Mercer Island planning commission hears staff briefing on infrastructure impacts of comprehensive-plan consistency amendments

The Mercer Island Planning Commission on May 27 received a staff briefing on agenda bill PCB26-06, a package of comprehensive-plan consistency amendments updating the transportation, utilities and capital facilities elements.

Allison Vancorp, deputy director for community planning and development, told commissioners the updates align the plan’s infrastructure sections with recent changes to land use, housing and economic development policies and that the current analyses focus on phase one of the station sub area plan. “We’re really looking to capture impacts on the city's infrastructure here,” she said.

Vancorp said the transportation element incorporates new traffic counts taken in mid-April, about two weeks after the light-rail station opened, and that staff used those counts to estimate 2044 traffic volumes and intersection levels of service. She said pandemic-era declines in commute traffic counterbalanced some projected growth from the upzone, and that modeling showed only minor changes in projected traffic volumes. “So we didn't need to add additional vehicle capacity projects beyond what was already in the plan,” Vancorp said.

Staff identified several intersections for future work in the plan: Island Crest Way at 63rd (currently LOS F and historically exempted from LOS-driven widening), an intersection near 68th (AM LOS D projected to decline), and an on-ramp to I-90. Vancorp said three capacity projects essential to maintaining the city’s LOS standard through 2044 remain in the document and a fourth (53rd and Island Crest) was retained as optional; one previously listed project — a right-turn lane at North Mercer Way and 80th Avenue — was removed after the new travel-pattern data showed it was no longer needed.

On utilities, Vancorp described preliminary sewer and water system modeling based on flow monitors and said the city’s water-plan update is scheduled for completion by the end of 2026 and the sewer plan in 2028. Those more detailed plans, she said, will identify specific capital projects and provide refined cost estimates. “These were more of an initial preliminary analysis,” she said, adding staff expect to refine projects and costs as the work continues.

Staff proposed substantive amendments to seven water policies and added three new water policies to provide more specificity about future needs. No policy changes were proposed for sewer goals because staff said the existing goals are broad and already emphasize maintaining service levels and upgrading systems as needed. For stormwater, Vancorp said two new policies require redevelopment in the town center and other constrained basins to provide on-site stormwater detention and treatment rather than relying on fee-in-lieu approaches, because the drainage basin there is constrained.

Commissioners asked whether capacity upgrades would be paid by developers or the city. Vancorp said the answer is “a mix.” She said some needs — such as upsizing pipes to meet fire-flow requirements — are city-level services that may be funded by the city, while other items, particularly sewer upsizing tied to added households, may be addressed with development charges, connection fees or capacity charges and will require further evaluation, especially for phase two.

Vancorp also summarized next steps: commissioners should submit comments on the three elements by the end of day Monday, June 1; staff will include responses and a redlined packet for the public hearing on June 3; the hearing may continue on June 10 if more time is needed; and the planning commission’s recommendation will go to the city council on June 16. The commission adjourned at 6:33 p.m.

Minutes: the commission unanimously approved the May 13, 2026 special meeting minutes in a roll-call vote earlier in the meeting.

What’s next: the public hearing on PCB26-06 is scheduled for June 3; staff will publish updated maps, the station sub area plan edits and proposed capital projects in the packet for that hearing.