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2015 Corridor Capacity Report

2015 Corridor Capacity Report (pdf 6.9 mb)
Annual Corridor Capacity Report (pdf 6.9 mb)

2015 Corridor Capacity Report Appendix (pdf 9.8 mb)
Corridor Capacity Report Appendix (pdf 9.8 mb)

WSDOT's Handbook for Corridor Capacity Evaluation (pdf 950 kb)
WSDOT's Handbook for Corridor Capacity Evaluation (pdf 950 kb)

New this year: Interactive maps help visualize data

Readers can explore this year's corridor performance data within interactive online maps. For an overview of Washington state's transportation capacity, click the image below or go to From there, readers can navigate to specific corridors to delve into areas of interest.

2015 CCR Statewide Map

The 2015 Corridor Capacity Report (pdf 8.9 mb) was created to help inform policy makers, planners and engineers as they examine the multimodal capacity opportunities for state highways. This report supports WSDOT's Practical Solutions and performance-based planning initiatives by reporting the multimodal capacity within 84 urban commute corridors. It also apprises WSDOT, the Legislature, stakeholders, educational and research institutions, the media and the public about highway system conditions and how we can work together to reduce congestion.

This year's publication has three parts:

  1. The 2015 Corridor Capacity Report (pdf 6.9 mb): Compares congestion conditions recorded in 2014 with those in 2012.
  2. A data Appendix (pdf 9.8 mb): Covers detailed performance measures in the form of tables, additional graphs, and information on all commute routes.
  3. An addendum (pdf 950 kb) to the detailed methodology report (first edition of the Handbook for Corridor Capacity Evaluation (pdf 5.7 mb) reflecting the new measures introduced with this year's report: This is a tool for implementing system performance measurement as part of the agency's accountability initiatives and the federal Moving Ahead for Progress in the 21st Century (MAP-21) requirements.


This report reflects WSDOT's partnerships with transit providers, other state agencies and regional planning organizations to implement a multimodal systems and performance analysis framework. The report provides multimodal system analysis, tracking not only congestion trends, but detailing usable capacity on state highways, mass transit, ferries and passenger rail. The report also considers congestion's impacts on air quality and people's wallets, providing a more complete picture of how traffic affects the state as a whole.

WSDOT expands multimodal capacity analysis statewide

With the 2013 edition, WSDOT introduced new performance measures and a new graphical display of information, all with an emphasis on multimodal and person-based metrics. These measures included park and ride lot utilization, routinely congested highway segments, greenhouse gas emissions, transit ridership, and the costs of congestion for commuters. Based on the positive feedback received for the 2013 report, WSDOT continued to enhance the multimodal aspect of system performance evaluation from a corridor perspective:

  • In the 2014 Corridor Capacity Report, multimodal capacity is evaluated along with travel time analyses for all major urban areas statewide where data is available.
  • With the 2015 report, WSDOT published additional multimodal measures including annual passenger miles traveled on transit, capacity savings due to transit, percent of transit seats occupied, and percent of park and ride spaces occupied. In addition, WSDOT introduced a pilot arterial corridor performance analysis of US 395 in the Tri-Cities region. There is little guidance for detailed performance measurement of arterials.

Highlights from the 2015 Corridor Capacity Report

Statewide and regional indicators

  • Although statewide traffic congestion (vehicle hours of delay) has been on an upward trajectory for the past five years, 2014 annual congestion (32.3 million hours) remained 8% below the 2007 pre-recession levels (35.1 million hours). The central Puget Sound region did not follow this 2014 trend and congestion there was 19% higher than pre-recession levels.
  • Vehicle hours of delay increased 4.6% between 2012 and 2014, mirroring the growth in the state's economy. This delay on state highways cost drivers and businesses $808 million in 2014 compared to $773 million in 2012, about $116 per Washingtonian in 2014 compared to $113 in 2012.
  • The amount of delay was influenced by Washington having more drivers on the road. Passenger vehicle registrations increased 6.9% while licensed drivers increased 7.6% between 2012 and 2014. More drivers in 2014 contributed to a 2.6% increase in the number of vehicle miles traveled (VMT) on all public roadways, up from 56.607 billion in 2012 to a new high of 58.060 billion miles.

Urban transit

  • More people are taking buses than before. Transit ridership on urban commute corridors during daily peak periods increased 7.8%, from 104,970 in 2012 to 113,200 in 2014.
  • The number of miles passengers traveled using transit during daily peak periods increased 10.4% statewide, from 1.3 million miles in 2012 to 1.5 million miles in 2014. Transit on I-5, between Federal Way and Everett, moved 56,331 people during peak periods on average weekdays. Without transit it would require five additional SOV lanes to meet the capacity demand on this stretch of I-5.
  • Daily greenhouse gas emissions avoided during peak periods due to transit ridership improved by 17.9%, from 629,673 pounds in 2012 to 742,177 pounds in 2014.

Ferries and passenger rail

  • The number of travelers using WSDOT Ferries continues its upward trend as annual ridership increased 4%, up from 22.2 million in 2012 to 23.2 million in 2014. Meanwhile, annual vehicle capacity utilization increased by two percentage points, from 59% in 2012 to 61% in 2014.
  • Amtrak Cascades riders took longer trips but less of them in 2014. While passenger miles traveled increased by 8.3% from 103.1 million miles in 2012 to 111.7 million miles in 2014, ridership declined 3.4% during the same period, from 725,000 to 700,000.

Multimodal performance measures

WSDOT uses more than 20 different metrics to define and describe statewide congestion trends to provide a comprehensive analysis of system performance. These metrics include delay, travel time, duration of congestion, travel time reliability, maximum throughput travel time index, and new multimodal measures (see above). For a complete list of key metrics, see page 5 of WSDOT's Handbook for Corridor Capacity Evaluation.

WSDOT targets maximum throughput for efficient system performance

WSDOT's goal is to maximize system efficiency by maintaining traffic flow at speeds that allow the greatest number of vehicles to move through a highway segment. Research shows that this maximum system throughput is achieved at 70%-85% of posted speed limit. Maximum throughput speed is a dynamic speed threshold that changes over time: it is affected by many other factors such as highway alignment, driver behavior, and weather. For more information, refer to page 4 of WSDOT's Handbook for Corridor Capacity Evaluation.

Data sources

WSDOT collects real-time data for 84 urban commute corridors on state highways that span 720 miles. In the central Puget Sound area alone, data is collected from about 6,800 loop detectors embedded in the pavement throughout 235 centerline miles (1,300 lane miles). Similarly, the south Puget Sound area has 128 active data sensors along 77 centerline miles (270 lane miles). WSDOT collects data from 165 Spokane area detectors, which stretch along 37 centerline miles (175 lane miles). Other urban areas of the state have loop detectors and other technologies used for traffic data collection such as automated license plate readers (ALPR), GPS National Performance Measures Research Data Set (NPRMDS) data, Bluetooth and vehicle detection.

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