From the Washington State HOV FAQ
Who can use HOV lanes?
Drivers can use HOV lanes whenever there are two or more people in their vehicle, including the driver. There are two exceptions to this rule:
SR 520, west of I-405. This is the only place on the HOV system where three or more people are required. The 3+ occupancy requirement on this segment has to do with safety concerns.
I-90 Express Lanes, west of Island Crest Way. Single occupant vehicles traveling between Seattle and Mercer Island and vise versa are allowed to use the I-90 HOV Express Lanes. This has to do with a Memo of Understanding between Seattle, Bellevue, Mercer Island, WSDOT, Sound Transit, and the Federal Transit Authority. Solo drivers may use the I-90 HOV Express Lanes between Exits #3 and #7 in both directions. They cannot use the HOV Express Lanes east of Exit #7 on Mercer Island.
The definition of \"people\" is not limited by age or licensing status. Single-occupant hybrid and other green vehicles are not allowed to use the HOV lane. There is no weight limit for recreational vehicles. Towing is allowed in the HOV lanes as long as an adequate speed can be maintained. Public buses, emergency vehicles, and motorcycles can use freeway HOV lanes at any time, regardless of how many people are on board.