VI. Signal Timing Examples

One way to illustrate how the tools and techniques presented in this document can be used to develop good signal timing plans on a shoestring (low budget) is to provide an example. Three different examples are discussed illustrating different measures that could be used depending on the available resources.

Moderate Signal Timing Budget

In this example, an agency retained a consultant to prepare signal timing plans for a suburban arterial three years ago when the arterial was widened and new controllers were installed. The new timing plans worked well in the beginning, but as development increased in the corridor, some obvious problems are beginning to emerge. Most of the problems can be related to the opening of the new shopping center with major signalized entrances at two of the 10 signals in the corridor. Although a consultant prepared the existing signal timing plans, the agency required the consultant to develop the timing plans using the same program that the agency uses and required the consultant to deliver all of the data input files for the program.

1—Identify the System Intersections

In the example, this is obviously the 10 intersections described in the arterial.

2—Collect and Organize Existing Data

This is a critical step in minimizing the cost of developing new plans. In the example, one can assume that there are turning movement counts available for all 10 intersections—the only problem is that they are 3 years old and there have been changes in the traffic demand in the corridor. Nevertheless, it is likely that some of the existing turning movement data can be updated and used in developing the new plans. However, new turning movement counts probably will be required for the two intersections that are the entrances to the shopping center.

Another type of data that will prove to be very useful is a 24-hour directional count on the arterial. The directional count can be used to estimate any overall change in the traffic demand during the last three years. Factors developed using these data can be used to adjust the three-year-old turning movement counts to reflect the current demand.

The agency is fortunate in that other data that would normally be collected (e.g., distance between intersections, intersection geometry, signal phasing, and the like is already available and coded in the optimization program input files). For the most part, the agency only has to update the traffic demand information and rerun the program to obtain updated signal timing plans.

3—Conduct Site Survey

Although many engineers are tempted to skip this step, experience has shown that a visit to the site during the hours for which the timing plans are being developed can be very beneficial.

Specific things to look for are unusual demand conditions. In the example, the timing at the two intersections affected by the shopping center needs to be updated—one might assume that the signal settings at the other eight intersections may be pretty good. This can be confirmed by a quick trip to the location during the peak periods. This field visit frequently exposes additional issues that can be readily addressed with new timing plans.

4—Obtain Turning Movement Data

This is the key step in minimizing the cost of developing signal timing plans. In the example, assuming that the 24-hour data can be used to update the old turning movement counts and that the site visit did not uncover a latent demand condition, new counts would be needed only at the two intersections that are heavily impacted by the shopping center. The 24-hour directional count data is used to reaffirm that the arterial should continue to operate with four timing plans.

To obtain the turning movement data, the agency will conduct short counts during the AM Peak, PM Peak, afternoon, and evening periods at the two locations. The times when the short counts are scheduled are determined from the 24-hour directional data. Factors developed from the 24-hour directional data are used to update the existing turning movement counts at the remaining eight intersections.

5—Calculate Local Timing Parameters

Because there is no significant change proposed to the operation of the individual controller, there is no need to change any of the local timing parameters.

6—Identify Signal Groupings

Because there is no need to change the structure of the system, there are no changes required in the grouping of the signals.

7—Calculate Coordination Parameters

Once the turning movement data are available, it is a simple matter to update the input files and rerun the optimization program.

8—Install and Evaluate New Plans

The physical installation of the new timing plan parameters is usually straightforward, but a task that is done manually offers the potential for significant errors. A major concern in the installation phase is to verification that the optimized parameters are, in fact, the parameters that are installed.

The second issue in this step is evaluation of how effective the new settings are. For agencies that have some resources that can be applied to signal timing issues, evaluation is an area that provides increasing benefits over time. To evaluate the timing plans developed in this scenario, two separate types of studies are recommended: travel time and stopped-time delay. 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. In the example, consider making delay studies at the two intersections where entirely new plans were installed. In addition, it is appropriate to make delay studies at the critical intersection in the group. The critical intersection is generally the most congested; it is the intersection 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.

Modest Signal Timing Budget

The second example uses a similar geometric situation with an entirely different system history. In this case, there is a 10-intersection arterial; but until now, all intersections have operated as full-actuated, isolated signals. The agency has just hired an electrical contractor to install new controllers, interconnect cable, and an on-street master. This is the classical installation of a “closed loop” system. To save scarce funding, the agency plans to develop the timing plans themselves and has limited resources for this effort.

