Project Impacts to Roads and Traffic

The City’s Water Supply Line project has kicked off on Gallagher Hill Road this week! Drivers and pedestrians on Mercer Island will begin to see impacts to traffic flow as construction crews break ground. This multi-phase project will impact the roadway differently over time:

• The first phase of work, which includes replacing the existing 10” AC water main, will impact drivers right around the middle of Gallagher Hill Road. For the first few weeks of work, traffic will remain open for two-way flow, assisted by flaggers onsite.
• Beginning in early November, the downhill lane of Gallagher Hill Road will close from SE 40^th St to 93^rd Ave SE. Drivers wanting to head downhill toward the businesses along SE 36^th St or to I-90 will be detoured down Mercerwood Drive to East Mercer Way.
• Visitors to the businesses or agencies near the intersection of SE 36^th St and East Mercer Way (such as the French American School of Puget Sound, the SJCC, or Bright Horizons and County Village Day School) will not be directly impacted by this phase of work, but may need to plan for a little extra time in travel.

These roadway impacts are expected to last up to six months.

Later phases of the project will include work near the intersection of SE 36^th St and East Mercer Way (estimated to begin in early 2027). This work will not result in closing lanes here, but will include temporary shifts and flagging.

City staff will communicate any significant changes to the detour as soon as possible, across platforms and directly to neighbors and the Mercer Island School District.