Despite common misconceptions about this issue, it is actually perfectly legal for a bicyclist to ride on a public sidewalk in Seattle, even if there is a designated and marked bicycle lane provided for bicyclists on that roadway.
Aside from addressing the fact that it is legal for bicyclists to use sidewalks in the city, Seattle's bicycle code simply states that cyclists are required to operate carefully and in a prudent manner "at a rate of speed no greater than is reasonable and proper under the conditions existing at the point of operation."
Bicyclists should take into consideration the size of the sidewalk as well as the number of people walking. In an article on the City of Seattle's "SDOT Blog," it is stated that bicyclists may also ride in the "bus only" lane.
More from the City of Seattle website:
"Every person operating a bicycle upon any sidewalk or public path shall operate the same in a careful and prudent manner and at a rate of speed no greater than is reasonable and proper under the conditions existing at the point of operation, taking into account the amount and character of pedestrian traffic, grade and width of sidewalk or public path, and condition of surface, and shall obey all traffic-control devices (signals, signs, road markings, etc.).
Every person operating a bicycle upon a sidewalk or public path shall yield the right of way to any pedestrian thereon, and shall give an audible signal before overtaking and passing any pedestrian."