Public Transit Agencies - Background

This page last reviewed March 31, 2011


The Air Resources Board (ARB or “the Board”) seeks to provide clean, healthful air to the citizens of California. California’s commitment to providing clean public transportation is an important part of achieving this goal. Public transportation has important societal benefits, including providing access to work and education, reducing traffic congestion, and meeting the mobility needs of the public, including the elderly and disabled.

California ’s transit agencies are responsible for providing basic transportation services for the public. Transit agencies provide both fixed-route services within urban places, such as traditional urban bus and neighborhood routes, and between urban places such as commuter routes, and non-fixed-route services such as paratransit, dial-a-ride and charter services.

Most types of public transportation, however, are also sources of polluting engine exhaust emissions. Oxides of nitrogen (NOx) and hydrocarbons (HC) contribute to the atmospheric formation of ozone and fine particles. Carbon monoxide (CO) is a colorless, odorless gas that reduces the ability of the body to transport oxygen to cells. Diesel particulate matter (PM) is a toxic air contaminant – a cancer-causing pollutant that also has significant short- and long-term negative cardiovascular impacts. These emissions often occur within California’s most populated areas. It is, therefore, vital to all Californians that the ARB continues its efforts to reduce engine exhaust emissions from all sources.

In February 2000, the Board confirmed its established a new fleet rule for transit agencies and more stringent emission standards for new urban bus engines and vehicles: The Public Transit Bus Fleet Rule and Emission Standards for New Urban Buses (ARB 1999, ARB 2000b). The multi-faceted regulations went beyond the federal requirements for urban buses. The rules were designed to reduce NOx and PM by setting fleet emission reduction requirements that encouraged transit agencies to purchase cleaner buses and retrofit their existing buses; and promoted advanced technologies by adopting requirements for zero emission bus (ZEB) demonstrations and acquisition that are applicable to larger transit agencies. New, more stringent mid- and long-term emission standards were adopted that apply to new urban bus engines.

As a result of these rules, many transit agencies have installed natural gas refueling infrastructure and purchased alternative-fuel urban buses; repowered old diesel engines to engines meeting cleaner exhaust emission standards; installed diesel particulate filters on diesel engines; and experimented with developing technologies, such as hybrid-electric buses, NOx after treatment systems and cleaner fuels. Many of California’s transit agencies consider themselves to be innovators and incubators for advanced technologies.

The Fleet Rule for Transit Agencies regulates urban buses that are owned or leased by public transit agencies. An “urban bus” is a bus that is powered by a heavy heavy-duty diesel engine, or of a type that would normally be powered by a heavy heavy-duty diesel engine. These buses are generally 35 feet in length or longer, although smaller chassis may have an urban bus engine installed if necessary, in which case a smaller bus could be an “urban bus.” Urban buses usually operate on a fixed route consisting of stops and starts as passengers are routinely picked up and delivered to their destinations. Commuter bus operations within metropolitan areas (such as the Yolo-Sacramento metropolitan area) that consist of more than a few pick-up and drop-off stops are also considered to fall within the definition of urban bus operation (ARB 2001b).

Diversification in public transit services has led to the increased use of buses not subject to the current Fleet Rule for Transit Agencies. These vehicles typically are greater than 8,500 lbs. gross vehicle weight rating (GVWR) but are smaller than a typical urban bus and use a medium heavy-duty engine. In addition, buses using heavy heavy-duty engines or buses that are charters or commuter buses having a few pick-up and drop-off stops may not meet the definition of an urban bus. Traditional paratransit and dial-a ride services also utilize smaller buses that are not currently subject to the rule. Several transit agencies only operate buses and vehicles that do not meet the definition of urban bus. Staff  has defined these vehicles as “transit fleet vehicles.” Today, transit fleet vehicles fueled by diesel and alternative fuel represent approximately one fourth of the vehicles operated by public transit agencies.


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