Carpool lanes are usually reserved only for major expressways, as opposed to small city roads. The lane farthest to the left of the highway is usually the carpool lane, and is oftentimes marked by either diamond shapes that appear in the center of the lane or by signs on the side of the road that indicate the boundaries of the carpool lane.
Most states require that you have at least two adults in the car in order to drive in the carpool lane. Small children will not usually count. Some states require more than two people. There are some states that allow single passenger vehicles to enter into express lanes, but only if passing a slower vehicle in the right lane. Single passenger vehicles should never remain in a carpool lane for more than a few seconds.
Even though the rate of travel is usually much quicker in a carpool lane, this is not because the speed limit is higher in the lane. Generally, carpool lanes have the same speed limit as the rest of the highway lanes (usually 65 miles per hour). The reason that the traffic moves better in the carpool lane is simply because there are less multi-passenger vehicles on the highway.
Not all states have carpool lanes, so if you're traveling across the country make sure to be aware of any signs that indicate whether or not a carpool lane exists on your highway. Arizona, California, Colorado, Connecticut, Florida, Georgia, Hawaii, Maryland, Massachusetts, Minnesota, New Jersey, New York, Oregon, Pennsylvania, Tennessee, Texas, Utah, Virginia, and Washington are all reported to enforce HOV lanes in some capacity.
Again, it depends on the state, but if you are driving without any passengers in a carpool lane, you could be fined. In most places, you'll receive at least a $300 fine. Depending on the situation, it could be more than $400. If you are unsure of whether or not you can get away with driving in the carpool lane, opt not to take the risk.
