According to the Bureau of Transportation Planning of the New Hampshire Department of Transportation ("Transportation Facts and Figures," January, 1999), three statistics have risen steadily since 1972: the state's population; the number of registered vehicles; and the number of licensed drivers in the state. In fact, the number of registered vehicles has grown at a faster rate than the number of drivers. The total number of miles traveled by each person has increased significantly since 1972, while light duty fuel economy has decreased steadily since 1987.

While the number of miles per driver has been increasing in recent yeas, in the late 1980s there was a sharp increase in millions of gallons of gasoline consumed. As vehicles became more fuel efficient, motorists became accustomed and more willing to pay higher prices for gasoline. However, in the 1990s more drivers owned more vehicles, drove more miles, and bought more gas than ever before. Vehicle-miles-traveled (or VMT) becomes a useful tool and an important measure for tracking levels of emissions from motor vehicles.

As can be seen below, mobile source emissions have decreased over the years thanks to technological advances such as catalytic converters for automobiles and cleaner fuels. However, these reductions are not as significant as they could have been due to the fact that there has been an increase in the vehicle miles traveled in New Hampshire and the country as a whole. EPA estimates that motor vehicle travel in the United States has quadrupled since 1950. The average daily vehicle miles traveled in New Hampshire has almost tripled from 1970 to 1999, with 32.5 million average daily vehicle miles traveled in 1999, compared to about 12 million in 1970

Recognizing the significant role mobile sources play in the overall air pollution picture, Congress gave EPA broad authority under the Clean Air Act of 1970 to regulate motor vehicle pollution, and the Agency's emission control policies have become progressively more stringent since the early 1970s. The advent of "first generation" catalytic converters in 1975 significantly reduced hydrocarbon and carbon monoxide emissions. In 1980, manufacturers began equipping new cars with even more sophisticated emission control systems. These systems generally include a "three-way" catalyst (which converts carbon monoxide and hydrocarbons to carbon dioxide and water, and also helps reduce nitrogen oxides to elemental nitrogen and oxygen), plus an on-board computer and oxygen sensor. This equipment helps optimize the efficiency of the catalytic converter. Vehicle emissions were further reduced by provisions of the 1990 Clean Air Act amendments, which included even tighter tailpipe standards, increased durability, improved control of evaporative emissions, and computerized diagnostic systems that identify malfunctioning emission controls.

There are other mobile sources besides on-road vehicles that contribute to air pollution. Marine engines have historically been inefficient in their use of gasoline and oil. You may be surprised to learn that off-road recreational vehicles, such as snowmobiles, off-highway motorcycles, ATVs, and other off-highway vehicles such as construction equipment, also produce harmful emissions. Even the small engines on lawnmowers, hand-held trimmers, and chain saws have been identified as significant sources of air pollution.