HomeMy WebLinkAboutTIS 2022-11-21 Item 2B - Discussion - Transportation Analysis for Temporary One-Way Operations of 42nd Avenue South BridgeMemorandum
To: Public Works Department, City of Tukwila
From: KPG Psomas
Date: November 11, 2022
Re: Transportation Analysis for Temporary One -Way Operations of the 42nd Ave. S Bridge
Introduction
The 42nd Ave. S bridge over the Duwamish River connects the Allentown Neighborhood with
Interurban Ave. S. In December 2021 the bridge was damaged by an over -height truck load and
was closed to all modes of travel. The City is designing a replacement bridge that will be
constructed in the same location.
Repairs to the bridge allowed it to open with load restrictions that would accommodate only
one-way travel, and on April 4th, 2022, the bridge was reopened in the southbound direction
only. The bridge is expected to continue to be open to one-way travel for the next 20 months,
when completion of a temporary bridge will allow travel in both directions. A new permanent
bridge is scheduled to open in late 2024 or in 2025.
Six months have passed since the repaired 42nd Ave. S bridge was reopened, and the City has
requested a reevaluation of the decision to operate the bridge with southbound -only traffic.
Traffic Volumes
Traffic counts of the bridge that were conducted from October 19th -25th, 2022 showed that
approximately 3,200 vehicles per weekday use the bridge in the southbound direction. The
counts also showed that 28 vehicles per weekday are illegally driving northbound across the
bridge, mostly at off peak times (7:00 pm to 4:00 am) when southbound traffic on the bridge is
lower. If bridge traffic direction was switched to the northbound direction, we expect that a
similar number of vehicles would illegally cross the bridge in the southbound direction.
Traffic Analysis
Currently, southbound traffic on the bridge can use the traffic signal at the 42nd Ave.
S/Interurban Ave. S intersection to exit the neighborhood, providing a protected signal phase
that allows southbound vehicles to efficiently and safely access Interurban Ave. S, or travel south
on Macadam Road S.
We analyzed traffic conditions for northbound -only traffic on the bridge. This would require the
3,200 vehicles per weekday currently traveling southbound on the bridge to drive north and exit
the Allentown neighborhood at the S 115th Street/East Marginal Way S intersection. This
intersection is unsignalized, north -south traffic on East Marginal Way S having the right of way.
Westbound vehicles on S 115th Street have a stop sign and are required to find gaps in both
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directions of traffic before turning left and traveling southbound on East Marginal Way S.
Currently, the westbound intersection approach operates at level of service (LOS) B during both
the AM and PM peak hours. Changing the bridge to northbound -only travel would significantly
increase the number of westbound left -turning vehicles at the S 115th Street/East Marginal Way
S intersection. This would result in increased delays and queuing, and the intersection worsening
to LOS F during both the AM and PM peak hours. In addition, westbound drivers at the S 115th
Street/East Marginal Way S intersection have limited sight distance to the south, making it more
difficult to turn left onto southbound East Marginal Way S.
Recommendation
We continue to recommend that the 42nd Ave. S bridge operate in the southbound -only
direction for the next 20 months until a temporary bridge is built. In comparison to exiting the
neighborhood at the unsignalized S 115th Street/East Marginal Way S intersection, using the
traffic signal at the 42nd Ave. S/Interurban Ave. S intersection is safer and more efficient.
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