End of the line for belching buses?

In an effort to reduce the negative impact buses have on urban air quality, many transit agencies already have begun switching their fleets from diesel fuel to natural gas. Now, some are turning to diesel hybrid technology.

Heavy-duty vehicles contribute 15% of the nation's emissions of smog-causing NOx and 22% of particulate matter (PM) emissions, according to the U.S. Environmental Protection Agency. In urban areas, the contribution to air pollution can be even greater, according to the EPA: an older, dirtier diesel truck or bus can emit almost 7.2 t (8 ton) of pollution a year.

Transit agencies increasingly are turning to alternative-fueled buses. Among the drivers of this trend are Clean Air Act mandates, politics, and grant programs available through federal, state, and local governments.