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SR 18 - Issaquah Hobart Road to I-90 - Preferred Alternative

In choosing the preferred alternative WSDOT’s design team evaluated six corridor-widening alternatives, two SR 18/I-90 interchange concepts and one Tiger Mountain Summit interchange concept. 

What’s the plan?

SR 18/I-90 Interchange

  • Features unique design that includes roundabout intersections and flyover ramps connecting SR 18 to I-90.
  • Provides enough capacity to keep traffic moving through at least 2035.
  • Fewer wetland effects than competing loop ramp design concept
  • Lower cost than competing design concept

thumbnail image of SR 18/I-90 interchange concept
View SR 18/I-90 interchange concept

Issaquah Hobart Road to Tiger Mountain Summit widening

  • Widens SR 18 from two to four general purpose lanes with an eastbound (uphill) truck lane approaching the summit
  • Widens to left (north) and right (south) sides of existing roadway to minimize effects on nearby Holder Creek
  • Uses concreted median barrier with shoulders to separate opposing traffic.    Using median barrier minimizes the overall "footprint" of the project.  A smaller footprint reduces effects on nearby streams and unstable slopes.
  • Enhances driver safety by lengthening curves and improving visibility

Tiger Mountain Summit to I-90 Widening

  • Widens SR 18 from two to four lanes with a westbound (uphill) truck climbing lane approaching the summit
  • Widens to left (north) side of existing roadway to minimize effects on stream and wetlands
  • Separates opposing traffic with a 51-foot grass median and concrete barrier at the summit to minimize effects on recreational and sensitive areas  
  • Enhances driver safety by lengthening and flattening curves, and improving visibility

Tiger Mountain Summit Interchange

  • Compact diamond interchange design
  • Includes bridge over SR 18 and on- and off-ramps
  • Works with all corridor widening alternatives

thumbnail image of Tiger Mountain Summit interchange concept
View Tiger Mountain Summit interchange concept