition to factors already considered in the HCM, the method should, at a minimum, be sensitive to downstream and upstream conditions, additional timing parameters, and nonrecurrent causes of variability in capacity. 

Task 7. 
Revise the HCM 2010 methodologies for traffic signal analysis so distributions are produced or characterized for the key performance measures identified in Task 2.  The methodologies should reflect how traffic signal controllers commonly operate.  Describe the data elements needed to use the methodologies, document techniques to collect the data, and discuss archival requirements. It is envisioned that, for example, an analyst could use these methodologies to assess the performance of a signal timing plan for base conditions (including normal variations in traffic demand), then try that base plan for special conditions such as snow, and then try a special snow day plan to determine its benefits. Guidance on making signal operation and timing more robust (i.e., ability to handle a broader range of conditions) should be included as appropriate.

Task 8. 
Demonstrate the benefits of considering performance reliability impacts in signal analyses by working with representative transportation agencies to analyze real locations. Document these experiences, including the impacts on agency resources, in the form of case studies, including one that looks at maintenance of signal timing.

Task 9. 
 Develop a computational engine for the methodologies, based on HCM 2010 requirements and building upon HCM 2010 modules as applicable. The computational engine should follow the documentation requirements for the HCM 2010.

Task 10. 
Submit a final report documenting the entire research effort and presenting an updated implementation plan that provides specific recommendations on how the methodologies should be presented to the TRB Committee on Highway Capacity and Quality of Service for consideration in a post-2010 HCM.

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