Central Puget Sound State Route 520 - Throughput
On this page:
Measures of throughput productivity on the SR 520 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/)
Multi-year trends
Throughput productivity at monitored locations remained steady in 2021 and 2022.
In 2020, there was no drop in vehicle throughput due to the statewide response to COVID-19 compared to previous years.
On SR 520 in the central Puget Sound region, WSDOT measures throughput productivity in both directions on the Evergreen Point floating bridge. From 2018 through 2019, there was lost throughput productivity during the morning commute on eastbound SR 520 and the evening commute on westbound SR 520.
Annual highlights
In 2022, eastbound throughput productivity was similar to 2021. There was no drop in vehicle throughput compared to the pre-pandemic level. Westbound throughput productivity began to show a drop: vehicle throughput could decrease to 84% of maximum throughput at the most congested times. 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 SR 520 in the central Puget Sound region, WSDOT analyzed vehicle throughput at the Evergreen Point floating bridge.
In 2021, throughput productivity was similar to 2020. There was no drop in vehicle throughput compared to the pre-pandemic level. 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 SR 520 in the central Puget Sound region, WSDOT analyzed vehicle throughput at the Evergreen Point floating bridge.
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 SR 520 in the central Puget Sound region, WSDOT analyzed vehicle throughput at the Evergreen Point floating bridge.
In 2019, vehicle throughput at this location ranged from 79% to 91% of maximum throughput at their most congested times. For example, in 2018 and 2019, 20% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling eastbound at the Evergreen Point floating bridge at 9:05 a.m. Throughput productivity loss started 15 minutes earlier in 2019 than it did in 2018, beginning at 7:50 a.m.
In 2018, vehicle throughput at this location ranged from 79% to 96% of maximum throughput at their most congested times. For example, in 2017 and 2018, 21% of the freeway's capacity was reduced due to slower speeds caused by congestion for commuters traveling eastbound at the Evergreen Point floating bridge at 9:05 a.m. Throughput productivity loss started 15 minutes earlier in 2019 than it did in 2018, beginning at 7:50 a.m.