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
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
View Tiger Mountain Summit interchange concept