1—Identify the System Intersections

In this second example, it is obvious that the 10 intersections will constitute the signal system.

2—Collect and Organize Existing Data

The use of existing data is even more critical in minimizing the cost of developing new plans. In this example, one can assume that there are turning movement counts available for all 10 intersections; but the turning movement counts were made during different years. Some of the existing counts are more than 10 years old. While there have been changes in the traffic demand in the corridor over the last 10 years, most of the development has taken place on the East side. It is likely that some of the existing turning movement data at intersections on the West side can be updated and used for developing the new plans. However, new turning movement data will likely be required for the five intersections on the East side.

As with the first example, the 24-hour directional count on the arterial will prove to be very useful. If a recent directional count is not available, one must be made as a first step. Hopefully, there are older 24-hour counts that are available and can be used to develop factors to adjust the existing turning movement counts to reflect the current demand.

Although the available funds are modest, the agency owns a signal timing optimization program and plans to use it. This decision may cost a little more in the short run, but it will allow the agency to easily revise the signal timing plans in the future when the input file is updated with new turning movement counts. In the meantime, it is necessary to collect all of the descriptive required by the program.

This descriptive data includes the following:

A condition diagram of each intersection showing the number of lanes and width of each lane on all approaches. The condition diagram must have a North arrow and show the street names.

A phasing diagram for intersections with existing controllers. It is important for the phasing diagram to include the NEMA phase number for each phase movement. The phasing diagram must also show all overlaps (if any).

Distance between intersections and the free-flow travel speed for the conditions under which the timing plan will operate. This information should be depicted on a map of the area showing the roads and signalized intersections. It is not necessary for the map to be drawn to scale, but it is important for each link on the map to be long enough to be able to show various data such as link length, speeds, and volume.

With this information in hand, particularly the 24-hour directional count, the Engineer can begin making some difficult decisions concerning turning movement data. A first step is to identify each intersection as either primary or secondary. As previously noted, the primary intersections are the ones that are the most congested and that drive the cycle length requirements. Another initial step is to determine how many timing plans are required for the facility. Using the methodology that compares total intersection and directional volume hour by hour is a good indication of when new timing plans are needed.

The engineer has three options to obtain the turning movement data that is necessary to run the optimization program: update the existing counts, make new counts using short-count techniques, or estimate the turning movements using a program like Dowling Associates TurnsW. With the provided scenario, one would tend to update the intersections on the West side (lowest growth) and make new short counts at the intersections on the East side. The estimated counts could be used at secondary intersections on the West side if data collection costs were a major limitation.

3—Conduct Site Survey

To save time and cost, many engineers are tempted to skip this step. This would be a mistake. Experience has shown that a visit to the site during the hours for which the timing plans are being developed will be beneficial.

4—Obtain Turning Movement Data

This is typically the most costly step in the signal timing process. After making the best possible use of the existing data as previously described, this step provides for the field data collection of turning movement counts.

5—Calculate Local Timing Parameters

Because the existing controllers are operating in the full actuated mode, the local timing parameters probably do not require any change.

6—Identify Signal Groupings

Because there is no need to change the structure of the system, there are no changes required in the grouping of the signals.

7—Calculate Coordination Parameters

Once the turning movement data are available, it is possible to run the optimization program.

8—Install and Evaluate New Plans

The physical installation of the new timing plan parameters is usually straightforward, but any task that is done manually offers the potential for significant errors. A major concern in the installation phase is to verify that the optimized parameters are in fact, the parameters that are installed.

The second issue in this step evaluation of how effective the new settings are. For agencies that have few resources, the evaluation frequently must be subjective. Nevertheless, this element is a very important part of the signal timing process. Rather than conducting formal travel time studies, the engineer may simply drive the arterial several times looking for discontinuities in the traffic flow or congestion where none was expected. These informal evaluations must be conducted during the periods for which the timing plans were developed. The engineer should pay particular attention to any instances of cycle failure (approach demand is not met during a green phase) as an indication of a serious deficiency.

Minimum Signal Timing Budget

The third example involves developing signal timing with the absolute minimum of resources. This example is similar to the previous example, except there are only five intersections on the arterial and all are currently operating as isolated, actuated controllers. However, the controllers have time-based coordination capability, and the engineer wants to use this function.

