Central Puget Sound Interstate 5 - Throughput
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Measures of throughput productivity on I-5 in the central Puget Sound region
Throughput productivity
Throughput productivity measures how efficiently existing highway capacity is being used. It can be reported for vehicles or travelers, making it a very adaptive metric. WSDOT uses the maximum throughput standard to assess travel delay relative to a highway's most efficient speed, which is 85% of the posted speed limit. As congestion increases and speeds decrease below maximum throughput, highways carry fewer vehicles, resulting in decreased throughput productivity.
Lost throughput productivity is the percentage of a highway's vehicle throughput lost due to congestion when compared to the maximum 5-minute weekday flow rate observed at a particular location of the highway for that calendar year. Refer to WSDOT's
Handbook for Corridor Capacity Evaluation (PDF 5.9MB) for details on how to calculate throughput productivity.
Sources: WSDOT Transportation Safety and Systems Analysis Division and WSDOT TRACFLOW (https://tracflow.wsdot.wa.gov/)
Note: 100% means a straight line that represents no loss in productivity.
Performance highlights
In 2023, throughput productivity at most locations in both directions of I-5 improved compared to 2022, likely due to the completion of I-5 construction near the Tacoma Dome. After construction was completed, delay in these sections decreased significantly.
In 2022, throughput productivity at most monitored locations was similar to 2021 while there were minor improvements at other locations. To gauge the lost productivity on I-5 in the central Puget Sound region, WSDOT analyzed vehicle throughput at three locations: near South 188th Street, near Northeast 103rd Street and at the I-90 interchange.
Throughput productivity varied by direction of travel, location and time of day. In 2022, for example, 20% of the freeway's capacity was reduced due to slower speeds caused by congestion from commuters traveling southbound at South 188th Street at 5:55 p.m. The duration of throughput productivity loss was 35 minutes longer in 2022 than it was in 2021, ending around 6:15 p.m.
In 2021, throughput productivity was similar to 2020. As congestion increases and speeds decrease below maximum throughput, highways carry fewer vehicles, resulting in a drop in throughput productivity. To gauge the lost productivity on I-5 in the central Puget Sound region, WSDOT analyzed vehicle throughput at three locations: near South 188th Street, near Northeast 103rd Street and at the I-90 interchange.
Throughput productivity varied by direction of travel, location and time of day. In 2021, 19% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling southbound at South 188th Street at 5:55 p.m. Duration of throughput productivity loss was almost same time in 2021 as it was in 2020, ending around 6:00 p.m.
In 2020, due to the statewide response to COVID-19, there was no drop in vehicle throughput on some of I-5 corridors, while some others followed the same pattern and intensity as 2019. As congestion increases and speeds decrease below maximum throughput, highways carry fewer vehicles, resulting in a drop in throughput productivity. To gauge the lost productivity on I-5 in the central Puget Sound region, WSDOT analyzed vehicle throughput at three locations: near South 188th Street, near Northeast 103rd Street and at the I-90 interchange.
In 2019 and 2020, vehicle throughput at these locations ranged from 61% to 84% of maximum throughput at their most congested times. Throughput productivity varies by direction of travel, location and time of day. For example, in 2019, 23% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling southbound at South 188th Street at 6 p.m. Throughput productivity loss was 55 minutes shorter in 2020 than it was in 2019.
As congestion increases and speeds decrease below maximum throughput, highways carry fewer vehicles, resulting in a drop in throughput productivity. To gauge the lost productivity on I-5 in the central Puget Sound region, WSDOT analyzed vehicle throughput at three locations: near South 188th Street, near Northeast 103rd Street and at the I-90 interchange.
In 2019, vehicle throughput at these locations ranged from 61% to 98% of maximum throughput at their most congested times. Throughput productivity varies by direction of travel, location and time of day. For example, in 2018 and 2019, 23% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling southbound at South 188th Street at 6 p.m. Throughput productivity loss was 10 minutes shorter in 2019 than it was in 2018, ending at 6:20 p.m.