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.
The signal timing development process involves eight key steps, ranging from the organization of existing data, to the collection of new data and the calculation of local timing and coordination parameters. The final step in the process is the installation and evaluation of the new signal timing plan. It is important to test the effectiveness of the new settings, so that any necessary adjustments can be made to maximize operational efficiency.
- Conduct travel time and stopped-time delay studies, when the budget allows. The travel time studies of a route up and down the arterial will provide a quantifiable measure of the traffic performance in the corridor. To complement this information, delay studies at selected intersections will provide insight to the operation of specific intersections. It is recommended that delay studies be conducted at the critical intersection in the grouping. The critical intersection is generally the one that is most congested and that drives the cycle length for the group. The combination of travel time studies and delay studies will provide an excellent baseline condition against which future changes in signal timing can be compared.
When only a minimum budget is available for signal timing projects (i.e. the shoestring budget approach), the evaluation phase of the project becomes even more important. With the absolutely minimalist approach, a different process is used for developing new signal timing plans. Instead of estimating the input data for a signal timing optimization computer program, the process involves estimating the parameters (cycle length, split, and offset) themselves using time space geometry and estimates based on engineering judgment. In these cases, when manual (rather than automated) techniques are used to generate timing plans, many steps that normally provide checks and balances are skipped, so the evaluation phase is essential. The practitioner should expect to spend 20% to 30% of the timing budget on this evaluation and fine tuning effort.
The following specific lessons learned are made with regard to evaluation of signal timing plans on a shoestring budget:
- Install the signal timing parameters in each controller.
- Test the plan during a benign traffic period, such as mid-morning after the morning peak rush hour. This will enable the agency to observe that the offsets are as expected and to identify any serious discrepancies without having a large negative impact on traffic operations.
- Place the plan in operation during the period for which it was developed. Once the plan has been tested during a period of low traffic demand, then it can be used during the period for which it was designed. Again, the offsets at each intersection should be observed. During peak periods, left turn bays should be checked for spillback.
Travel time and delay studies are an effective means for testing and evaluating new signal timing plans, but on a shoestring budget, such methods may not be an option. In cases where the budget is minimal, agencies will still need to devote adequate resources to evaluating their signal timing plans. In particular, tests of the new signal timing plans should be made during periods of low traffic demand. Through evaluating the new signal timing plans, agencies will be able to identify any problems and make adjustments as necessary. In this way, they can maximize operational efficiency and achieve the desired mobility benefits.
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