Summary

The City of Portland has optimized traffic signal timing at 135 intersections on 16 streets in Portland. This optimization work has resulted in saving motorists over 1,750,000 gallons of gas each year. This reduction in gasoline consumption is equivalent to 15,460 tons of CO2 each year.

What is it?

Traffic signal optimization is the process of changing the timing parameters relative to the length of the green light for each traffic movement and the timed relationship between signalized intersections using a computer software program known as Trafficware Syncro Studio. Optimizing traffic signal timing reduces both idling and the acceleration of vehicles, leading to less fuel being burned and less carbon dioxide emissions.

How does it work?

Traffic engineers perform the optimization process, either as employees of the municipality or as employees of a consulting engineering firm.Each set of traffic signals operates under a unique set of timing parameters, computerized into a traffic signal controller located in a field cabinet at the intersection. The timing parameters include minimum and maximum green durations, pedestrian indication requirements, gap and extension times, overlaps and the phase change intervals (yellow light). Each traffic signal also has fixed cycle, offset and split timing parameters.

  • Cycle Parameter: time needed to serve all phases.
  • Offset Parameter: time from one reference point to another reference point (from one green light at one intersection to the start of green light at the next intersection). This allows for progressive movement: vehicles moving at the proper speed to advance from intersection to intersection without stopping.
  • Split Parameter: time allowed for each specific phase.

As traffic patterns change throughout the day and the week, it is appropriate to modify the signal timing parameters to reflect the needs of these unique traffic As traffic patterns change throughout the day and the week, it is appropriate to modify the signal timing parameters to reflect the needs of these unique traffic patterns. Engineers gather data on existing traffic volumes and variations throughout the day.

The engineer enters the data into a computer software program that models the existing signal operation and recommends improved signal timings. The improved signal timing strategies will lead to the minimization of stops and delays, fuel consumption and air pollution emissions, and the maximization of progressive movement through the system.

There are several traffic signal timing optimization software programs available. The City uses Trafficware’s Synchro Studio program. (More information can be found at: http://www.trafficware.com/products.html)

Staff then install the recommended timing information into each intersection’s traffic signal controller.

The computer software program then re-evaluates the effectiveness of the changes and develops different timing parameters for the morning and evening peak periods, the midday off-peak periods, weekends, and nights.The computer software program computes the changes in stops and delay. The software program then uses internal algorithms to determine fuel consumption. The change in fuel savings between the initial and improved signal timing is converted into metric tons of CO2 using the conversion factor of 8.8 metric tons of CO2 saved for every 1,000 gallons of gasoline not consumed.

The cost to re-time one intersection ranges from $1,000 to $3,000 per intersection. The cost of the signal optimization work was $533,000, which was provided by the Climate Trust of Oregon. Due to budget constraints, the City of Portland would not have been able to do this work without funding from The Climate Trust.

Next steps

The City of Portland is seeking another $50,000 from The Climate Trust to continue the effort. The Climate Trust is a non-profit organization based in Portland that invests in projects that generate CO2 offsets. The Climate Trust then sells those carbon offsets to power plants, regulators, businesses, and individuals.

Application

  • Optimizing traffic signal timing is a fairly straightforward process.
  • Agencies operating traffic signals can either having their traffic engineering staff perform the retiming process or they can hire a traffic-engineering consultant.
  • A computer software program is required that models existing signal operations and recommends improved signal timings. There are several computer software programs to chose from, including Trafficware Ltd. which Portland uses.