Motor
vehicles create a substantial amount of air pollution. In fact,
transportation is responsible for nearly 80 percent of carbon monoxide
and 50 percent of nitrogen oxide emissions in the U.S. Not surprisingly,
many metropolitan areas do not meet the air quality standards specified
in the 1990 Clean Air Act Amendments. Although individual cars are
much cleaner today than they were in earlier years, if total traffic
continues to grow, overall air quality will deteriorate. Moreover,
cars and trucks burn millions of barrels of oil, a non-renewable
energy source, every day.