2018 Corridor Capacity Report

Average daily weekday delay in general purpose (GP) lanes on freeway corridors (Interstate 5, I-405, I-90, State Route 520 and SR 167) in King and Snohomish counties grew roughly 7.3% from 29,747 daily vehicle hours of delay in 2015 to 31,918 hours in 2017. 

Despite the overall increase, three of the corridors in the region, I-405, I-90 and SR 520, saw decreases in GP delay—of 1.3%, 28.7% and 33.3%, respectively. WSDOT projects expanded managed lanes (HOV or Express Toll Lanes) between 2015 and 2017 on all three of these corridors. 

The other two corridors in the central Puget Sound region saw increases (15.6% on I-5 and 27.4% on SR 167) in daily vehicle hours of GP lane delay between 2015 and 2017. 

Vehicle delay on the I-5 corridor in the central Puget Sound region between Federal Way and Everett was higher in 2017 than in 2015, with the magnitude of the difference varying by location and direction of travel (see graph above). In 2017, the worst vehicle delay on the I-5 corridor occurred northbound between the I-90 interchange and Northgate (including around downtown Seattle), as well as southbound at SeaTac and the SR 520 interchange.  The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

Vehicle delay along the I-405 corridor was higher at most locations in 2017 than in 2015, except for the section between Kirkland and Bothell, where delay decreased substantially in both directions, and the northbound section north of Bothell. Delay on the I-405 corridor increased from 2015 to 2017 between SR 167 and I-90 in both directions (up 20%), and northbound between SR 520 and Kirkland (up 32%). Delay decreased by 58% over the same period between Kirkland and Bothell. Northbound delay north of Bothell also decreased, likely due to the opening of the peak use shoulder lane in fall 2017.  The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

WSDOT estimates with high certainty that vehicle delay on the SR 520 corridor trended downward from 2015 to 2017. Data quality issues resulting from construction activities and the replacement of the Evergreen Point Floating Bridge required the agency to estimate delay on parts of the corridor that lacked reliable loop detector data (indicated by dotted lines in the graph above). As a result, the magnitudes of the delay estimates and the percent change should be treated as the agency’s best approximations.

The addition of HOV lanes across the bridge decreased delay on both the eastbound and westbound approaches. It also shifted the westbound bottleneck from Medina to the west side of the bridge (approaching Montlake Blvd.), which is now the most congested location on the corridor (see graphs above). WSDOT will make improvements to this congested section of SR 520 between the Evergreen Point Floating bridge and I-5, which currently has two GP lanes and no HOV lanes, during its “ Rest of the West ” project.” This $1.6 billion project will improve interchange connections between SR 520, Montlake and I-5, add bicyclist and pedestrian facilities (including a bicyclist/pedestrian land bridge over SR 520), and add a new bridge to carry three lanes of eastbound traffic at the Montlake Cut. The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

The I-90 corridor in the central Puget Sound region between Seattle and Issaquah experienced an overall reduction in vehicle delay of 29% between 2015 and 2017 (see graph above). Locations on eastbound and westbound I-90 saw decreases in delay between 2015 and 2017, including westbound I-90 at Mercer Island (down 49%) and eastbound I-90 at Seattle (down 58%). However, westbound delay at the I-405 interchange was 43% higher in 2017 than in 2015.  The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

Overall delay on SR 167 between Auburn and Renton increased by 27% from 2015 to 2017. Because person miles traveled increased by only 0.9% over the same period, this increase in delay suggests that capacity constraints are impacting traffic on the corridor during peak periods. In addition to delaying commuters, this congestion directly impacted the movement of goods in Washington as trucks accounted for 8% of the total daily traffic volume on the corridor in 2017. The worst delay, as well as the greatest increase in hours of delay, occurred on northbound SR 167 in Renton (see graph above). This delay may have been influenced by construction at the interchange. The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

WSDOT measures vehicle throughput in the central Puget Sound region's general purpose (GP) lanes at nine locations on five corridors. Throughput is a measure of highway efficiency that varies by location and time of day. In 2015 and 2017, productivity at these locations ranged from 35% to 100%.

WSDOT also measures person throughput for high occupancy vehicle (HOV) lanes in the central Puget Sound region. HOV lanes play a key role in maximizing the system's capacity. WSDOT collects data on how many people are being moved per lane at ten locations. At these locations, the  HOV lanes moved up to three times as many people as each adjacent GP lane.

Approximately 38% of all person miles traveled on freeways in the central Puget Sound region were in HOV lanes, which make up 27% of the region’s lane miles.

Much of the 225 miles of highway tracked in the central Puget Sound region in the Corridor Capacity Report was routinely congested in 2017.

Between 2015 and 2017, many routinely congested segments in the central Puget Sound region saw changes in both length and duration--some segments improved, while others worsened. As routine congestion changes, travel times are also impacted. See travel times between major commute destinations changed in the  2018 Corridor Capacity Report Appendix (pdf, 5mb) .

