Mountains to Sound Greenway
Mountains to Sound Greenway,  Interstate 90 over Snoqualmie Pass in the state of Washington is unique in the Interstate Highway system in the US, in that it is a designated National Scenic Byway. The Mountains To Sound Greenway Trust has acted to protect the route of I-90 from Seattle to Thorp, WA, which is the portion of I-90 through the Cascade Mountain Range. Many scenic and unique places in the Cascades can be quickly and easily accessed from the Seattle area via the Mountains to Sound Greenway and I-90. For example, Snoqualmie Pass is only a one hour drive from downtown Seattle (in good weather).
Make sure you have tire chains and basic emergency travel supplies in winter! I-90 is often closed during storms, and has even experienced landslides in the last few years! The westbound lanes from the Pass down about 3 miles are on elevated roadway that ices over easily.
I-90 is the main corridor through the Greenway. One hour's drive is all it takes to get from Seattle to Snoqualmie Pass, but you can easily spend several days exploring and hiking.
The Mountains to Sound Greenway Trust website features a geology-based travel guide to the corridor, at http://www.mtsgreenway.org/newsandpubications