Central Puget Sound Interstate 90 - Throughput
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Measures of throughput productivity on the I-90 corridor 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.
Lost throughput productivity is the percentage change of highway throughput in respect to maximum throughput due to congestion and other events that cause vehicles to travel below a threshold speed. See WSDOT's
Handbook for Corridor Capacity Evaluation (PDF 5.9MB)
Sources: WSDOT Transportation Safety and Systems Analysis Division and WSDOT TRACFLOW (https://tracflow.wsdot.wa.gov/)
Note: Eastbound and Westbound I-90 at the floating bridge 2018 and 2019 data was unavailable.
Multi-year trends
Throughput productivity remained steady between 2020 and 2022.
In 2020, due to the statewide response to COVID-19 there was no drop in vehicle throughput compared to previous years.
On I-90 in the central Puget Sound region, lost throughput productivity occurred at similar locations and times of day from 2018 through 2019. On the floating bridge, I-90 saw similar (but longer-lasting) throughput losses during both morning and evening commute peaks in 2018 and 2019.
Annual highlights
In 2022, throughput productivity was similar to 2021; there was no drop in vehicle throughput compared to previous years. 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-90 in the central Puget Sound region, WSDOT analyzed vehicle throughput at two locations: the SR 900 interchange and the I-90 floating bridge across Lake Washington.
In 2021, throughput productivity was similar to 2020, there was no drop in vehicle throughput compared to previous years. 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-90 in the central Puget Sound region, WSDOT analyzed vehicle throughput at two locations: at the SR 900 interchange and at the I-90 floating bridge across Lake Washington.
In 2020, due to the statewide response to COVID-19 there was no drop in vehicle throughput compared to the previous year. 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-90 in the central Puget Sound region, WSDOT analyzed vehicle throughput at two locations: at the SR 900 interchange and at the I-90 floating bridge across Lake Washington.
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-90 in the central Puget Sound region, WSDOT analyzed vehicle throughput at two locations: at the SR 900 interchange and at the I-90 floating bridge across Lake Washington.
In 2018 and 2019, vehicle throughput at these locations was 100% 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 eastbound at I-90 at SR 900 at 6 p.m. Throughput productivity loss was 10 minutes shorter in 2019 than it was in 2018, ending at 6:20 p.m.
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-90 in the central Puget Sound region, WSDOT analyzed vehicle throughput at two locations: at the SR 900 interchange and at the I-90 floating bridge across Lake Washington.
In 2017 and 2018, 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 2017 and 2018, 23% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling eastbound at I-90 at SR 900 at 6 p.m. Throughput productivity loss was five minutes longer in 2018 than it was in 2017, ending at 6:30 p.m.