Actually it is a state law. Last year on the way home from fishing for gators, I got pulled over also but no ticket. I started a thread here, and this is what I found out.

Any vehicle in combination -- pulling a trailer -- must obey the posted truck speed limit. Also, any vehicle in combination may not use the far left lane for any reason (even to pass) on a three lane highway.

However, if the highway is two lanes, you can use the left lane. Also, if the highway is three lanes, and the third lane is a carpool lane, you CAN use the carpool lane if you have enough passengers. The carpool lane does not count as a third lane.

What I do not know, however, is if the highway is four lanes, and the fourth is a carpool lane, whether or not you can use the carpool lane. Since you cannot use the third far left lane, how can you get to the carpool lane?

You need a law degree to understand Washington's driving laws!!