The most severe morning routine congestion occurred heading toward the Seattle or Bellevue areas. Continue scrolling to see evening congestion.

Significant evening routine congestion occurred throughout the central Puget Sound region

Transit ridership on urban commute corridors in King and Snohomish counties during daily peak periods increased 14%, from roughly 93,201 in 2015 to 106,645 in 2017. Transit on I-5 between Federal Way and Everett moved 59,724 people during peak periods on average weekdays. Without transit, more than 5.1 additional GP lanes would be needed to meet demand on this stretch of I-5. 

The number of miles passengers traveled using transit during daily peak periods on urban commute corridors in King and Snohomish counties increased 10.7%, from 1.21 million miles in 2015 to 1.34 million miles in 2017.   See details on transit travel time changes between 2015 and 2017(pdf, 5mb). 

Park & rides are an essential part of the transit system. On the five tracked corridors in the central Puget Sound region, utilization ranged from 26% to 107% of capacity in 2017. (Rates over 100% denote vehicles parked in unmarked spaces.)

2018 Corridor Capacity Report

Delay worsened at some locations on the I-5 corridor in the South Puget Sound region between 2015 and 2017, but improved at others. Morning and evening weekday commutes experienced moderate to heavy congestion on a daily basis in both years.

Northbound delay was impacted by lane changes that occurred during the construction of new ramps and lanes between SR 16 and I-5, and near the Tacoma Dome. As a result, the majority of northbound delay on I-5 occurred between South 38th Street (Tacoma Mall) and Fife. From 2015 to 2017, delay on northbound I-5 increased significantly in the Tacoma Mall area, but decreased between the Tacoma Dome and Fife (see graph at above).

In the southbound direction, most of the delay occurred between the King County line and Fife in both years. This area of delay, which worsened between 2015 and 2017, appears to have been caused by the lane reduction near the end of the southbound HOV lane, which terminates at Fife (see graph above). Continued construction in the Tacoma Dome area (which was in progress in both 2015 and 2017) likely also impacted southbound delay.

Delay was also influenced by increases in population and employment in the region. Both Pierce and Thurston counties saw their populations grow by 3.5% from 2015 to 2017. Employment in the Tacoma-Lakewood metropolitan area also increased significantly (by 5.5%) between 2015 and 2017. The  heatmap graph (pdf, 6mb) in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

As traffic increases and speeds drop below maximum throughput, congested roads carry fewer vehicles, resulting in a drop in throughput productivity. In order to gauge the lost productivity on I-5 in the south Puget Sound region, WSDOT analyzed vehicle throughput at five locations: near 14th Avenue in Olympia, near Joint Base Lewis-McChord (JBLM), near the Tacoma Dome, at Fife and near SR 18. In 2015 and 2017, productivity at these locations ranged from 64% to 100% at their most congested.

Throughput productivity loss in the northbound direction near the Tacoma Dome remained relatively stable from 2015 to 2017. 

The graph above shows how productivity on northbound I-5 at the Tacoma Dome varies by time of day. For example, 24% of the freeway’s capacity was unavailable due to congestion at 6:45 p.m. on the average day in 2017— slightly less than the 25% that was unavailable in 2015.

The most routinely congested segments of the I-5 corridor between Olympia and Federal Way in 2017 were located between SR 18 and Tacoma, and near JBLM. The locations where routine congestion occurred were similar in 2015 and 2017, but many of them experienced increases in the duration of congestion between the two years

Morning routine congestion occurred north and south of the Tacoma Dome, as well as approaching SR 18. Continue scrolling to see evening routine congestion.

Evening routine congestion occurred in Olympia and JBLM, while some of the routine congestion around Tacoma Dome dissipated.

On an average weekday in 2017, transit moved approximately 9,226 people on I-5 between Olympia and Federal Way during the morning and evening peak periods—a decrease of 0.5% from 2015 (9,277 riders). Daily transit passenger miles traveled dropped by 1.6% over the same period. Ridership from Olympia to Tacoma during the morning peak period and from Tacoma to Olympia during the evening peak period decreased by 7% and 8%, respectively, from 2015 to 2017.

Peak period transit ridership on the I-5 corridor in the south Puget Sound region was equal to 85% of an extra GP lane of capacity in 2017 (when compared to the peak efficiency of the GP lanes, which is a conservative approach to this measurement). Additionally, transit use on the south Puget Sound I-5 corridor during peak periods avoided approximately 53,353 pounds of GHG emissions per day in 2017, a 5.2% decrease from 2015 (56,289 pounds).

In 2017, 44% of available transit seats on south Puget Sound I-5 commutes were occupied during the morning and evening peak periods, and five of the 334 daily transit trips during the peak periods were over 90% of seating capacity on a typical weekday. These figures indicate that transit use in the south Puget Sound region is not constrained by capacity.

