Transportation reports identify no new impacts, loss of mobility to Mercer Island

On April 5, the city of Mercer Island was notified that Sound Transit and the Washington State Department of Transportation (WSDOT) had published a State Environmental Policy Act (SEPA) addendum for the East Link light rail extension, which will traverse Mercer Island along the center roadway of Interstate 90, and an updated “I-90 and Mercer Island Mobility Study,” along with a memorandum from WSDOT.

Sound Transit told the city that an update to the Final Environmental Impact Statement (EIS) was underway at a hearing in King County Superior Court on March 10. Mercer Island sued Sound Transit and WSDOT over I-90 access in February, and was met with countersuits from both agencies. The mobility study was required as part of the 2004 amendment to a 1976 agreement regarding access to I-90.

The addendum updates the EIS issued in July 2011 — and 2013 and 2016 addenda — by providing additional analysis of the East Link project. It describes changes in the operation of the I-90 high-occupancy vehicle (HOV) lanes and project refinements associated with transit integration on Mercer Island, evaluates the potential impacts of these changes and identifies changes to mitigation measures.

“The East Link Final EIS assumed that single-occupant vehicles (SOVs) traveling between Seattle and Mercer Island would be able to use the R-8A HOV lanes in both directions of I-90 between Seattle and Island Crest Way on Mercer Island, similar to how they currently use the center roadway between Seattle and Mercer Island,” according to the document.

However, in August 2016, the Federal Highway Administration sent a letter to WSDOT and the city of Mercer Island stating that the U.S. Department of Transportation does not have legal authority to grant temporary or permanent SOV access to the I-90 HOV lanes, meaning that SOV Island traffic could not use the R8-A lanes or the HOV-only westbound Island Crest Way ramp.

The city of Mercer Island prepared an analysis of local street impacts with the westbound Island Crest Way ramp operating as HOV only, and Sound Transit reviewed the city’s report and considered it in its analysis, according to a footnote in the document.

The addendum examines three options for operating the R-8A lanes on I-90: Mercer Island SOVs allowed in the HOV lanes, Mercer Island SOVs prohibited from HOV lanes and HOV lanes converted to general purpose lanes, only for the construction period. It also looks at “Mercer Island Bus Transit Integration Configurations.” The Mercer Island City Council rejected the latest bus intercept proposal in May 2015.

“A No Build condition was also analyzed in the transportation section for the construction and operations periods,” according to the addendum. “It is provided for comparison purposes only as the East Link Extension Project is already approved.”

Overall, I-90 travel times for all modes to and from Mercer Island, including SOV, HOV and transit riders, would be similar to or improved for Options 1, 2 and 3 compared to the No Build condition during construction (2020) and for Options 1 and 2 compared to the No Build condition during operations (2035), according to the document.

At the end of the executive summary, the report concludes that “impacts from the I-90 options and Transit Integration configurations are within the range of impacts and alternatives evaluated in the Final EIS and subsequent Addenda, and can be mitigated … No new probable significant adverse environmental impacts would arise and a supplemental EIS is not warranted.”

The mobility study states “there is no loss of mobility to or from Mercer Island, there is no need to propose implementing measures to address a loss of mobility” and concludes that “the overall mobility for people traveling to or from Mercer Island will be similar to or slightly improved compared to existing conditions during the six-year East Link construction period, and will be improved once East Link light rail service begins in 2023.”

It also analyzes an “Option 4,” which would allow temporary use of the HOV lanes for SOVs using the westbound ramp, but would require an immediate merge into the general lanes. According to the report, this option would increase travel times and crash risk, and does not comply with federal law.

The addendum, supporting materials, memorandum and mobility study can be found on the city’s website at

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