Seems there's a bit of confusion with the bikeability of the freeway bridges, so I'll clarify:
There are two freeway bridges that cross the Columbia River between Portland and Vancouver, WA. Both are bikeable.
- I-5 (Interstate Bridge) is on the west. It's less than a mile long. The bike path is narrow sidewalks on the sides. A tight fit so take it a bit slow.
- I-205 (Glenn Jackson Bridge) is on the east. About two miles long. The wide bike path is in the middle. More of a slog heading northbound as it's uphill.
The two freeway bridges in central Portland across the Willamette, the I-5 (Marquam) and I-405/US 30 (Fremont) are NOT bikeable, as there are no facilities. However, all other bridges in the central city (from south to north: Broadway, Steel, Burnside, Morrison, Hawthorne, Tillikum, Ross Island) are bikeable, some better than others. The only one of those at this time that doesn't have good biking facilities is the Ross Island. But if you were heading down from Vancouver, that would be the least likely bridge to use.