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Air Quality

Our commitment to cleaner fleet technologies contributes to healthier communities.

SamTrans is delivering the highest and cleanest levels of service in its history. Or put another way – driving down emissions to their lowest level ever all while driving more routes.

As ridership continues to grow and service levels increase, the District is also operating more efficiently than ever before, moving more people while reducing emissions per vehicle mile traveled and per passenger trip. In FY2025, SamTrans achieved its most efficient service since 2018 based on emissions per VMT, while operating the highest level of service recorded since tracking began in 2010. Much of this progress is the direct result of operational improvements that have enhanced service delivery while reducing emissions intensity.

By reducing single-occupancy vehicle trips on local roads, SamTrans’ service supports cleaner air and regional traffic congestion. During this reporting period, SamTrans achieved a major milestone: for the first time in the agency’s history, the air quality benefits from displaced vehicle trips exceeded the criteria air pollutants generated by SamTrans operations. In FY2025, SamTrans generated approximately 26.1 tons of criteria air pollutants and displaced approximately 27.2 tons, resulting in a net reduction of about 1.2 tons.

This milestone reflects the growing impact of transit in reducing regional emissions, driven by a combination of increased ridership and more efficient operations. And these impacts are only anticipated to increase as the District continues to expand its zero-emission bus fleet.

Let's visit GHG Emissions

Approximately 40% of California's greenhouse gas (GHG) emissions stem from surface transportation. The choice between driving alone and riding public transit is one of the most significant climate choices Californians make every day. SamTrans’ sustainability program supports the District’s goal of helping residents reduce their carbon footprint, improve air quality, and reduce reliance on cars for transportation.

Let's visit GHG Emissions

Generated 27,668 MTCO₂e

Let's visit GHG Emissions

Generated 27,668 MTCO₂e

Displaced -6,082 MTCO₂e

Let's visit GHG Emissions

Generated 27,668 MTCO₂e

Displaced -6,082 MTCO₂e

Net Total 21,586 MTCO₂e

More than 90% of SamTrans' annual greenhouse gas emissions result from revenue fleet operations, fixed-route bus, paratransit, and shuttle services currently powered primarily by diesel, gasoline, and a small share of other fuels. That mix is shifting as battery-electric and hydrogen fuel cell buses enter service. A smaller share of emissions stems from non-revenue vehicles and facility natural gas use.

SamTrans purchases 100% GHG-free and renewable electricity through WestLight Energy (formally Peninsula Clean Energy (PCE)), resulting in zero greenhouse gas emissions from electricity consumption. The District also calculates indirect emissions from waste disposal, water consumption, and employee commuting, as well as displaced emissions from riders choosing transit over private vehicles.

FY2020 GHG Emissions by Source ^{[Footnote #1]}

Pie chart showing FY2020 GHG emissions by source. Revenue Fleet 89.6%, Waste and Diversion 4.9%, Employee Commute 4.6%, Non-Revenue Fleet 0.9%

No emissions from water have been generated since FY2020. ↩︎

Down the Road

As SamTrans continues its journey to a zero-emission fleet and incorporates electric vehicles into its non-revenue fleet, GHG emissions will decrease significantly and eventually displace more emissions than generated.

Go Further: View GHG Annual Data

SamTrans' net GHG emissions = generated emissions - emissions displaced.

SamTrans' net GHG emissions = generated emissions - emissions displaced. A line graph of SamTrans' net GHG emissions with generated and displaced emissions highlighted as bars for each fiscal year. This figure shows the trends in generated GHG emissions decreased during the pandemic period. By FY2025, service levels exceeded pre-pandemic levels, while generated GHG emissions remained below pre-pandemic levels.

In FY2025, SamTrans generated approximately 27,668 MTCO2e and displaced approximately 6,082 MTCO2e through avoided private vehicle trips, resulting in net emissions of approximately 21,586 MTCO2e.

From FY2023 to FY2025, generated emissions increased as service levels rose to support continued ridership recovery. Displaced emissions changed only modestly over the same period. As SamTrans incorporates electric vehicles into its revenue fleet, generated GHG and criteria air emissions are expected to decrease significantly due to the procurement of renewable energy

GHG Emissions - Generated, Displaced and Net

This figure illustrates how much CO2e SamTrans generates per vehicle mile operated and per passenger boarding. In FY2025, generated GHG emissions intensity improved compared to FY2024. These improvements reflect the combined benefits of ridership recovery, service productivity, and the early integration of zero-emission vehicles into the revenue fleet.

