Most Bay Area residents use vehicles for most if not all of their transportation needs. In fact, Bay Area auto use is projected to grow significantly faster than the population through 2010. One consequence of that growth is increased air pollution. According to the Bay Area Air Quality Management District, motor vehicles produce nearly half of the emissions that combine to produce ozone, the principal component of smog. Cars are also a major source of carbon monoxide, fine particulates and certain toxic contaminants that, along with ozone, are detrimental to human health.
Cars also produce a significant amount of carbon dioxide, which contributes to global warming.
However, unlike air pollutants, carbon dioxide can't be removed easily or inexpensively by attaching emission-control devices to vehicles. The only way to cut carbon dioxide is to improve fuel economy, reduce the number of vehicles on the road, or switch fuels, none of which have much popular support. Bay Area homes, job sites, schools, shops, and services are frequently scattered over great distances. They may be bordered by vast parking lots and wide, high-speed roadways that make public transit, walking, and cycling between sites impractical. Newer developments are often built at densities too low to support frequent, convenient transit service.