Along the I-5 corridor in the south Puget Sound region in 2017, park and ride (P&R) utilization rates ranged from 22% to 99%. The SR 512 Lakewood P&R and the Tacoma Dome Station both saw utilization rates of 99%. Any P&R lot that has 85% or more utilization is identified as operating at capacity. Lakewood Station and the DuPont P&R have utilization rates of 82% and 74%, respectively, which are considered to be nearing capacity. 

2018 Corridor Capacity Report

As traffic increases and speeds drop below maximum throughput, congested roads carry fewer vehicles, resulting in a drop in throughput productivity. 

In order to gauge the lost productivity on I-90 in the Spokane region, WSDOT analyzed vehicle throughput at several locations between Division Street and Argonne Road. In 2015 and 2017, productivity at these locations was as low as 70% at their most congested. 

For example, at 5:45 p.m., 27% of the corridor’s capacity was unavailable due to congestion on eastbound I-90 near Freya Street.

In 2017, the eastbound evening commute on I-90 in the Spokane region experienced nearly four miles of congestion lasting over an hour-and-a-half between Monroe/Walnut and Custer Road. This is an increase in length of 1.3 miles (to 3.8 miles) and an increase in duration of 35 minutes (to 1 hour 35 minutes) over 2015. Notably, the westbound I-90 evening commute is now experiencing 15 minutes of congestion over a length of 1.2 miles—measurable evening westbound congestion did not exist in 2015. For westbound morning commuters, the duration of congestion increased from 25 minutes to 55 minutes and the length increased by 2.3 miles.  See how travel times between Division Street and Argonne Road changed in the 2018 Corridor Capacity Report Appendix (pdf, 5mb). 

Those traveling on I-90 between Division Street and Argonne Road experienced increasing congestion in 2017. Westbound travelers had a 63% chance of experiencing congestion during the morning peak period and a 50% chance during the evening peak period. These are increases of 34 percentage points and 18 percentage points, respectively, over 2015. Eastbound travelers had a 53% chance of experiencing congestion during the evening peak period, which is an increase of 15 percentage points over 2015. 

Commuters on this route also experience severe congestion (speeds less than 36 mph). The westbound commute had a 24% chance of severe congestion in the morning and an 18% chance during the evening peak period, while eastbound evening travelers experienced severe congestion 10% of the time.

Morning routine congestion occurred from N Elton Rd to near S Regal St. Continue scrolling to see evening routine congestion..

The eastbound I-90 evening commute experienced nearly four miles of congestion lasting nearly 1.5 hours between S Wall Street and Custer Road.

Transit moved nearly 800 people on the corridor during the morning and evening peak periods on an average weekday in 2017.  Peak period transit ridership on the Spokane region I-90 corridor was equal to about 7% of an extra lane of capacity in 2017 (when compared to the peak efficiency of the roadway, which is a conservative approach to this measurement).

2018 Corridor Capacity Report

Traffic at some locations on the monitored corridors in the Vancouver area (segments of I-5, I-205 and SR 14) improved from 2015 to 2017, while other locations worsened. The combined delay on these corridors increased 11% between 2015 and 2017. 

 Traffic at some locations on the corridors improved from 2015 to 2017, while other locations worsened. The combined delay on these corridors increased 11% between 2015 and 2017. The  heatmap graph (pdf, 6mb)  in the 2018 Corridor Capacity Report shows delay for 2017 by milepost and time of day.

WSDOT analyzed vehicle throughput at three locations: near Fourth Plain Boulevard on I-5, near 10th Street on I-205, and near South Lieser Road on SR 14. In 2017, productivity at these locations ranged from 59% to 100%. Throughput productivity varies by direction of travel, location and time of day. 

For example, in 2017, 41% of the corridor’s capacity on southbound I-5 near Fourth Plain Boulevard was unavailable due to congestion during the morning peak 5-minute period. 

The segments of the analyzed corridors in the Vancouver area that experienced the most routine congestion in 2017 were on southbound I-5 and southbound I-205 between SR 500 and the state line, primarily during the morning commute.

Combined routine congestion on the I-5, I-205, and SR 14 corridors in the Vancouver area increased in length from 9.0 miles in 2015 to 9.5 miles in 2017.  See travel times between major commute destinations changed in the 2018 Corridor Capacity Report Appendix (pdf,5mb). 

Significant congestion (not shown in the map below) also occurred in the Portland area on the I-5 and I-205 corridors heading into Washington during the evening commute. 

Morning routine congestion occurred heading across the I-5 and I-205 bridges toward Oregon. Continue scrolling to see evening routine congestion.

In the Vancouver region in 2017, park and ride (P&R) utilization rates ranged from 16% to 99%. The Andresen P&R had the highest average utilization rate (99%). The Fisher’s Landing Transit Center P&R was expanded in September 2016 from 563 spaces to 761 spaces, causing a lower utilization rate in 2017 (66%) compared to 2015 (90%) despite having the same number of spaces occupied.