2  Methods and data sources

2.1 Methodological framework

Transportation emissions are available for motorcycles, passenger cars, intercity buses, light commercial trucks, single unit long-haul trucks, refuse trucks and transit buses using EPA data sources. We use a geographic, or territorial accounting method, aligning with Scope 1 in the Greenhouse Gas Protocol. Geographic methods account for any transportation emissions taking place within a geographic boundary, regardless of origin or destination (Fong et al. 2021).

Geographic emissions are essential for quantifying air pollution experienced by people living in the area, but they do not give information on the logistic decisions of individuals.

2.2 Data sources

2.2.1 EPA emissions data

The EPA releases various emissions estimates as part of several programs and initiatives.

All datasets are compiled from Sparse Matrix Operator Kernel Emissions (SMOKE) Flat File 10 (FF10) formatted data downloaded from the EPA website. SMOKE FF10 is a standardized format regularly released by the EPA for NEI, EQUATES, and Air Emissions Modeling platforms (CMAS 2024, sec. 2.2.3).

SMOKE FF10 files were processed using read_smoke_ff10(), which reads in the raw data, records relevant metadata, filters to only include relevant counties and pollutants, and saves an intermediary dataset. These intermediary datasets are read back in, combined, and saved.

SMOKE FF10 data were aggregated to include all MOVES processes for on- and off-network vehicle operation, including running, starting, and idling exhaust, tire and brake wear, evaporative permeation, fuel leaks, and fuel vapor venting, and crankcase exhaust (CMAS 2024, sec. 2.7.4.9). 1

Direct URLs and download information are available in the EPA downloads guide.

Table 2.1: Intermediary datasets and processing scripts
Data source Dataset Processing script
National Emissions Inventory epa_nei_smoke_ff.RDS data-raw/epa_nei_smoke_ff.R
EQUATES equates_cmas_mn_wi.RDS data-raw/epa_equates_read.R
Air Emissions Modeling onroad_mn_wi.RDS data-raw/epa_air_emissions_modeling_onroad.R

Each data source and year uses a different MOVES edition. These are listed in Table 2.2.

Table 2.2: On-road pollutants available by year and EPA data source
Data source MOVES edition Years
Air Emissions Modeling MOVES4 2021, 2022
National Emissions Inventory MOVES3 2020
EQUATES MOVES3 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019

Various pollutants are available.

Table 2.3: On-road pollutants available by year and EPA data source
Data source Years Pollutants
Air Emissions Modeling 2021, 2022 CO2, CH4, N2O, CO, NO, NOx, SO2, PM2.5, PM10, NH3
National Emissions Inventory 2020 CO2, CH4, N2O, CO, NOx, SO2, NH3, PM2.5, PM10
EQUATES 2018, 2019 CO2, CH4, N2O, CO, NO, NOx, SO2, PM2.5, PM10, NH3
EQUATES 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017 CO2, CH4, CO, NO, NOx, SO2, PM2.5, PM10, NH3

Pollutant descriptions

Table 2.4: Pollutants provided by EPA datasets
Pollutant Pollutant code Description
CH4 CH4 Methane
CO CO Carbon Monoxide
CO2 CO2 Carbon Dioxide
N2O N2O Nitrous Oxide
NH3 NH3 Ammonia
NO NO Nitric Oxide
NOx NOX Nitrogen Oxides
PM2.5 PM10-PRI PM10 Primary (Filt + Cond)
PM10 PM25-PRI PM2.5 Primary (Filt + Cond)
SO2 SO2 Sulfur Dioxide
VOC VOC Volatile Organic Compounds

