http://www.openstreetmap.org/?lat=47.63632&lon=-122.2276&zoom=17&layers=0B00FTF

Way #29502024 (oneway eastbound from NE 30th St intersection to I-405 westbound) is restricted to motorcycles and 3-person carpools.* Note that there are two on-ramps, the one heading from northbound is open to all vehicles.

Interestingly, there’s a lane restriction leading up to it: only the right lane can be used for using the northbound on-ramp, starting from NE 24th St. I’ve gotten stuck a couple of times when the right lane was crowded and I didn’t merge in time, I had to loop around.

*SR-520 is, I think, the only highway in Washington state with 3-person restrictions, the other High Occupancy Vehicle lanes have 2-person restrictions, although the High Occupancy Toll lanes on SR-167 add another wrinkle.