14  Reduce transportation-related air emissions

14.1 Ambient air quality

In 1970, the Clean Air Act authorized the federal Environmental Protection Agency (EPA) to establish standards for six pollutants known to cause harm to human health and the environment; these were given the name criteria pollutants. The Clean Air Act requires each state to monitor these pollutants, then to report the findings to the EPA. In Minnesota, the Minnesota Pollution Control Agency (MPCA) is responsible for these actions (MPCA 2022).

The criteria pollutants are

  • particulate matter (currently PM2.5 and PM10)
  • lead (Pb)
  • ozone (O3)
  • nitrogen dioxide (NO2)
  • sulfur dioxide (SO2)
  • carbon monoxide (CO)

For each of these pollutants, the EPA has developed National Ambient Air Quality Monitoring Standards (NAAQS). Primary standards are set to protect public health, while secondary standards are set to protect the environment and public welfare (i.e. visibility, crops, animals, vegetation, and buildings).

The region is currently in attainment for all the pollutants regulated by the EPA. PM10 was the most recent pollutant that was unacceptably high in a small portion of Ramsey County, mostly due to non-transportation related sources. Until 2022, the region was still in a 20-year maintenance period due to these past high PM10 levels. This PM10 maintenance period expired in September 2022, bringing the region in full NAAQS conformity. 1

Figure 14.1 shows the maximum pollutant level for each year as a percentage of the National Ambient Air Quality Standards from all sources (not just transportation). Except for PM10, the maximum level of pollutants shown in the chart have generally trended downward over the past 20 years. Many factors could contribute to these decreases in maximum pollutant level, including changes in energy production, industrial practices, building trends, land uses patterns, and transportation behavior. The weather also plays a big role in ambient air quality. Examples of transportation-related changes that might decrease pollutant levels include decreased vehicle travel, changes in vehicle emissions technology and increased consumer adoption of these newer vehicles, and growing use of alternative fuel sources.

To learn more about NAAQS, see Section C.2.

Learn more about air quality monitoring on the Minnesota Pollution Control Agency (MPCA) air quality monitoring webpage.

Figure 14.1: Air pollution in Twin Cities (maximum value as percent of air quality standard)

14.1.1 Air Quality Index (AQI)

The Air Quality Index (AQI) is an EPA measurement system that converts pollutant measurements into categories that describe their possible effects on human health. These categories span categories from good to unhealthy or very unhealthy. The MPCA uses daily forecasts of AQI for small particulate matter (PM2.5) and ozone to inform residents about potential health risks from air quality conditions. The Minnesota Pollution Control Agency (MPCA) issues an air quality alert when the pollutant with the highest AQI approaches or exceeds 101.

Figure 14.2 shows the number of days in each year that ozone or PM2.5 levels exceeded an AQI of 100. While the figure includes some fluctuation, the general trend shows a decrease in annual days with an AQI over 100 between 2000 and 2020. The MPCA does note that many changes in the AQI are not necessarily due to short-term changes in emissions from things such as transportation. Much of the fluctuations in AQI come from meteorological conditions such as temperatures, winds, precipitation, and air pressure.

Figure 14.2: Number of days exceeding air quality standards, by PM2.5 and Ozone

14.2 Regional Vehicle Miles Traveled (VMT)

Vehicle miles traveled (VMT) per person remained stable at approximately 25.5 VMT per person per day from 2010 through 2019. In 2020 with the onset of the COVID-19 pandemic, this fell to approximately 20 VMT per person per day. However, in 2021 it was already beginning to rebound toward the previous long running rate. As population has increased in our region from 2010 through 2019, daily regional VMT has continued to increase from approximately 73 million in 2010 to 81 million in 2019. Like VMT per person per day, total regional VMT fell in 2020 and has partially rebounded in 2021.

Regional VMT is important to the transportation system as it is an indicator of transportation’s contribution to greenhouse gas emissions and negative public health impacts from burning fossil fuels. As VMT increases, congestion becomes more prominent with its own direct impacts and those of the highway improvements that often result. It also indicates how well our transportation system provides options to driving alone that can reduce household transportation costs and improve public health and the climate. Options other than driving alone can be especially important to low-income populations and those who don’t have access to a private vehicle.

The regional population 2 has steadily grown since 2010, from 2,938,394 to 3,292,036 in 2021 (a total increase of 12%).

Figure 14.3: Population growth of the metropolitan planning area from 2010 to 2021

From 2010 to 2019, VMT increased from 73.1 to 80.9 million miles per day. VMT fell in 2020, but rebounded somewhat in 2021.

Figure 14.4: Average daily vehicle miles traveled in the metropolitan planning area from 2010 to 2021. 2010, 2011, and 2012 VMT data include only the 7-county metro.

From 2010-2019, per-capita VMT varied in a narrow range from 22.3 to 25.5 miles per person, per day. In 2020, per-capita VMT plunged to 20.3 miles per person per day, a 18.8% decrease. It rebounded in 2021 to 22.3 miles per day per person.

Figure 14.5: Average daily vehicle miles traveled per person in MPO area. 2010, 2011, and 2012 VMT data include only the 7-county metro.

Increases in population and VMT kept pace with each other from 2010 to 2019, with total population growing by 10% from 2010-2019, and VMT growing by 8% 3.

