Formally known as High Occupancy Vehicle lanes (HOV), carpool lanes are the lanes on a highway reserved for cars with a driver and at least one passenger, emergency vehicles, and motorcycles. What's the point of having it, though?
If you come across a carpool lane on the highway it will most likely be the one furthest to the left, and if you're lucky enough to be driving with other people in the car, you can drive on it. The main purpose of a carpool lane is to move more people in fewer cars compared to the other lanes of traffic. Since carpool lanes can remain fairly empty during high volume traffic periods, drivers have an incentive to use the carpool lane, either by riding with a co-worker or taking the bus, in order to avoid congestion.
This not only benefits the drivers but also the environment, and with global warming on the rise a carpool lane is just one way to reduce pollution by reducing the amount of cars on the road.