Alternative fuel vehicles
Public transit is often called upon as a measure to reduce environmental impacts of travel,<ref>Shaheen, Susan and Timothy Lipman, "Reducing Greenhouse Gas Emissions and Fuel Consumption: Sustainable Approaches for Surface Transportation." IATTS Research, Volume 31. http://innovativemobility.org/wp-content/uploads/2015/07/Reducing-Greenhouse-Emissions-and-Fuel-Consumption.pdf</ref> both by consolidating travelers from single-occupant vehicles into one environmentally-efficient vehicle, and by using modern technology for cleaner propulsion. The American Public Transportation Association (APTA) estimated that by 2011, about 35% of the transit fleet in America was using alternative fuels or hybrid technologies.<ref>Miller, V. (2013). "More than 35% of U.S. Public Transit Buses Use Alternative Fuels or Hybrid Technology." American Public Transportation Association</ref> Many technologies have been adapted for bus and rail transit, including electricity and battery, natural gas, and hydrogen.
Standard and Bio-fuels: Gasoline and Diesel
Gasoline and diesel remain the most common fuels for all vehicles. Federal regulations attempting to reduce the impact of these fossil fuels on the environment have mandated supply of ultra-low sulfur diesel and the use of ethanol (also known as E85) in gasoline.<ref>US Environmental Protection Administration. "Biofuels and the Environment: Basic Information."</ref> Biodiesel fuel blends can typically be used in any modern diesel engine, making an attractive opportunity for agencies to use alternative fuels while avoiding the high cost associated with other technologies such as hybrid-drive buses. However, in a 2011 report to Congress, the EPA warned that increased production of biomass, especially corn, to blend with fuel and decrease dependence on fossil fuels may not have overall positive effects on the environment.<ref>National Center for Environmental Assessment. (2011). "Biofuels and the Environment: First Triennial Report to Congress." US Environmental Protection Agency.</ref>
Diesel Environmental Concerns
Although diesel engines are particularly efficient and one of the most common combustion-engine choices for buses and other commercial vehicles, they also cause significant harm to the environment in the form of particulate matter (PM) from engine exhaust. Research suggests that long-term exposure to diesel exhaust is linked to increases in asthma in children, exacerbation of allergies, and possibly premature death.<ref>National Center for Environmental Assessment. (2002). "Health Assessment Document for Diesel Engine Exhaust." Environmental Protection Agency.</ref> In response to research conducted by the California Air Resources Board (CARB) and others in the early 2000s, new regulations were placed into effect for diesel engines requiring fitting of diesel particulate filters (DPF). However, transit agencies are subject to different regulations than other buses and trucks<ref>CARB. "Fact Sheet: Fleet Rule for Transit Agencies Urban Bus Requirements."</ref>, which went into effect earlier than the recent standards for retrofitting DPF to trucks operating in California.<ref>Knee, R. (2013). "DPF Retrofits Growing Due to California Rule." Transport Topics.</ref>
Regulations pertaining to transit agencies (defined as "urban bus") are found in title 13 of the California Administrative Code (13 CCR § 2020 - 2023.4), provided here by CARB.
Practically all bus manufacturing firms offer diesel options, and cutaway buses are commonly available in either gasoline or diesel configurations. Cummins is an example of an engine manufacturer for transit buses that certifies their products for use with biodiesel fuel.<ref>Cummins. "Biodiesel FAQ."</ref>
Natural gas is used as a fuel in both liquid (LNG) and compressed-gas forms (CNG). Santa Monica, California's Big Blue Bus includes a fleet of buses powered by LNG. Los Angeles County Metropolitan Transportation Authority (LACMTA, or Metro) operates the country's largest fleet of CNG buses.
Bus Manufacturers with Natural Gas Offerings
The Gillig Corporation introduced a CNG option for their buses in 2011. New Flyer and subsidiary NABI provide CNG vehicles.
Liquid Propane Gas (LPG) should not be confused with LNG, above.
Electric power for buses is one of the oldest propulsion technologies, adapted from electric streetcars. Buses powered by overhead wires are commonly called "trolley-buses" and still operate today in some cities such as Seattle, San Francisco, Dayton, Boston, and Philadelphia. Buses can also be powered by electric battery without external power such as overhead wires, but the range of these vehicles tends to be limited. San Francisco Municipal Transportation Agency (SFMTA) and King County Metro in Seattle jointly purchased new electric trolley-buses from New Flyer to replace aging fleets.<ref>Metro Magazine. (2013). "King County Metro purchase all-electric New Flyers."</ref>
The most common application of electric power for buses today is the hybrid-electric. SFMTA and Long Beach Transit operate fleets of hybrid-electric buses.<ref>SFMTA. "MUNI Hybrid Buses."</ref>. The Long Beach buses were purchased from New Flyer in 2005 for a published cost of $550,000 per vehicle.<ref>Long Beach Transit. "Hybrid E-Power Bus Fact Sheet."</ref><ref>Long Beach Transit. "Environmental Issues."</ref> Gillig and New Flyer both offer hybrid-electric bus options.
In 2017, the Los Angeles Department of Transportation introduced the first all-electric bus in its DASH system.<ref>Linton, J. (2017). "Electric DASH Buses To Begin Service In DTLA Next Week." Streetsblog Los Angeles.</ref>
Hydrogen Fuel Cell
Hydrogen fuel cells has been researched as a power source for buses using Federal funding. AC Transit of California has participated in a hydrogen fuel cell bus testing program since 2000 using Van Hool buses and a power plant developed by UTC Power of Connecticut. In 2013, UTC Power was sold to ClearEdge Power, and the future of the fuel cell bus program is unknown.<ref>Kane, B. (2013). "UTC Power sold to Oregon fuel cell firm." Hartford Business Journal.</ref>