Vehicle and fuel types

Table 2.5: Vehicle and fuel types provided by EPA datasets
Vehicle weight label Fuel types Vehicle types
Passenger Gasoline Motorcycles, Passenger cars, Passenger trucks
Passenger Diesel Passenger cars, Passenger trucks
Passenger Electric Passenger cars
Passenger Ethanol (E-85), Electric Passenger cars, Passenger trucks
Passenger Ethanol (E-85) Passenger cars, Passenger trucks
Buses Gasoline Intercity buses, Transit buses, School buses
Buses Diesel Intercity buses, Transit buses, School buses
Buses Compressed natural gas (CNG) Intercity buses, Transit buses
Buses Gasoline Transit buses, School buses
Buses Diesel Transit buses, School buses
Trucks Gasoline Light commercial trucks, Refuse trucks, Single unit short-haul trucks, Single unit long-haul trucks, Motor homes, Combination short-haul trucks
Trucks Diesel Light commercial trucks, Refuse trucks, Single unit short-haul trucks, Single unit long-haul trucks, Motor homes, Combination short-haul trucks, Combination long-haul trucks
Trucks Compressed natural gas (CNG) Single unit long-haul trucks
Trucks Compressed natural gas (CNG) Refuse trucks, Single unit long-haul trucks
Trucks Compressed natural gas (CNG), Ethanol (E-85), Electric Refuse trucks, Single unit long-haul trucks, Light commercial trucks
Trucks Compressed natural gas (CNG), Ethanol (E-85) Refuse trucks, Single unit long-haul trucks, Light commercial trucks
Trucks Gasoline Light commercial trucks, Refuse trucks, Single unit short-haul trucks, Single unit long-haul trucks, Motor homes
Trucks Compressed natural gas (CNG), Ethanol (E-85), Electric Single unit long-haul trucks, Light commercial trucks
Trucks Gasoline Light commercial trucks, Single unit short-haul trucks, Single unit long-haul trucks, Motor homes, Combination short-haul trucks

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 2023b).

NEI data were pulled using the EnviroFacts API and processed in R scripts: epa_nei.R and epa_nei_envirofacts.R.

NEI SMOKE FF10 data are processed in epa_nei_smoke_ff.R.

NEI on-road regional summaries are processed in epa_nei_onroad_emissions.R.

Ultimately, NEI data used in the Metropolitan Council inventory were compiled from SMOKE FF10 for year 2020.

Verification and validation

NEI data were cross-verified by comparing county level emissions totals compiled from NEI EnviroFacts, NEI data summaries by region, and compiled SMOKE FF10.

epa_verify_nei_envirofacts_smoke.R found that data compiled from SMOKE FF10 and regional summaries aligned exactly for year 2020 and closely for other years. Similarly, data compiled from EnviroFacts also aligned closely with SMOKE FF10 and regional summaries.

Data published on the EPA website are subject to change at any time. Every effort was taken to align versions, model runs, and other opportunities for differentiation.

EQUATES

EQUATES (EPA’s Air QUAlity TimE Series) is a set of modeled emissions and supporting data developed by EPA scientists spanning years 2002 to 2019. EQUATES is particularly useful in that it uses modern source classification codes (SCCs) to provide a continuous time series (K. M. Foley et al. 2023).

Between the 2008 and 2011 NEI releases, the EPA completed major changes to their source classification codes (SCCs), which rendered direct comparison between 2008 and prior years with 2011 and later years impossible.

EQUATES is based on the 2017 NEI and uses MOVES3 (K. M. Foley et al. 2023).

EQUATES data are available for years 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018 and 2019.

EQUATES SMOKE FF10 data are processed in epa_equates_read.R.

Verification and validation

Though EQUATES datasets are available on the EPA file transfer site and the CMAS Data Warehouse Google Drive, individual file names and file contents were identical.

Limitations

In addition to limitations described in Section 2.2.1.4, EQUATES has its own set of limitations.

  • EQUATES does not contain emissions estimates for N2O (nitrous oxide) for years 2002-2017. N2O was added to the EPA Emissions Modeling Framework (EMF) after EQUATES was compiled. N2O does not affect air quality monitoring and so was not included in older emissions work (K. Foley, Eyth, and Allen 2024). When compared with the NEI and Air Emissions Modeling, including N2O in total CO2e resulted in a maximum difference of around 3% for some counties and years. See epa_verify_n2o_differences.R for more detail.
  • EQUATES includes only on-road emission sources.

Air Emissions Modeling Platforms

The EPA continually works on emissions inventories for various projects.

Air Emissions Modeling data are available for several years, but years 2021 and 2022 are used in the final inventory.

Both the 2021 and 2022 estimates are based on the 2020 NEI USEPA (2024a).

Air Emissions Modeling SMOKE FF10 data are processed in epa_air_emissions_modeling_onroad.R.

Verification and validation

Air Emissions Modeling data are only available from a single consistent website, and so verification across locations was not necessary.

Limitations

In addition to limitations described in Section 2.2.1.4, Air Emissions Modeling has its own set of limitations.

