WSDOT continues to deploy innovative solutions and projects to reduce congestion as part of the agency’s Moving Washington strategies. The 2011 Annual Congestion Report (pdf 2.9 mb), shows that congestion on Washington's highways between 2008 and 2010 has in many ways mirrored both the economic recession and the recovery. The trends of decreasing congestion and lessening delay that prevailed from 2008 through 2009 appear to have slowed, as both delay and vehicle miles traveled increased in 2010. It remains unclear whether congestion will worsen as economic conditions improve. The report includes analysis of Before and After results from projects deployed within WSDOT's Moving Washington strategies: Operate Efficiently, Manage Demand, Add Capacity Strategically.
Trends in this year's report, which analyzed data from 2008 and 2010, show that most congestion performance metrics for 2010 are below 2008 levels, but higher than in 2009. New this year, the report features five-year trend analysis covering 2006 to 2010.
Highlights from the 2011 Congestion Report
The report provides in-depth analysis for 52 central Puget Sound and two Spokane commute routes, and information on statewide transportation system performance. Selected highlights include:
- Vehicle miles traveled has increased 3.1% since 2008, as Washington residents drove more in 2010 compared to 2008. The average driver added 65 miles and 57 of the extra miles were on state highways.
- In 2010, statewide delay, measured by vehicle hours, was down 9% compared to 2008. On the other hand, when compared to 2009, vehicle hours of delay in 2010 increased by 13%.
- Per person, people in Washington spent 12% more time delayed in traffic in 2010 compared to 2009, but 4% less time than they did in 2008.
- When quantifying this delay in terms of total dollars, the cost to drivers and businesses was over $750 million in 2010 based on maximum throughput speed thresholds.
- In 2010, delay on the key Puget Sound region commute corridors was down 14% from 2008, but has increased 3.6% since 2009.
- Between 2008 and 2010, changes to travel times and reliability were modest on most of the 40 high demand Puget Sound region commutes. 45 out of 48 high occupancy vehicle (HOV) commutes provided more reliable travel times than the corresponding general purpose (GP) commutes.
- 2009 was the least congested year for the transportation system of the five years studied since 2006, due in part to the economic recession.
- Moving Washington projects are being implemented at strategic locations on the state highway system to help fight congestion. An analysis of the “Before and After” results for the completed projects shows measurable benefits to commuters on such corridors as I-5, I-405, SR 520, US 2, and SR 519. For example, since the December 2010 opening of a new interchange and an additional lane in each direction on I-405 between SR 167 and SR 169, congestion no longer occurs between NE Park Drive and I-5, and the southbound evening travel time improved from 20 minutes to about 15 minutes from Coal Creek Parkway to I-5.
Maximum throughput as the basis for performance analysis
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 geometrics, driver behavior, and weather.
Congestion Performance metrics
In order to provide a comprehensive analysis of system performance, WSDOT uses more than 20 different metrics to define and describe congestion trends in Washington. These metrics include delay, travel time, duration of congestion, travel time reliability, and maximum throughput travel time index. For a list of key metrics, please see page 9 of the 2011 Congestion Report. Thresholds for calculating congestion metrics can be viewed on page 11.
WSDOT analysis primarily uses measured, archived, real-time data, collected across the state using more than 6,800 in-pavement induction loop detectors. Other data capture technologies include Automated License Plate Reader (ALPR), moving test vehicles, and tolling transponder data. In addition, this report also uses some Highway Performance Monitoring System database to calculate the statewide performance metrics where loop data does not exist. Other data sources used in this report include economic data, collision data, and transit ridership data, to name a few. Overall, WSDOT performs extensive data quality control on all data sets used in this analysis.