There's also a lot of recreational paths that end up making areas quite a lot more bike accessible. And I mean rail trails extending for tens of miles along transport corridors, not just trails to nowhere in parks.
Unfortunately, even with wide shoulders and good visibility, "accidents" still happen thanks to unalert/hostile drivers (check out the recent terrible death of Mike Hall in Aussieland)
Rail trails are pretty cool to be honest, and there is a good and much needed network here. Of course, it will never be as extensive or well maintained as auto roads are here in the US for political reasons
They are allowed on Interstates, frex on I90 from just outside of Issaquah to Spokane. It's not all that pleasant or fun, but they are allowed.
Shoulders suck as a place to ride though, it's where all the road debris hangs out, waiting to puncture a tire.
http://www.legislature.mi.gov/(S(disednjhhuuyovzmbvhdxfys))/...
Are you sure that Washington doesn't just have a provision in state law requiring action to close a shoulder?
Yes, that's why I said "(not limited access roads, just highways)", because of the ambiguity in the word "highway". A couple of states do allow bike access on a large portion of limited access highways, mostly in the west.
We don't have bike trails because they are expensive and little used (because of population density more than anything). That's a political reason, it isn't exactly a surprising or awful reason.