Appendix C. Frequency of Signal Retiming

The need for signal retiming or rephasing may result from the following:

• Change in population level and car ownership in the region
• Change in land use that changes traffic demand.
• Change in traffic patterns resulting from changes in the highway network.
• Change in traffic operations such as corridor management plans.
• Change in motorist driving patterns resulting from increased emphasis on corridor management, use of transit or car-pooling.

Many agencies have reported a wide range of benefits and benefit to cost ratios resulting from signal retiming. Because of differences in evaluation methods and periods between retiming it is difficult to provide a general benefit figure except to say that it usually exceeds the cost of retiming by many times and usually results in a significant reduction in delay and fuel consumption.

Figure 1 (1) shows a general concept for assessing the benefits of signal retiming as a function of time. The figure shows that after three years the benefits begin to increase significantly.

Swayampakala and Graham (2) studied the benefits of retiming signals at varying time intervals for a number of intersections in several cities in North Carolina. They calculated delay using SYNCHRO runs based on collected turning movement counts. The benefit is computed as the saving in delay less the cost of retiming. Figures 2, 3 and 4 show the results of the research. Retiming intervals in the figures are shown in months. The results vary widely from location to location. No pattern can be observed with different rates of change in traffic volumes.

Swayampakala and Graham conclude that the suggested time period for signal retiming is 24 to 30 months. Parsonson (3) recommends a one to three year time interval.

The NTOC survey data responding to the question "Does your agency conduct a comprehensive review of area-wide or corridor signal timing at least every 3 years or sooner if justified" was analyzed. A score of 5 corresponds to retiming every 3 to 5 years. The scores are as follows:

Score	Percentage
5	12
4	24
3	25
2	19
1	18

Thus, a relatively small number of agencies retime the signals at 3 to 5 year intervals or sooner. A 30 to 36 month time period for the constrained system would therefore appear to capture most of the benefits indicated by prior research, while still setting an achievable goal for operating agencies.

REFERENCES

1. Sunkari, S. "The Benefits of Retiming Traffic Signals", ITE Journal, April 2004, pp 26-29.
2. Swayampakala, R.K. and J.R. Graham, "Optimum Time Intervals fir the Traffic Signal Re-Timing Process, presented at the 2004 Annual Meeting of the Transportation Research Board.
3. Parsonson, P. "Signal Timing Improvement Practices." In Synthesis of Highway Practice, No. 172, TRB, pp 1-10.

Figure 1. Savings in User Costs due to Signal Retiming

Figure 2. Annual Savings vs. Re-Timing Intervals Intersections with Significant Rate of Change in Traffic Volumes

Figure 3. Annual Savings vs. Re-Timing Intervals Intersections with Moderate Rate of Change in Traffic Volumes

Figure 4. Annual Savings vs. Re-Timing Intervals Intersections with Minimal Rate of Change in Traffic Volumes

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