SPUR’s report Critical Cooling recommends 42 options for reducing local carbon emissions. This is one of them. To learn about all 42 ideas, read the full report

Retime traffic signals to reduce idling and congestion

Urbanist Article May 1, 2009
Annual savings potential:
Annual public cost:
Public cost per ton:
Implementing agency:
Horizon year:
5,900 metric tons
$17 million
Municipal Transportation Agency


  • Cost of $230,000 per intersection.
  • There are 1,162 signalized intersections in San Francisco
  • There are 2.8 million VMT per day on driven on San Francisco streets.


Fine-tuning traffic signal timing can reduce traffic congestion, cutting short-run CO2 emissions from traffic through the signalized area by about 2 percent. However, doing so requires the investment of time by city engineering staff, and sometimes capital improvements as well. The cost of retiming traffic signals does not appear to justify the practice for emissions purposes alone.

The emissions benefit from retiming traffic signals should also be treated as a short-term benefit. In the long run, the growth in San Francisco’s motor vehicle traffic is substantially constrained by congestion. To the extent that signal retiming is able to reduce congestion, latent demand will make use of newly available roadway capacity, overwhelming the original emissions abatement. It should also be noted that the signal timing that most efficiently accommodates drivers is not always the best timing for pedestrians, cyclists, or transit. Reductions in these modes of travel would increase overall emissions. Signal timing should balance the needs of all modes.

What we do now
Traffic congestion can slow vehicle travel and increase emissions per mile, both for private vehicles and for buses. City traffic engineers work to time signals in a way that reduces congestion. As travel patterns change over time, signals must periodically be retimed.

What we could do
One way to reduce emissions for transportation citywide would be for the city to increase its investment in fine-tuning signal timing, retiming each intersection more often.

Cost and cost components
SFMTA traffic engineers estimate that the cost of signal retiming varies widely, but that $230,000 per intersection is a typical figure. With 1,162 intersections in San Francisco, the cost of fully retiming all intersections would be about $267 million.1 Considered as a capital investment and annualized at 5 percent over 30 years, this signal retiming project would cost approximately $17 million per year. There would be an annual fuel cost savings of $1.3 million accruing to San Francisco drivers. Note that this estimate is for illustrative purposes only: The City does not have the resources or the need to retime all signals at once.

Carbon savings potential
Using a methodology developed by the Center for Clean Air Policy2, and assuming 2.8 million VMT per day in San Francisco and a fleet-wide fuel economy of 25 miles per gallon, we estimate that the retiming would save 1,811 gallons of fuel per day, a VMT equivalent of 45,000. The CO2 reduction would be 5,900 metric tons per year, and the cost per ton of CO2 emissions abatement would be just less than $3,000.SPUR logo

1 Brain Deausault, San Francisco Department of Parking and Traffic.
2 CCAP Guidebook Emissions Calculator. http://www.ccap.org/guidebook


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