Air quality

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Public transit can have an effect on human health by reducing air pollution created by trips taken by private automobiles. The emissions from cars contribute to asthma, cancer, and other diseases. This pollution also has serious implications for equity, as a growing body of evidence demonstrates that low-income people, more likely to live near heavily traveled highways, are disproportionately impacted by these diseases. Emissions from vehicles of all types also have implications for climate change, which ultimately has an effect on human health by raising temperatures and causing or exacerbating extreme weather events.

This is the ribbon-cutting ceremony for San Jose's newest zero-emissions bus. Photo by Flickr user congressman_honda.

Strategies for Improving Air Quality

Increasing Ridership

Transit agencies have an interest in increasing ridership for cost-effectiveness reasons - serving more passengers per vehicle mile, for example. But environmental agencies and regional governments rely on public transit as an alternative to the single-occupant vehicle as a strategy for reaching air quality goals. However, studies conducted by transit agencies on their work’s effect on air quality were not readily available, possibly because those agencies are charged with many other responsibilities and do not have resources to produce independent studies. The Centers for Disease Control, though, recommend expanding public transportation as one of its eight transportation policies that could drastically improve public health. The CDC also offers a toolkit to aid planners and decisionmakers in conducting health impact assessments to measure the benefits and costs for health of transportation projects.[1]

Modernizing Vehicles

Most California transit agencies have had to modernize their buses for some degree because of the California Air Resources Board’s Fleet Rule. For example, transit agencies in California are required to replace vehicles early beginning in 2015 and as of January 2012, must add particulate matter filters to buses.[2] But many transit agencies have exceeded these expectations because they see modernizing their vehicle fleets as a part of their mission to improve air quality and reduce their dependence on volatile and expensive traditional gasoline and diesel fuel. Los Angeles’ Metropolitan Transportation Authority switched many of its buses to compressed natural gas (CNG) in 2005, and retired its last diesel bus in 2011.[3] An alternative to retiring buses is retrofitting them. Because diesel fuel is also a major contributor to particulate matter in the air, the Metropolitan Transportation Commission (MTC) and the Bay Area Air Quality Management District (BAAQMD) worked together to support Bay Area transit agencies in retrofitting buses with filters that capture 85 percent of diesel exhaust particulate matter.[4]

Important Policies

Congestion Mitigation and Air Quality standards (CMAQ)

Air quality and public transit are linked in many ways, but one of the most direct connections made by any agency is through the Congestion Mitigation and Air Quality (CMAQ) standards. CMAQ provides a source of funding to states with air quality that does not meet the Clean Air Act’s air quality standards - this funding can be used for public transit projects and is distributed through the state’s department of transportation.These funds may be used by transit agencies to provide new service, to expand public transit service, and/or to provide incentives to use existing services.[5]

Transportation Improvement Program (TIP)

In the interest of attaining federal air quality standards, every four years, or when a regionally significant project is approved, regions across the country must prepare a Transportation Improvement Program (TIP). The TIP must be approved first by the metropolitan planning organization where the non-attainment area is located, then by the Federal Highway Administration (FHWA) and the Federal Transit Administration (FTA).[6] When a planning agency develops a new Regional Transportation Plan (RTP), they must also prepare a conformity analysis to demonstrate that the transportation plans meet air quality standards and do not exceed the ‘budget’ of emissions allocated to the area by the State Transportation Improvement Plan (STIP).[7]

Proposition 1B (California)

Approved in 2006, Proposition 1B, The Highway, Safety, Traffic Reduction, Air Quality, and Port Security Bond Act, set aside billions of dollars of bond money to establish accounts to accomplish a variety of transportation goals, including modernizing transit systems, improving air quality, and improving intercity rail systems. The California Department of Transportation (Caltrans) disburses these funds to local transit agencies for different projects.[8] In 2012, about 80 projects were awarded a total of about $350 million in grants through this program.[9]


  1. Centers for Disease Control. CDC Transportation Recommendations. 2010.
  2. California Air Resources Board.“Public Transit Agencies.” 2011.
  3. Metropolitan Transportation Authority. “Metro Retires Last Diesel Bus, Becomes World’s First Major Transit Agency to Operate Only Clean Fuel buses.” 2011.
  4. Metropolitan Transportation Commission and Bay Area Air Quality Management District. "Bus Filters Remove Tons of Soot from Bay Area Air." 2006.
  5. Federal Highway Administration. “Air Quality.” 2005.
  6. Federal Highway Administration and Federal Transit Administration. The Transportation Planning Process; A Briefing Book for Transportaiton Planning Decisionmakers, Officials, and Staff.” 2007.
  7. Metropolitan Transportation Commission. ”Transportation-Air Quality Conformity Analysis for the Transportation 2035 Plan and 2011 Transportation Improvement Program.” 2010.
  8. California Department of Transportation. “Transportation Programming - Proposition 1B - Transportation Bond Program.” 2011.
  9. California Department of Transportation. “Caltrans Awards $350 Million in Grants to Improve Public Transit and Air Quality.” 2012.

Additional Reading

Los Angeles County Metropolitan Transportation Authority. “Metro Orange Line Mode Shift Study and Greenhouse Gas Emissions Analysis.” 2011.

When Los Angeles Metro built its Orange Line bus rapid transit line, it included a bikeway and other facilities for bicyclists alongside the dedicated lane. The purpose of this study was to establish the benefits of that bikeway in terms of greenhouse gas emissions avoided. The bikeway was created in order to complement the busway and both have exceeded expectations in terms of use. Bicycle facilities are an important tool that transit agencies have for pursuing sustainability and promoting public health and this study offers one example of quantifying these benefits.

Environmental Protection Agency. [ “Transportation Conformity: General Information.”

This guide from the EPA provides links to a guide for understanding the transportation conformity process, along with resources for technical assistance, and a guide for state and local officials. It also includes a report with case studies of cities implementing the conformity requirements through the 1990s, including San Francisco.

California Environmental Protection Agency Air Resources Board. “Final Diesel Risk Reduction Plan with Appendices.” 2000.

This report from the California Air Resources Board outlines some strategies for reducing the particulate matter in the state’s air. It describes the risks and strategies from a variety of diesel engine types (stationary, and mobile engines in school buses, transit buses, and trucks). However, transit providers may be interested in the report because it includes a summary of existing policies governing diesel engines, methodology for measuring particulate matter, and reviews of technologies for reducing particulate matter.

Metropolitan Transportation Commission. “Transportation, Land Use, and Greenhouse Gases: A Bay Area Resource Guide.” 2009.

This guide reviews a wide range of strategies for meeting California’s greenhouse gas emissions reductions over the next couple decades. The guide evaluates strategies based on how well they will be able to reduce emissions, as well as how cost-effective they are. Increasing ridership on public transit, facilitating biking and walking, and changing travel behavior are all included as components of the overall plan to reduce California’s emissions.