Major Signal Retiming Improves Traffic Flow on Key Center City Corridors

SDOT today released the initial results of its recently completed traffic signal retiming project in the downtown core. A comparison of travel times before and after the Next Generation Intelligent Transportation System (ITS) Center City Signal Timing Project was implemented shows notable travel time reductions as well as more consistent travel times on downtown streets. These signal timing improvements, completed and modified over the past several months, join recent transit, walking and biking enhancements to improve travel through Seattle’s Center City.

Seattle Skyline Day

Below is a snapshot of weekday travel times from March 2015 (before signal retiming) to March 2016 (after):

  • Central Business District:
    • Fourth Avenue northbound from Jackson Street to Virginia Street:
      • Peak morning times improved 6% or by 25 seconds.
      • Peak afternoon times improved 36% or by 3:09 minutes.
      • Overall improved 19% or by 1:25 minutes.
    • Third Avenue southbound from Stewart St. to Yesler Way (a main commute transit route):
      • Peak morning times improved 4% or by 19 seconds.
      • Peak afternoon times improved 11 % or by 1:14 minutes.
      • Overall improved 5% or by 28 seconds.
  • Denny Way Corridor:
    • Denny Way eastbound from Western Avenue to Dexter Avenue:
      • Peak morning times improved 9% or by 20 seconds.
      • Peak afternoon times improved 24% or by 1:41 minutes.
      • Overall improved 11% or by 33 seconds.
  • Additional travel time reductions during off-peak hours:
    • Northbound Fourth Ave from Jackson St to Virginia St improved by 39 seconds.
    • Southbound Second Ave from Denny Way to Stewart St improved by 3:32 minutes.

SDOT implemented its Next Generation ITS Center City Signal Timing Project in December 2015 and completed work in January 2016. The project divided downtown Seattle into zones and retimed traffic signals in each zone, such as the Central Business District (CBD), Pioneer Square, Belltown, and the Denny Way Corridor.

Highlights of the ITS project:

  • Retimed 260 traffic signals in the downtown core.
  • Divided the existing traffic signal system into a multi-zone network, allowing SDOT greater flexibility to control signal timings for different sections of the network while keeping other zones constant.
  • Adjusted pedestrian crossing times at every downtown intersection to meet current national standards and increase the time pedestrians have to clear intersections before vehicles start moving.

The cost of the signal project was $1.35 million, paid for using REET (Real Estate Excise Tax) funding. Comprehensive signal retiming programs have documented benefits of a 7% to 13% reduction in overall travel time, a 15% to 37% reduction in delay and a 6% to 9% fuel savings (Institute of Transportation Engineers, 2009).

In addition to its major signal timing project, SDOT has made a number of other transportation improvements to keep people moving in downtown Seattle:

The extension and splitting of the RapidRide C Line to South Lake Union and the RapidRide D Line to Pioneer Square was funded by the City of Seattle and began in March of 2016. These changes were designed to improve the reliability of the two lines, which carry more than 21,000 riders each weekday, while connecting riders to growing employment markets.

Thanks to this investment and on-street improvements, C Line on-time performance increased from 80.7% in April 2015 to 84.9% in April 2016. D Line on-time performance increased from 81.4% in April 2015 to 86.7% in April 2016. As a result, for the same time period:

  • C Line ridership increased 27%, about 2,300 new daily rides; and
  • D Line ridership increased 23%, over 2,600 new daily rides. 

The Second Avenue Protected Bike Lane (PBL) was upgraded in April 2016 between Denny Way and Pike Street to improve safety and efficiency for people biking, walking and driving. The project included new traffic signals with dedicated left turns, planter boxes to clarify and buffer the bike lane, and raised driveways to encourage travelers to look out for each other. After the initial PBL installation occurred on Second Avenue the rate of bicycle collisions dropped by 82% and the rate of serious bicycle collisions (involving an injury or fatality) dropped by 79%.

In addition to these transportation improvements, the Director’s Rule for Pedestrian Mobility In and Around Work Zones provides ongoing safety standards for pedestrians around construction zones in Seattle. The Director’s Rule took effect January 1 and makes navigation around construction sites easier and safer for everyone, including those with disabilities. Important standards outlined in the new rule include the requirement that contractors use solid, cane-detectable barriers instead of cones to define the outer edge of pedestrian reroutes, that clear and consistent signage be used, that specifications for meeting ADA sidewalk ramp requirements are met, and that sidewalk and lane closures are only used as a last resort.