Research and experience have shown that retiming traffic signals is one of the most cost-effective means for improving traffic flow, yet agencies often do not have the budgetary resources to conduct a signal retiming project. The conventional approach to signal timing optimization and field deployment requires current traffic flow data, experiences with traffic optimization models, familiarity with the signal controller hardware and knowledge of field operations, including signal timing fine-tuning. Setting new signal timing parameters for efficient traffic flow can be time consuming and expensive; however, there are practitioners who have developed practical and cost-effective means to shortcut tasks and to still generate efficient signal timing plans.
This report assembles a body of knowledge related to signal timing that is structured to be useful to those who are responsible for making signal timing adjustments. The report defines the signal timing steps that are common to any signal retiming effort and provides a “tool box“ of resources for the practitioner, including techniques that may be used when cost constraints prohibit more traditional solutions to signal retiming. In addition, signal timing examples or scenarios are presented that illustrate how the tools and techniques can be used to develop good signal timing plans on a shoestring.
When developing new signal timing plans, conducting a site survey may be one of the most important steps of the process. Although it is possible to generate both local and coordinated signal timing parameters without ever seeing the intersection, this practice should be avoided. Physical constraints that may or may not be noted on a plan sheet, but that may have an obvious impact on traffic flow, are immediately obvious during a site survey. Vegetation sight distance obstructions, adverse approach grades and curvature, and fading pavement markings are examples of factors that affect traffic flow and that are apparent during a site survey.
On a shoestring budget, the site survey becomes absolutely vital to the process of developing a signal timing plan. The practitioner must carefully observe each of the intersections for several cycles during the period for which the signal timing plan is being developed. Specific issues to look for are unusual demand conditions, pedestrian demands, and any congested movements. The practitioner will need to estimate the average speed for the traffic between intersections on the artery, and depending on what data are available, may need to measure the distance between intersections. The primary output of the site visit is the identification of the primary intersection of the group.
The following lessons learned are presented with regard to conducting the site survey:
- Conduct the site survey after all the existing data have been collected and organized. In this way, the site visit can be used to confirm the accuracy of the existing data and to collect any additional data that may be needed.
- Visit each intersection during the hours for which the timing plan is being developed. For example, if four timing plans are being developed, then the site should be visited during the peak AM period, during a typical day period, during the peak PM period, and during a low volume night period. Site visits during extreme conditions of high and low traffic will provide useful insights into signal operations.
- Collect (or verify) the following information:
- Condition diagram. The diagram should include an intersection sketch, approach lane configurations, sight distance restrictions, and curb restrictions.
- Phasing diagram. The phasing diagram should include the National Electrical Manufacturers Association (NEMA) phase number for each phase movement and should identify the NEMA phase number with the corresponding traffic movement by direction.
- Detector locations. Existing detector locations, type and phase assignment data are necessary to determine the phase interval settings.
- Existing controller settings. The practitioner should assess whether or not the settings are reasonable for traffic conditions and should reconcile any differences among the different sets of local controller timing data.
- Traffic flow. The practitioner should record the typical free flow speed observed on each link and for different timing plans. In addition, it is practical to verify information on link length.
Conducting a site survey is critical to the process of developing a signal timing plan, particularly if the budget is minimal. The site visit enables the practitioner to identify unusual demand conditions and frequently exposes additional issues that can be addressed with new timing plans. On a low budget, the practitioner does not have the analytical tools to evaluate intersection performance, so the observations and experience during the site visit must serve as a substitute. The site visit provides valuable information that enables practitioners to develop signal timing plans that maximize operational efficiency on the arterial, resulting in mobility benefits to the user.
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