I've ridden over Snoqualmie many times. You have an 8 foot shoulder almost the whole way. Your main dangers are all the stuff--boards, metal, car parts, miscellaneous items off trucks, even furniture on the riding lane. The cars have never really been a problem especially with a ripple strip between the biker and the car lane. There is one other hairy situation, and that's the snowshed westbound along the side of Lake Keechelus about 3 or 4 miles before Snoqualmie summit. The shoulder is reduced to a foot or two and the section is too long to get through before the cars come, because you can't see the road back far enough when entering the tunnel. Also, shoulders are reduced a lot over bridges, but for those you can see far enough back to wait for an opening. Watch out, because if there's debris clogging that reduced lane width, you've got nowhere to go. Despite all of this, I like the ride--great grade, nice views, good stops in Easton, the Summit, and North Bend. Also, use a mirror!
May the wind be at your back!