Despite improvements from the days of notoriously smog-ridden Los Angeles, California is still home to some of the worst air pollution problems in the country. Every major metropolitan area in the state fails to meet fully federal air quality standards, exposing more than 80 percent of California's population to unhealthy levels of air pollution. Transportation is one of the major contributors to air pollution because the number of trucks, buses and cars on the road traveling further and further distances is overwhelming any recent technological advances in cleaner fuels and engines.