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TCRP Synthesis 84: Current Practices in Greenhouse Gas Emissions Savings from Transit


Transit agencies have a key role to play in reducing the greenhouse gas (GHG) emissions that contribute to climate change. Buses, trains, vans, and ferries can move passengers using less fuel than private vehicles can. Less fuel used generally means fewer GHGs emitted. Most U.S. transit agencies are already helping to reduce GHG emissions just by operating their current services, but transit agencies can further reduce GHG emissions and achieve other important goals by implementing strategies to increase ridership and improve the efficiency of their operations.

This study describes the role of transit agencies in reducing GHG emissions and catalogs the current practices of a sample of agencies. Research for this study included a literature review, a survey of 62 transit agencies, with 41 responding (66%); and interviews with three agencies.

Climate change is the broadest environmental challenge of the 21st century. Consequences of climate change expected in the coming years include rising sea levels, increases in average temperatures, changes in patterns of precipitation, and increases in the intensity and frequency of severe weather. These climatic shifts may reduce crop yields, increase the risk of invasive species, exacerbate drought conditions, and threaten endangered species. The built environment is also at risk. Human settlements in coastal and low-lying areas are particularly vulnerable to changes in sea level and to storm and precipitation events. Increases in global concentrations of GHGs, largely the result of human activities, are the predominant cause of climate change. Transportation is one of the largest sources of GHG emissions in the United States.

Public transportation stands out as an important partial solution to the problem. Passenger travel in cars and trucks alone generates nearly two-thirds of transportation’s GHG emissions in the United States. Public transportation can reduce these emissions by transporting passengers more efficiently than private vehicles can. Transit reduces GHG emissionsin four principal ways. Transit displaces emissions from other modes by:

1.Reducing miles traveled in private vehicles;

2.Reducing on-road congestion, thereby reducing fuel burned when vehicles idle on congested roadways; and

3.Facilitating compact development patterns that lead to less GHG-intensive travel.

Transit agencies can also:

4.Reduce the emissions that they generate from their vehicles and facilities.

The net impact of transit on GHG emissions depends on the balance of emissions displaced and emissions released by vehicles and facilities. A crucial determinant of transit’s net impact is the passenger load on individual transit services. Ridership on vehicles must be high enough that more emissions are displaced from private travel than are emitted from the tailpipe of the transit vehicle. Balancing emissions produced and displaced, many transit agencies are already net reducers of GHG emissions. The U.S. transit industry as a whole produces an annual net reduction of GHG emissions roughly equivalent to emissions from all transportation in the state of Washington.

In addition to the benefits of their existing services, every transit agency surveyed is planning or implementing strategies that can further reduce GHG emissions. Interest in these strategies is widespread across agencies, and agencies are generally aware of the impact such strategies can have on GHG emissions. Types of strategies are as follows:


  • Expanding transit service (78% of respondents planning or implementing)—Agencies can increase ridership by expanding route coverage, increasing service frequency, and extending operating hours. These strategies will reduce GHG emissions as long as displaced emissions are not outweighed by higher emissions from transit vehicles.
  • Increasing vehicle passenger loads (93% of respondents planning or implementing)— Strategies that boost passenger loads allow agencies to increase the emissions they displace without increasing emissions from transit vehicles, and without substantial new capital and operating expenditures. These strategies include improving transit access, comfort, and safety; improving service speed and reliability; providing transit information, marketing, and incentives to use transit; and optimizing existing transit routes (which could include reducing service).
  • Reducing roadway congestion (88% of respondents planning or implementing)—Most transit strategies that mitigate congestion are the same strategies that increase transit .ments to provide transit service targeted to reduce congestion on specific corridors. •Promoting compact development (70% of respondents planning or implementing)—Tran.sit agencies can promote compact development in specific nodes around transit stations and by contributing to local and regional development and planning processes. These types of strategies typically require cooperation with other local and regional agencies.
  • Alternative fuels and vehicle types (90% of respondents planning or implementing)— Some alternative propulsion technologies emit fewer GHGs per mile of travel than do .sion technologies. In some cases, alternative fuels can be used in existing vehicles.
  • Vehicle operations and maintenance (90% of respondents planning or implementing)— Improvements to existing vehicles and changes to operating practices can increase the fuel economy of vehicles and thereby reduce GHG emissions.
  • Construction and maintenance (73% of respondents planning or implementing)— Strategies that reduce emissions from construction and maintenance are those that reduce the use of virgin materials or reduce the use of fossil fuels in construction and maintenance processes.
  • Reducing emissions from facilities and nonrevenue vehicles (83%)—Agencies can reduce the use of fossil-based energy in their facilities through a variety of energy-saving measures and by using electricity generated from renewable sources. Agencies can help employees reduce their own GHG emissions.

