Yes it is very narrow. It can be difficult with the wind and you might need to stop if you pass a bike or a pedestrian. Be alert when you go off the bridge, on both side it is a bit tricky, one side has a small step and one side has a tight spot with some metal near your elbow.
This has been an annual Pedalpalooza event, this is the listing from last year's calendar:
RU ABRIDGED CURIOUS?! GUIDED COMMUTER TOUR ACROSS I5
West 8th St @ Esther St, Vancouver WA (North end of Vancouver Farmers Market)
10:30am, Ride leaves park at 11AM. Returns by 12.30PM
This is a causual guided ride for recreational and commuter bicyclists who want to learn how to cross the Interstate Bridge to Portland, load their bike on the MAX and bus, and find bike routes up to Delta Park/Kenton. A free MAX ticket ('Patch Kit') will be offered to the first 10 riders who RSVP. Bring a bike and helmet, water, and lunch money.
There is only one lane on the south bound side, that is why you need to stop when you pass or cross somebody else, it is narrow, you might see on an areal view of the bridge. Also look for the local bike maps on: http://tripplanner.bycycle.org/ it works very well, I think.