Generated GHG Emissions Per Boarding and Vehicle Mile

Employee Commuting

Emissions associated with employee commuting were calculated as part of SamTrans indirect GHG emissions. As shown in Figure X, employee commuting contributes an average of approximately 8% of SamTrans total GHG emissions during FY2023-25.

Annual employee vehicle miles traveled were estimated based on anonymized employee zip code information, workplace location, and the conservative assumptions that all employees are full-time and commute to work five days a week by passenger vehicle. Emissions associated with commute were calculated based on Bay Area carpooling trends from the American Community Survey (ACS) and vehicle emissions from the California Air Resources Board Emission Factor Database (EMFAC 2021). With these assumptions, the average employee commute emissions during FY2023–FY2025 were approximately 11.6% lower than the pre-pandemic baseline of 1,981 MTCO₂e in FY2019.

EMFAC2021 represents a significant update to California's on-road vehicle emissions model compared to EMFAC2014, incorporating updated vehicle fleet characteristics, travel activity data, emissions testing, and regulatory assumptions. The model reflects increased adoption of zero-emission vehicles, revised emissions methodologies, improved temperature corrections, and expanded vehicle classifications, resulting in more representative emission factors for current and future fleet conditions. EMFAC2021 is the current regulatory model approved for transportation conformity analyses and other official emissions inventories in California, replacing the older assumptions and projections used in EMFAC2014.

To support sustainable commuting, SamTrans continues to promote its Employee Clean Commute (ECC) Program, which provides incentives for employees who commute to work by walking, biking, carpooling/vanpooling, or using public transit. Eligible employees also receive free rides on SamTrans and Caltrain services, along with additional commute benefits such as transit subsidies, carpool/vanpool subsidies, and a Guaranteed Ride Home program.

Beyond employee-focused commute programs, the District supports a variety of commute alternatives to reduce single-occupancy vehicle trips and promote sustainable transportation across San Mateo County. The District encourages multimodal commuting by providing bike amenities that support first- and last-mile connections, subsidized and pre-tax transit passes to local employers, funding community shuttle services, and supporting regional vanpool and carpool commuting programs.

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Criteria Air Pollutants

Criteria air pollutants (CAPs) include pollutants that cause smog and acid rain and have been linked to negative health effects. SamTrans' vehicles emit CAPs when they burn fossil fuels like diesel and gasoline, but they also displace CAPs that otherwise would have been emitted if passengers had chosen to drive alone instead of taking public transit. Approximately 98% of the CAPs emitted by SamTrans are from the diesel revenue fleet, with the remaining 2% attributable to the non-revenue fleet.

SamTrans' transition to cleaner vehicles is already reducing CAP emissions. In 2018, the District replaced 2002-era articulated buses with 55 heavy-duty clean-diesel models, cutting NOx by 87% and overall CAPs by approximately 18%. And SamTrans isn’t stopping there. With the planned replacement of existing buses with new, zero-emission vehicles, generated CAPs will continue to decrease towards zero.

As we transition our fleet, zero-emission buses will measurably
improve the air quality in San Mateo County

Go Further: View CAPs Annual Data

Net reduction in CAPs is shown in a line graph with the generated and displaced CAPs shown in bars above and below the line. SamTrans' transition to cleaner vehicles and to zero-emission buses is expected to continue to reduce CAP generation

Criteria Air Pollutants - Generated, Displaced and Net

SamTrans' generated CAPs normalized by SamTrans VM and boardings. SamTrans' CAPs per VM and CAPs per rider have been declining over the past several years.

Criteria Air Pollutants Generated Per Vehicle Mile and Boarding

SamTrans' CAPs per rider, as shown in the graphs above, have been declining over the past several years. This is primarily due to a cleaner fleet and engines (the transition to electric vehicles), increased ridership, and operational efficiency.

Air Quality

Let's visit GHG Emissions

Let's visit GHG Emissions

Let's visit GHG Emissions

Let's visit GHG Emissions

Footnotes

Down the Road

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