Analytical and planning processes related to GHG emissions are still nascent fields in the transit industry, and in the transportation industry as a whole. Major findings from the literature review and survey include the following:


  • GHG emissions are still a peripheral concern for transit agencies. Less than half of survey respondents said that reducing GHG emissions was a principal factor in pursuing any given strategy. Increasing ridership, reducing costs, and complying with environmental regulations were generally more important factors. Agencies are unlikely to .gies that reduce GHG emissions have substantial co-benefits.
  • Guidance on calculating GHG emissions displaced by transit is still under development. The most robust methodologies use separate calculations for emissions displaced by mode shift, reduced congestion, and compact development. APTA’s “Recommended Practice for Quantifying Greenhouse Gas Emissions from Transit” .tainty around techniques to estimate the impact of transit on compact development. New and better guidance may lead to greater recognition of displaced emissions by reporting organizations.
  • Many agencies have estimated some part of their impact on GHG emissions, or have had calculations performed by a partner agency. More than one-third of survey .tions. Nearly half of respondents have estimated or are estimating some displaced emissions. Agencies most commonly estimate the mode shift effect of their services. Fewer agencies have estimated the benefits they provide through reduced congestion or compact development.
  • More research is needed on methodologies to estimate changes in emissions from specific improvements to transit. Most studies that have analyzed the impact of transit on GHG emissions have focused on existing services. Many of these are limited to analyses at the state or national levels. Very few analyses have covered a full array of strategies that transit agencies can implement to reduce GHG emissions. Even fewer have assessed the cost-effectiveness of such strategies.
  • Some transit agencies have initiated formal or semiformal efforts to address GHG emissions. A handful of agencies include GHG emissions in internal sustainability plans or have joined sustainability efforts organized by APTA and the International Association of Public Transport. A few agencies have drafted or plan to draft their own climate action plans. More than two-thirds of agencies have participated in talks or joint efforts with other transportation stakeholders on the topic of climate change.
  • A study on best practices, opportunities, and challenges for integrating climate change into transit planning would be helpful. Many transit agencies are struggling with how objectives to reduce GHG emissions will fit with their traditional planning .cies and state departments of transportation integrate climate change into planning objectives and practices. No parallel research has been conducted on transit agencies and transit planning.

Transit agencies can expect federal, state, and local policies on GHG emissions to affect the way they do business in the future. Nearly two-thirds of survey respondents are located in states and cities that have policies related to GHG emissions, including GHG reduction .tion on GHG emissions is expected in the near future. These policies present challenges, as well as funding opportunities, for transit agencies. The uncertainty of future regulations could be addressed in research studies:


  • Transit agencies could benefit from focused research and guidance on new funding opportunities related to GHG emissions. A few agencies are actively considering new funding opportunities that might be created by emissions trading schemes or government grant programs. Such opportunities could become an important source of funding.
  • Some agencies are unclear about how reporting their emissions might affect their ability to receive credit for current or future reductions. A research study could describe the risks and opportunities that reporting of emissions provides to transit agencies. Such a study might also engage third-party reporting agencies to think more critically about the needs of transit agencies in reporting their emissions.

Many transit agencies see addressing GHG emissions as a challenge. Survey respondents see funding and staffing for GHG planning initiatives as the biggest obstacles. Uncertainty .cerns. Still, many agencies are taking important first steps to further their role in reducing GHG emissions. Using existing research, agencies can begin to account for the benefits that their services provide to GHG emissions. Transit agencies can also develop new strategies that both reduce GHG emissions and meet other agency priorities.