TIS 2022-11-21 Item 2B - Discussion - Transportation Analysis for Temporary One-Way Operations of 42nd Avenue South Bridge

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Memorandum 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 3131 Elliott Avenue, Suite 400, Seattle, WA 98121 206.286.1640 www.kpg.com 17