In many cities it is common to see 7 or 8 lanes of traffic totally stopped during rush hour. One thing that can be seen in US cities is that the vast majority of the cars have only one person inside, resulting in many cars on the road.
As mentioned before, carpool lanes are meant to encourage people to ride to work with multiple people in the same car. They are specially designated lanes of traffic on the same highways that, by law, are accesible only to cars with more than one person in them. The incentive to drive in these lanes is that they are supposed to offer faster travel speed and quicker travel times.