For your reference, I believe this is the law that Jeff is referring to. Any lawyers or law enforcement officers care to chime in?
Law section
Okay, I printed out this section of the vehicle code and had a nice chat with the watch commander of the local CHP substation. He said this code does not apply to Carpool lanes.
With regards to this section of the code on highways, he said that the left lane is a passing lane. The idea is to get around traffic as quickly as possible, at a speed within reason (which is left up to the CHP officer if observed). Technically, he said that a motorist cannot use the left lane as a travel lane whether going the posted speed limit, less than the speed limit, or more than the speed limit. You run the risk of being ticketed in any of these scenarios. The only exception is that if the right lane is so impacted with traffic that it is unsafe to move to the right lane, then it is perfectly legal to proceed in the left lane at the posted speed limit, not less than, not more than.
Regarding the freeway system in Southern California with 4/5/6 travel lanes in one direction, the code is difficult to enforce. He says they find that sometimes, a motorist will be going less than the speed limit in #1 lane and a motorist going more than the speed limit in #6 lane. He said rarely would CHP give a ticket for impeding traffic in this scenario since the freeway system tends to be a no man's land.
Regarding speed limit, the law is the law. However, the California Court System generally throws out speeding tickets that are 10 mph or less over the posted speed limit. They do this due to inaccuracies in radar systems, inaccuracies in onboard speedometers, changing tire sizes, and giving the driver the benefit of the doubt. Consequently, CHP officers will 'look the other way' if the motorist is not driving recklessly, if the speed is reasonable according to the individual officer (and this judgement can vary widely among officers), and the local conditions - weather, traffic density, time of day, etc. It's a judgement call by the officer.
Regarding Carpool lanes, the posted speed limit is the speed limit in the car pool lane. A motorist cannot be cited for driving the speed limit in the car pool lane. A motorist can be cited for driving slower than the speed limit particularly if he can move safely into an adjacent regular lane that is moving at reasonable speed. A motorist can be cited for driving faster than the speed limit in the carpool lane if done in a reckless manner - tailgating, weaving in and out of traffic, etc. HOWEVER, the Court System wields a double edged sword! Speeds 10 mph or less over the speed limit in the carpool lane are generally thrown out, and speeds 10 mph or less than the speed limit in the car pool lane are generally thrown out. So it is left to the Officers judgement, but usually speeds that vary from the posted speed limit, by 10 mph one way or the other, even in the carpool lane are usually overlooked. What will get you a citation in the carpool lane, he said, is tailgating, not having a sticker for a single occupant vehicle (having it in the glovebox doesn't count), or driving in a reckless manner - slow or fast. Everything is left to the officer's judgement.
He said that it appears that there are going to be fewer and fewer carpool stickers, for single occupants, issued and limited to 2 years per vehicle, due to the congestion in the carpool lanes during rush hours (which in Southern California is at least 12 hours a day!). He said personally it will make him happy because it will make his job easier.
Did I leave anything out?