Case Study

TriMet Plots a Course to Decarbonizing Its Fleet

Guidehouse supports TriMet in addressing its operational footprint with a resilient, future-proof solution


Decarbonization and resilience are closely linked. As well as helping to mitigate transportation’s contribution to climate change and extreme weather events, decarbonized fleets are less vulnerable to price volatility during the fossil fuel transition. While mass transit use reduces the overall carbon footprint in Portland, Oregon, TriMet—a public agency providing bus and rail services in the area—had the ambitious target of transitioning its entire bus fleet to all alternative fuels and was planning trials of electric buses. But the agency wanted to explore the opportunities and costs of transitioning from diesel to different alternative fuels so it could make an informed decision on which to pursue.


Guidehouse estimated that TriMet’s bus fleet would generate more than 2.8 million metric tons of greenhouse gas emissions (GHGs) from 2019 to 2055 if no changes were made to its fleet, fuel, and vehicle mix. Costs, including capital, fuel, and pollution harms, approached an estimated $4 billion, while leaving the agency open to the risks of regulatory changes and fuel price volatility. A report explored a suite of possible strategies: hydrogen, electricity, biofuels, and natural gases. Each option carried costs and risks beyond the price of fuel and vehicles alone. To understand these lifetime costs and GHG emissions, Guidehouse had to pull together a diverse range of data and streamline it into a single analysis. Some data points were in the public domain, others required surveys. The team looked at costs, including vehicle purchase, infrastructure development, energy, and operation, as well as additional factors, such as the ambient environment and technology maturity.

TriMet learned that a battery electric bus transition fleet would be the most cost-effective option for maximally reducing the agency’s GHG emissions. The organization found that new vehicles would not begin to pay themselves back within a 16-year period until around 2025. By this time, vehicle costs would also have decreased, while a wider range of both buses and charging technology would be on the market. In addition to these cost savings, delaying transition to 2025 would allow TriMet to calibrate real-world metrics from its electric bus trials. 

Building a resilient, decarbonized fleet entails more than transitioning to electric vehicles. TriMet also needed to address its legacy bus fleet, which is expected to operate until 2035. Given that the fleet is equipped with internal combustion engines, the natural route to reduce GHG emissions is biofuels. Guidehouse modeled costs and emissions reductions over the period 2019–2035 for diesel blends containing 5%, 20%, and 99% renewable diesel. With a 99% mix, TriMet could avoid up to 495,000 metric tons of CO2e emissions at a cost of more than $46 million. Trading the current 5% biodiesel blend for a 5% renewable diesel blend would produce a small decrease in emissions and minor cost reductions, increasing efficiency somewhat while still shrinking maintenance costs. A 20% blend would significantly reduce emissions yet also increase costs.


In December 2021, TriMet transitioned all buses covering routes through the Portland metro area to a mix of 99% renewable diesel, lowering its bus fleet GHG emissions by about 61%. Additionally, as of 2022, despite reduced rider numbers and revenue due to the pandemic, the agency was testing four different types of electric buses, gathering vital information to define its future fleet, and powering them all on 100% renewable electricity.


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