Here's a link to the Oregon Bicyclist Manual. You can read it for yourself- maybe print out the pertinent pages and put 'em in your seat bag.

http://www.oregon.gov/ODOT/hwy/bikep...ike_manual.pdf

It would surprise me greatly if there wasn't Oregon law supporting this document, put out by the Oregon Dept. of Transportation. The verbiage in that manual is quite specific. If not based on law...... what?