The next time you get behind the wheel of your car, turn to the passenger seat. Chances are, it's empty. In most of the U.S., the single-occupant driver still reigns supreme. Nearly 80% of people drive to work alone; about 38% drive alone in general. In some places, that's starting to change. As part of its Clean Air Act, Washington State appealed to business with incentives to encourage employees to drive less or at least stop driving alone. A state tax credit benefits companies that encourage their employees to carpool, ride the bus, walk or bike to work, or work a compressed workweek. The result: about 20,000 fewer vehicle trips each morning since the program started in 1991, saving commuters $13.7 million and 5.8 million gallons of gas, and reducing 78,000 tons of air pollutants and CO2-equivalent gases.