Emily Eros is the Director of Advanced Research at StreetLight, and leads the data science team behind our new GHG Measurement product.
The Federal Highway Administration recently published a new performance measurement rule for tracking greenhouse gas (GHG) emissions on highways.1 This new rule requires state DOTs and MPOs to establish a 2022 baseline level of GHG emissions produced by travel on the National Highway System (NHS). Agencies must set 4-year declining targets and report on the percent change in emissions. State DOTs must establish and report their baseline and targets by Feb 1, 2024, while MPOs must report their targets no later than 180 days after their State DOT establishes its 4-year target.
Overall, we’re encouraged and excited to see this rule established; it’s an important step towards reducing GHG emissions and meeting climate goals. However, FHWA outlines a calculation method that prioritizes consistency and simplicity but has some limitations — particularly for MPOs. In this blog post, we unpack that and explain what we recommend for MPOs, who have more flexibility in how they calculate the performance measure.
The FHWA Method for States
The FHWA requires state DOTs to use a specific approach to calculate baseline 2022 emissions:
In other words, each state’s CO2 estimate is calculated by multiplying the gallons of fuel sold in that state by the CO2 emissions for each fuel type. This is then multiplied by the proportion of state VMT that occurs on the NHS, in order to estimate emissions on NHS highways within the state.
Fuel consumption is taken from monthly state fuel sales reporting, available for gasoline/gasohol and special fuels (diesel). VMT is taken from November 2023 HPMS data.
This method is straightforward, simple, and achievable with data already reported by state agencies. The calculations will be consistent across states and across different time periods. Nationwide, these emissions will amount to the correct, empirical total. At the state level, these may be primary considerations. However, basing emissions on fuel sales has important limitations, particularly for smaller jurisdictions like MPOs.
Limitation: Fuel is not necessarily consumed where it is purchased
This is a general concern when it comes to emissions from trucking, and in areas where travelers may be purchasing fuel across state lines.
Heavy-duty trucks make up a significant portion of highway traffic. For long-distance trucks, it’s typical to refuel roughly every 1000 miles — though many can travel closer to 2000 miles before refueling, depending on fuel tank size and fuel economy. Therefore, for these vehicles in particular, fuel is often consumed in a different state or region from where it was purchased. It’s difficult to tell whether or not these individual patterns balance out; state diesel sales may or may not correspond to state diesel emissions.
StreetLight explored this issue in a recent GHG Emissions Methodology and Validation White Paper. We compared emissions from 2022 state fuel sales against the EPA’s 2020 National Emissions Inventory (NEI), which we adjusted to 2022 according to year-over-year traffic volume trends published by the FHWA. We found inconsistency between these two datasets, particularly for emissions from diesel sales/trucking, where some states had differences of up to +/- 75% depending on the approach used.
These differences are shown in the graph below. For diesel sales (shown in blue), the distribution is centered close to zero but the spread is reasonably wide. This means that many states had very substantial differences between emissions calculated using these two approaches.
Differences were smaller for gasoline-based emissions (shown in gold). The graph below shows a tighter distribution with a smaller range, meaning that the reference datasets were more similar — though the right-skewed distribution indicates that NEI was generally slightly higher than the fuel-sales-based approach. For diesel sales, the left-skewed distribution indicates that NEI was slightly higher than the fuel-sales-based approach.
Emissions from gasoline and diesel reported in the National Emissions Inventory (NEI) are compared with emissions calculated from gas and diesel sales.
Neither the fuel sales nor the NEI approach is a ground-truth measure of emissions, but the variation between diesel/trucking emissions estimates supports our concern that diesel sales in particular may not accurately and consistently measure trucking emissions in all states or regions. Since the federal performance measure specifically focuses on highways, diesel sales variation could have significant impacts on emissions estimates.
Limitation: Fuel sales data are not (typically) available at the MPO level
Motor vehicle fuel sales are tracked and reported at the state level, but often not at more granular levels. So, as the FHWA notes in their final rule, MPOs lack a data source comparable to the Fuels & Financial Analysis System for Highways (FASH) database. Therefore they must estimate CO2 emissions using an approach different from the state DOTs.
This could involve estimating the proportion of state fuel sales consumed within the MPO — but this would compound the uncertainty of the fuel sales approach, and it would not account for fuel economy variation across the state.
Solution: StreetLight provides accurate, reliable GHG metrics for the FHWA performance measure
StreetLight offers a robust solution for MPOs (or other jurisdictions) interested in measuring GHG emissions. StreetLight offers two different GPC-compliant methods (Within Boundary and Induced Activity) to ensure that agencies get the metrics they need to support their goals.2 For each of these methods, we leverage our best-in-class transportation data and analytics to calculate GHG emissions for either individual road segments or entire areas — or both. Our methods use StreetLight’s trusted volume models along with local vehicle registration data to calculate accurate emissions estimates and breakdowns.
For the new GHG performance measure, StreetLight can calculate 2022 baseline emissions metrics for NHS highways within a region, with separate emissions breakdowns for passenger vehicles and trucking.
In other words, rather than using fuel sales to estimate highway emissions, StreetLight’s method drills down into highway VMT by vehicle type. We then factor in vehicle registration data to understand how much gasoline and diesel are actually consumed on NHS roadways. This enables us to calculate GHG emissions by vehicle type.
In addition, StreetLight offers area-level metrics that offer a complete picture of GHG emissions for your region. Area-level metrics include emissions from all roads within a census tract, county, or MPO. They include breakdowns by vehicle type (light-, medium-, and heavy-duty), as well as by trip type, to distinguish between internal and trans-boundary trips. Trip type breakdowns are important for understanding the type of emissions that are occurring, and what reduction strategies might be most appropriate. We also provide metrics for avoided emissions from electric vehicles, so you can track progress towards tailpipe emissions reductions.
StreetLight’s GHG Emissions Metrics show the proportion of total emissions produced by light-, medium-, and heavy-duty vehicles for counties in Maine.
For example, the Twin Cities Metropolitan Council used StreetLight data to develop an online tool to share emissions data with local governments. This way, individual jurisdictions and townships could easily access publicly available data on local emissions rather than relying on national averages alone. Along the way, they used O-D analyses to solve over-counted emissions for small communities that intersected major highways, helping to clarify where emissions reduction projects would be most impactful.
Similarly, Southern Maine Planning & Development Commission used StreetLight’s Origin-Destination analytics to access hyper-localized VMT insights that would have been too expensive and computationally intensive to access otherwise. They then used these insights to calculate emissions for each community in southern Maine and identify where mitigation measures would have the greatest impact.
StreetLight’s GHG offering enables agencies to meet the new GHG performance measure reporting requirements, and it goes further by providing a more complete picture of regional emissions.
Download our GHG Metrics and Methods Whitepaper to learn more, or contact us to discuss your needs.
1. Federal Highway Administration. “GHG Performance Measure.” November 22, 2023. https://www.fhwa.dot.gov/environment/ghg_measure/
2. Greenhouse Gas Protocol. Global Protocol for Community-Scale Greenhouse Gas Inventories. https://ghgprotocol.org/sites/default/files/standards/GPC_Full_MASTER_RW_v7.pdf
