Actually, there is a law. (But there might as well not be.)
Here's how it reads:
A. No person shall operate a bicycle on a roadway against the flow of motorized traffic, except where permitted by official signs or pavement markings.
B. Every person operating a bicycle upon a two-way roadway shall be entitled to use the right-hand lane and shall proceed in the same direction of travel as other vehicles in that lane. On one-way roadways a bicycle may be operated in any existing lane.
C. The operator of a bicycle traveling at a rate of speed which delays a vehicle or vehicles following in the same lane shall be required, when it is unlawful or unsafe for the following vehicle to pass, to move as far to the right of the traveled roadway, or to the left where the bicycle is in the left lane of a one-way roadway, as is safe under the conditions then existing; provided, however, that when the bicyclist is within fifty feet (50') of an intersection, he shall not be required to move to the right or left until he has moved through the intersection.
(From the Boise City Code, Section 10-14-06. "It's in the book!" I'm sure most jurisdictions have a very similar law.)
One of my ongoing pet peeves is that bicycle laws are not enforced. In fact, they are routinely ignored by many totally-irresponsible cyclists, and by many totally-irresponsible law enforcement officers, unless an accident is involved.
(Now... you read some laws, and you wonder what the lawmakers were thinking. But Section 10-14-06 makes perfect sense - it is a VERY well-crafted law!)
I've even called and complained about against-traffic cyclists (because they routinely place me in very hazardous situations!) - I've been told, very matter-of-factly, "That isn't a priority violation for us."
Well, folks... I decided to do some research.
I submitted an official request... asking the folks at the Boise Police Department to tell me how many times "Section 10-14-06" has been enforced over the last five years.
I got the response in the mail yesterday.
Five tickets (!!) have been issued in the City of Boise since November 2002. Three in 2003, one in 2005, and one in 2006. (And just one of 'em was for the "riding against traffic" part.)
That does a lot to explain why I see so many doofuses on bikes, riding against traffic. The Police Department should be ashamed of their totally lax enforcement. I'm betting if the police chief rode a bike for transportation, against-traffic bike riding would get higher-priority attention.