  • Air Emissions Modeling datasets are in active development and subject to change.

Consistent limitations

  • The NEI, EQUATES, and Air Emissions Modeling platforms are based on MOVES, which does not account for activity on local roads.
  • NEI, EQUATES, and Air Emissions modeling use different MOVES editions (see Table 2.3), which may result in discrepancies between years.
  • To reduce run times, the EPA uses fuel months to represent summer and winter fuels. The month of January represents October through April (winter), while July represents May through September (summer) (USEPA 2023a, sec. 5.6.6.2). Variation within the summer and winter months is not accounted for using this method.
  • The 2020 NEI had particular challenges due to the COVID-19 pandemic
  • Minnesota did not submit custom data inputs for the 2020 NEI, meaning that inputs to MOVES were based on national default values. Wisconsin submitted custom data for VMT, vehicle population, and road type distribution. Both Minnesota and Wisconsin submitted data for 2017, 2014, and 2011 USEPA (2015).
  • The NEI augmented vehicle miles traveled (VMT) data for Minnesota and Wisconsin in 2020 using federal and state-level datasets due to data availability issues (USEPA, Godfrey, and Eyth 2022).
  • To reduce model run-time, the EPA groups counties together and only runs MOVES on a single representative county. The resulting MOVES emissions factors are multiplied by county-specific activity data (including VMT, vehicle population, hourly speed distribution, among others) to get county-specific emissions (USEPA 2023a, sec. 5.6.2.1). Effectively, emissions factors are generated on a single representative county, and are then applied to similar counties.
Nitrous oxide (N2O) availability

Though nitrous oxide N2O has a high global warming potential (Section A.2), the amount of N2O released is relatively small when compared to other sectors.

N2O is unavailable in EQUATES, except years 2018 and 2019.

2.2.2 State DOT data

As required by federal law, Minnesota and Wisconsin state departments of transportation (MnDOT and WisDOT) report various traffic measures for planning, forecasting, and various analysis endeavors.

Vehicle miles traveled

Vehicle miles traveled (VMT) is a standardized measure created by multiplying average annual daily traffic (AADT) by centerline miles. AADT is an estimate of the total vehicles on a road segment on any given day of the year in all directions of travel. VMT and AADT are common traffic measures and standardized across the United States.

MnDOT and WisDOT derive VMT using traffic counts from continuous and short term traffic monitoring sites. These raw counts are adjusted by multiplying seasonal, day-of-week, and axle adjustment factors WisDOT (2023). Data is not collected for every site every year, but the data are sufficient for year-over-year comparisons.

County vehicle miles traveled

We consider county-level data to be of the highest quality and most reliable measure of VMT.

These data were compiled from MnDOT and WisDOT county level reports. MnDOT provides Excel workbooks with VMT by county and route system on their website. These were downloaded, filtered to include the relevant counties, and aggregated to the county level by summing VMT by county/route system. Processing code can be found in mndot_vmt_county.R.

VMT data for 2015 were interpolated at the county and year level using the midpoint method.[^ MnDOT VMT for year 2015 is unavailable due to significant and fundamental changes in underlying data structure that make directly comparing data prior- and post-2015 inappropriate. However, our interpolation here is based on the county level summary of all VMT and use for comparison purposes only. We used the midpoint method, which is the average of the observation directly before and directly after the missing data point.]

WisDOT publishes PDF tables with county-level VMT. These were downloaded and data was extracted using {tabulapdf}, an R package interfacing with the Tabula PDF extractor library. Processing code can be found in wisdot_vmt_county.R.

Figure 2.1: County annual vehicle miles traveled
City vehicle miles traveled

City VMT is available only for a select number of cities, townships, unorganized areas (CTUs).

These data were compiled from MnDOT city and route system reports available on their website. Reports were downloaded and aggregated at the CTU level by summing VMT up for all route systems. Processing code can be found in mndot_vmt_ctu.R.

Due to limitations in data availability and consistency, not all CTUs in the 7-county metro region are included.

