The guidelines on HOVs in WA:
http://www.wsdot.wa.gov/HOV/FAQ.htm
Should I get out of the HOV lane if the vehicle behind me wants to pass?
There is no law requiring either one to move over in such a situation, and you do not have a legal obligation to move out of the HOV Lane to allow speeding vehicles by. However, the Washington State Patrol suggests that it is usually best for a vehicle in the HOV lane to move over, when safe to do so. This allows the aggressive driver by and could prevent a road rage incident.
The HOV lane is a separate facility from the rest of the freeway and is not the “fast lane.” Since the HOV lane may be the only one moving during congested periods, however, some drivers do consider it to be the fast lane and think they should be moving at the speed limit or faster. It is usually not worth risking a road rage incident by blocking faster-moving cars, even if you are going the speed limit. Use common sense and pull over into the adjacent general purpose lane (not the shoulder) only when it is safe to do so.
It is best to drive in the HOV lane only when you are able to keep up with the flow of traffic. Patrol officers will sometimes pull a legal HOV over to suggest this action if they see that the vehicle is not keeping up with the flow of traffic or has several vehicles queued up behind it. This is usually for educational and safety purposes, although a ticket may be issued if you are impeding “normal and reasonable movement of traffic” in the lane per Revised Code of Washington 46.61.425.
And [Revised Code of Washington 46.61.425][1] states
No person shall drive a motor vehicle at such a slow speed as to impede the normal and reasonable movement of traffic except when reduced speed is necessary for safe operation or in compliance with law: PROVIDED, That a person following a vehicle driving at less than the legal maximum speed and desiring to pass such vehicle may exceed the speed limit, subject to the provisions of RCW 46.61.120 on highways having only one lane of traffic in each direction, at only such a speed and for only such a distance as is necessary to complete the pass with a reasonable margin of safety.
[1]: http://app.leg.wa.gov/rcw/default.aspx?cite=46.61.425
So, you can drive in the HOV lane at the speed limit, and you don’t have to let people past (try it! It’s fun!), but you might get pulled over anyway for ‘education’ if enough people are trying to speed behind you, and the advice is to pull over to the right (into the general purpose ‘fast’ lane!) to let people past you.
Reads to me like (funnily enough!) you can only ever be ticketed for impeding the normal flow of traffic if you’re going below the maximum speed limit. i.e. the maximum limit is really the recommended speed for the road.
This is interesting. Speed limits appear to be defined so that 15% of people are speeding.
http://www.wsdot.wa.gov/Operations/Traffic/speedlimits.htm