South Puget Sound Interstate 5 - Throughput
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Measures of throughput productivity on the I-5 corridor in the south Puget Sound region
Throughput productivity
Throughput productivity measures the efficient use of the existing highway capacity. It can be reported for vehicles or for persons, making it a very adaptive metric. WSDOT uses the maximum throughput standard as a basis for measurement to assess travel delay relative to a highway's most efficient speed of about 85% of posted speed.
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.
See WSDOT how to calculate throughput productivity (PDF 5.9MB). for throughput productivity calculation method.
Sources: WSDOT Transportation Safety and Systems Analysis Division and WSDOT TRACFLOW (https://tracflow.wsdot.wa.gov/)
Multi-year trends
In 2022, both directions of I-5 throughput productivity slightly improved compared to 2021. This is probably the result of the completion of construction on I-5 at the Tacoma dome. After the completion of construction, the delay in these sections were significantly reduced.
In 2021, throughput productivity worsened from 2020 due to the recovery of traffic volume in the wake of COVID-19 but was still lower than the pre-pandemic level.
In 2020, due to the statewide response to COVID-19, there was a significant improvement in vehicle throughput compared to previous years.
From 2018 to 2019, most locations on I-5 in the south Puget Sound region experienced a significant decrease in throughput at typical choke points on the corridor, such as the major cities of Olympia and Tacoma, Fife, Joint Base Lewis McChord, and near ramps to State Route 18 and SR 16.
Annual highlights
In 2022, throughput productivity at most locations in both directions of I-5 improved compared to 2021, probably due to the completion of I-5 construction near the Tacoma dome. After the completion of construction, delay in these sections was significantly reduced.
In 2021, throughput productivity worsened from 2020 due to the recovery of traffic volume in the wake of COVID-19, but still didn't reach the pre-pandemic level. Vehicle throughput at these locations ranged from 77% to 99% of maximum throughput at the most congested times. Throughput productivity varies by direction of travel, location and time of day. For example, in 2021, 27% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling northbound on I-5 (Tacoma Dome, MP 133.5) at 7:50 a.m. compared to the same area in 2020 was 23% at 7:50 a.m.
In 2020, due to the statewide response to COVID-19 there was a significant improvement in vehicle throughput compared to previous years. Vehicle throughput at these locations ranged from 77% to 99% of maximum throughput at their most congested times. Throughput productivity varies by direction of travel, location and time of day. For example, in 2020, 23% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling northbound on I-5 (Tacoma Dome, MP 133.5) at 7:50 a.m compared to the same area in 2019 was 50% at 8:00 a.m.
In 2018 and 2019, vehicle throughput at these locations ranged from 41% to 99% 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, 28% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling southbound at I-5 at Fife at 8:00 p.m. Throughput productivity loss was an hour and 35 minutes longer in 2019 than it was in 2018, starting at 10:40 a.m and ending at 8:10 p.m.
In 2017 and 2018, vehicle throughput at these locations ranged from 52% to 88% 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, 25% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling southbound at I-5 at Fife at 7:55 p.m. Throughput productivity loss was 15 minutes longer in 2018 than it was in 2017, starting at 11:55 a.m and ending at 8:15 p.m.