In the previous examples, the cost of preparing the plans was minimized by reducing the cost of the data collection effort necessary to run the optimization software, specifically, the turning movement counts. With this absolutely minimalist approach, a different process is used. Instead of estimating the input data, the process involves estimating the parameters (cycle length, split, and offset) themselves using time-space geometry and estimates based on engineering judgment.

1—Identify the System Intersections

In the example, the system consists of the five intersections.

2—Collect and Organize Existing Data

As with the previous examples, the practitioner must make maximum use of existing data, especially existing timing data and system layout information.

3—Conduct Site Survey

This step is absolutely vital with the minimalist approach. The practitioner must carefully observe each of the five intersections for several cycles during the period for which the timing plan is being developed. In this survey, the practitioner must estimate the average speed of the traffic between intersections on the artery, and if not available in the existing data, the practitioner must measure the distance between intersections.

As with the previous examples, specific issues to look for are unusual demand conditions, pedestrian demands, and any congested movements. The primary output of this effort is the identification of the primary intersection in the group.

4—Obtain Turning Movement Data

With the minimalist approach, the practitioner must obtain current turning movement information for the primary intersection usually by using short-count techniques as with the previous examples. In addition, the practitioner must obtain 24-hour directional counts on each of the four major approaches to the primary intersection.

At secondary intersections, the practitioner must observe the operation of the intersection for several cycles and record the average phase time for each phase during the cycle.

5—Calculate Local Timing Parameters

Unless a problem is identified during the Site Survey, there is usually no need to change any of the local timing parameters.

6—Identify Signal Groupings

Because there is no need to change the structure of the system, there are no changes required in the grouping of the signals.

7—Calculate Coordination Parameters

In the previous two examples, this step involved running an optimization program. With the minimalist approach, this step involves estimating the coordination parameters directly.

Cycle Length—The cycle length must be selected for the group based on meeting three criteria: optimum cycle length, resonant cycle length, and pedestrian constraints.

Using the turning movement information at the Primary intersection, calculate the optimum cycle length using Webster’s equation:

Cycle Length = (1.5 * L + 5) / (1.0 -Y_i).

Using the intersection layout and phasing, determine the minimum pedestrian cycle length:

Ped Minimum Cycle = LT + 14 + W_m / S_P +W_c / S_P+Y

Where:

LT = Time required for the left turn movements.

14 = (2 * 7) Time that the “Walk” is displayed to cross each street.

W_m / S_P = Width of the Main Street divided by the pedestrian speed.

W_c / S_P = Width of the Cross Street divided by the pedestrian speed.

Y = Yellow and All Red time for critical phases.

Using the average distance between the five intersections, calculate the potential resonant cycles using the following equations:

Cycle = 2 * Distance / Speed

Cycle = 4 * Distance / Speed

Cycle = 6 * Distance / Speed

Cycle = 8 * Distance / Speed.

Select the shortest resonant cycle that is longer than the optimum cycle and the pedestrian minimum cycle. If none of the resonant cycles are longer than the optimum cycle, select the longest resonant cycle.

Offset—The offset is calculated using the Kell method.

Split—The split is calculated for the primary intersection using the Critical Movement method. Once the splits are calculated based on demand, they must be checked to verify that the pedestrian requirements are met. For example, assume that the calculated splits required 15 seconds for phases 4 and 8. If the minimum pedestrian time were 19 (7 + 48/4) seconds, then the intersection split would have to be increased to 19 seconds to accommodate the pedestrian movement.

At intersections other than the primary intersection, the split is based on the average phase time observations. These averages are converted to a percent of the cycle, and new splits are calculated by multiplying these percent splits by the new cycle length. The final split is based on the pedestrian requirement or the average phase time, whichever is longer.

8—Install and Evaluate New Plans

As with the other methods, the physical installation of the new timing plan parameters is usually straightforward, but any task that is done manually offers the potential for significant errors. A major concern in the installation phase is to verify that the optimized parameters are, in fact, the parameters that are installed.

This minimalist approach takes many short cuts compared to the steps that would be followed in a traditional signal timing effort. It is very important for the Practitioners to carefully verify these plans in the field. In fact, in spite of the objective of minimizing costs, more time and effort should be planned for this task under this approach than either of the two other approaches that use optimization software. The new plans should be tested during a period of low traffic demand conditions before they are used in the period for which they are designed. For example, a plan for the AM rush hour might be tested in the late morning or early evening when the demand is usually lighter. This test period should allow the Practitioner to identify any serious discrepancies without having a large negative impact on traffic operations.