  • Shoreview, Blaine, and West Saint Paul are split among more than one county. For some CTU/county/year combinations, only data from 2016 onward were available. For consistency in the time series, we assigned 2016 VMT data to year 2015 for these CTU/county combinations.
  • Due to geographic data source differences, MnDOT reports a small amount of VMT invalid CTU/county combinations (i.e., Minneapolis, a Hennepin County CTU, centerline miles and VMT reported in Anoka County). We discussed these anomalies with MnDOT staff and determined this to be a non-issue. The county designations for each CTU were corrected such that summing to the CTU by the CTU name determines the total VMT for each CTU No changes to county designation were made to CTUs known to be split across multiple counties (Blaine, Chanhassen, Hastings, Saint Anthony, Shorewood, Spring Lake Park and White Bear Lake).
  • 2015 data were interpolated in the same manner as the county VMT data.
Figure 2.2: City annual vehicle miles traveled

Cities without a complete time series from 2010 to 2023 were modeled.

Gap-filled city vehicle miles traveled

Modeling took place in mndot_vmt_ctu_gap_fill_model.R.

All counties have the majority of their VMT accounted for when totaling up reported city level VMT.

For cites without reported annual MnDOT VMT, we estimated VMT from 2010 to 2022 using a model trained on population, households, employment, county, and Imagine 2050 designation.

Limitations

  • AADT/VMT data rely on modeling, and not every site will have new observed data every year.
  • AADT/VMT are generally estimated for high-use arterial roads and highways, leaving most local roads out.
  • We may want to consider using non-permanent counters and/or counters from just outside the study region to increase the total number of calibration roads.

2.2.3 Regional Travel Demand Model

VMT forecasts for counties and cities are generated from our regional travel demand model.

The current regional travel demand forecast model (TourCast) is an activity-based model, which means that it simulates transportation decisions made by individuals ranging from long-term (e.g. regular work/school location, whether to own an automobile), day-level (e.g, what activities to engage in, with whom, where, and when), and trip-level (what transportation mode to use, what route to take) in order to evaluate policy and investment choices at a high level of detail.

Model inputs include

  • 2010 Travel Behavior Inventory results
  • Observed/existing vehicle volumes
  • 2022 population, employment, and other demographic characteristics
  • Demographics from Council long-range forecasts
  • Road networks based on all projects programmed through year 2025. The projects generally include
    • any project that has a change in capacity (number of lanes) or major interchanges
    • any regionally significant project
    • long-range capital projects

The base-year model outputs best represent year 2023 and forecast out to year 2050.

Calculating VMT from the model network

The regional travel demand model network is made up of nodes and segments. We use the network segment-level information to calculate VMT.

The network based approach is based on attributing all the vehicle traffic that occurs within a given city or county to that city or county, regardless of where the trip starts or ends. VMT is calculated by multiplying segment vehicle volume (vehicles) by segment length (miles traveled). Network segments are attributed to cities by a spatial join. When a segment crosses more than one city boundary, the segment is split at the boundary. The segment total volume is attributed to both sub-segments, and the segment length is re-calculated for each sub-segment. Thus, no volume is lost. All time periods are road types are aggregated to represent average daily vehicle miles traveled. Daily VMT are expanded to annual VMT using an annualization factor of 340 (Castigliego et al. (2019)).

Code for processing RTDM outputs relies on internal file system access and is not available publicly. Please contact us for more information and reproducible examples.

Limitations

  • The regional travel demand model, by definition, is built to function at a regional level. Scaling down to smaller geographies is stretching the limits of what it can do.
  • The model outputs a base year estimate (2023) and future year estimate (2050). All intermediary years (2024-2049) are interpolated linearly between the two points.

2.3 Limitations

  • Geographic accounting methods
    • Geographic accounting methods do not account for the decisions or travel behavior of individuals within the geographic boundaries.
    • Within the Twin Cities region, this method will show high emissions per capita in low population areas with significant vehicle traffic, such as a small town with a major freeway. The residents of the small town are not responsible for the emissions of vehicles passing through their town without stopping, but they are subject to the air pollution associated with those trips. Additionally, the city governing body cannot expect to reduce emissions from freeway traffic, as the road is out of their jurisdiction.
    • We will take these limitations into account and plan to mitigate wherever possible for CTU-level inventories and forecasts.

2.4 Validation


  1. All six MOVES emissions processes, including rate per distance (RPD), rate per vehicle (RPV), rate per hour (RPH), rate per profile (RPP), rate per start (RPS), and rate per hour for off-network idling (RPHO) were summed for each vehicle type, fuel type, and pollutant (Beidler and Eyth 2024)↩︎