6 Mobile combustion
6.1 Introduction
This is the transportation section for estimating on-road emissions.
Note that this does not include buses, trains, freight rail, light rail, and other transportation emissions.
6.2 Methods
The general workflow for transportation emissions is to find vehicle miles traveled for a given geography,
\[Emissions = miles \times \frac{\text{GHG grams}}{mile}\]
6.2.1 Vehicle Miles Traveled
To find the estimated number of vehicle miles traveled (VMT), we used the estimated number of vehicles and the average trip length in miles for all origin - destination pairs and vehicle types (passenger, medium-duty, and heavy-duty). VMT is calculated as follows:
\[VMT = trips \times length\] where
\[trips = \text{number of trips}\] \[length = \text{average trip length in miles}\]
6.2.2 Emissions factors
You can read more about emissions factors we used in Section 7.3.
\[Emissions=\frac{1}{2}T_{o} + \frac{1}{2}T_{d} + T{_s}\]
where
\[T_o=\textrm{grams of emissions for each GHG from all trips originating in the county}\]
\[T_d=\textrm{grams of emissions for each GHG from all trips ending in the county}\]
and
\[T_s=\textrm{grams of emissions for each GHG for trips originating and ending within the county}\]
Transportation VMT and emissions were calculated in vmt_emissions.R, using functions calculate_emissions() and calculate_vmt().
6.3 Results
We can also view this data broken out by vehicle weight in Figure 6.2.
6.3.2 Comparison with other inventories
6.3.2.1 National Emissions Inventory
The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of criteria pollutants, criteria precursors, and hazardous air pollutants from air emissions sources. The county-level GHG emissions included in the NEI for this category are calculated by running the MOVES model with State-, Local-, and Tribal-submitted activity data and EPA-developed activity inputs based on data from FHWA and other sources (USEPA 2023).
NEI data were pulled using the EnviroFacts API and processed in R scripts: epa_nei.R and epa_nei_transportation.R
We expect that the NEI data will show higher emissions, because it is based on overall activity, not explicit origin-destination travel.
6.3.2.2 Local Greenhouse Gas Inventory Tool (LGGIT)
EPA’s Local Greenhouse Gas Inventory Tool (LGGIT) was developed to help communities across the United States to evaluate their greenhouse gas emissions. We used the LGGIT to validate our calculations and datasets (US EPA 2017).
We used the LGGIT Excel workbook tool with the following inputs in the “Mobile-Data” sheet. Several calculations and assumptions were made
- Passenger VMT was split between gasoline and diesel powered vehicles based on regional fleet composition in the Travel Behavior Inventory (TBI).
- Passenger vehicle age was determined from the median vehicle model year in the TBI. See Section 7.4.1 for more detail.
- Fuel consumption was calculated by dividing VMT by average vehicle miles per gallon (MPG), specific to vehicle type and fuel type.
- Fuel efficiency data were taken from the LGGIT tool and verified with the EPA Emission Factors Hub (2021 edition) (USEPA 2021).
- All medium-duty VMT were classified as “Light Truck”, while all heavy-duty VMT were classified as “Heavy-Duty Vehicle”.
- Commercial trucks were assumed to be year 2007, based on a 2018 report that found the average age of trucks in the US to be 14.2 years (Brusseau 2019) (
2021 - 14 = 2007).
LGGIT entries and resulting output tables were generated and processed in epa_lggit.R.
| ID | Unit Description | Sector | Vehicle Year | Vehicle Type | Vehicle Model (optional) | Fuel Type | Fuel Consumption | VMT |
|---|---|---|---|---|---|---|---|---|
The results from the LGGIT tool estimate a regional emissions total of 8,891,741 metric tons CO2e, while our calculations estimate 8,757,962 metric tons CO2e. These values are sufficiently close, and may be explained by differences between nationwide vehicle emission rates and fuel efficiency and region-specific values from EPA MOVES. Additionally, LGGIT uses global warming potential (GWP) values from the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) (IPCC and Core Writing Team 2014), while our calculations use slightly higher values from the 6th Assessment Report (AR6) (IPCC 2023). See Section 7.3 for a more detailed comparison.