Ah. The restrictions for that bit of road permit access to:
[]1. three or more people riding in a car or recreational vehicle
[
]1. three or more people riding in a truck that weighs less than 10k lbs GVW (gross vehicle weight)
[]motorcycles
[
]buses (er, PSV)
[*]emergency vehicles

and deny access to:
[]two or more people in a car or recreational vehicle
[
]trucks that weigh more than 10k lbs GVW
[*]bicycles, pedestrians, horses

More details at http://www.wsdot.wa.gov/hov/

So, given http://wiki.openstreetmap.org/wiki/Key:access perhaps:
access=designated
hov=3
motorcycle=yes
psv=yes
emergency=yes
bicycle=no
horse=no
foot=no
maxweight=4.5

Does that seem right? I’m thinking that highway=motorway_link (and highway=motorway) might imply horse=no and foot=no (and perhaps even bicycle=no) but I’m not sure. Also access=designated might not be right.