These trends diverged in 2020, with a substantial decrease in VMT.

In 2021, VMT was just 1% lower than 2010 levels: the pandemic had essentially re-set VMT to 2010 levels. Future years of data will be needed to see if the reduction is permanent.

Figure 14.6: Change in population and average daily vehicle miles traveled per person since 2010. 2010, 2011, and 2012 VMT data include only the 7-county metro.

In comparison,

Table 14.1: Freeway and arterial street vehicle miles traveled per person in peer regions, arranged by 10-year average
Peer Region 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 10-year Average
St. Louis 22.7 22.8 22.1 22.6 22.6 22.5 22.9 23.4 23.2 23.2 19.5 22.5
Dallas 20.3 20.3 20.2 19.8 20.3 20.9 21.5 21.2 20.8 20.7 17.7 20.3
Cincinnati 19.8 20.1 19.6 20.5 20.3 20.1 21.1 20.1 19.8 19.8 16.9 19.8
Minneapolis-St. Paul 19.7 19.7 18.9 19.7 19.9 19.8 20.3 20.7 20.6 20.5 16.3 19.6
Milwaukee 18.9 19.0 18.3 17.9 17.8 18.5 19.9 20.3 20.4 20.4 16.4 18.9
Baltimore 17.9 17.9 17.8 18.5 18.3 18.6 19.0 19.2 19.2 19.4 15.1 18.3
Denver 18.2 18.1 16.8 16.3 17.7 18.2 18.5 18.8 18.9 19.0 16.2 17.9
Cleveland 17.4 17.3 16.4 17.4 17.4 17.4 17.9 18.3 18.4 18.6 15.3 17.4
Seattle 18.6 19.0 17.9 17.1 16.7 17.0 17.2 17.1 17.1 16.8 13.1 17.1
Pittsburgh 15.7 15.6 13.8 13.5 14.0 14.7 14.9 14.8 15.8 16.4 13.2 14.8
Portland 15.1 15.1 14.1 13.8 14.2 14.4 14.4 14.4 14.5 14.6 11.7 14.2
Source: TTI Urban Mobility Report 2021

14.3 Electric vehicle registrations

Plug-in Hybrid Electric Vehicle (PHEV) registrations have grown 48% from 2018 to 2022 while Battery Electric Vehicles (BEV) have grown 241% in the same timeframe from a similar starting point (MnDOT 2022). PHEVs are those with a traditional internal combustion engine and fuel tank, and an electric motor and a small battery that can provide 30-50 miles of range. This allows the initial miles after being charged to use electricity while the internal combustion engine and fuel tank allow for greater range. BEVs have an electric motor and battery to provide all the vehicle’s propulsion. As the available range on new BEVs continues to improve, they are rapidly becoming a more common choice in new vehicle purchases. In 2022, battery electric vehicles made up 5.8% of new car sales in the U.S. (Tucker 2023).

Both types of vehicles often result in more greenhouses when they are manufactured. However, due additional efficiency in their energy use and the common sources of that electricity, they result in fewer greenhouse gases and criteria pollutants over the life of their use. The benefits to climate change and public health of electric vehicles will only increase as our electricity generation continues to use fewer fossil fuels and more renewable sources.

Figure 14.7: Number of EV registrations, separated by BEVs and PHEVs

14.4 Fuel consumption

Fuel consumption, as shown in data from the Minnesota Department of Revenue, was flat from 2018 to 2019 but fell in 2020 with the beginning of the COVID-19 pandemic and partially recovered in 2021. Fuel consumption follows the vehicle miles traveled pattern in the short term, but in the long term is reduced by improved fuel efficiency and the increasing rate of electric vehicle adoption. Like VMT, fuel consumption is an indicator of transportation’s contribution to climate change and public health. It is also important for transportation finance as the gas tax is an important revenue source for state and local agencies to be able to maintain and improve the transportation system.

Figure 14.8: Gallons of gasoline consumed per year, not including aviation, from 1965 to 2022. Source: Minnesota Department of Revenue

14.5 Greenhouse Gas Emissions (GHG) analysis

According to the Met Council greenhouse gas inventory, the transportation sector is the regions’s largest emitter of greenhouse gases (GHGs). Many residents in the metro region use some types of transportation that do not release GHGs, including walking and bicycling. However, other modes of transportation like cars, motorcycles, trains, buses, and airplanes do emit GHGs. The shipping industry also contributes to GHG emissions in the region as goods are transported within and pass through the metropolitan area on trains and trucks.

Figure 14.9: Regional greenhouse gas emissions by sector, 2018.

Figure 14.10: Regional transportation greenhouse gas emissions as a percentage all greenhouse gas emissions, 2018.

Learn more about how the transportation greenhouse gas inventory was calculated on the project methodology page.

  1. Air quality data based on nine metro ambient air quality monitoring sites: Apple Valley, Blaine, Inner Grove Heights, Lakeville, Minneapolis, Rosemount, Shakopee, St. Paul, and St. Paul Park. The maximum value is the maximum value as observed across all monitoring sites. The standard value is the lowest of the EPA-established primary and secondary standards. Averaging time and lowest standard can be found in Section C.2↩︎

  2. The MPO area includes the 7-county core Twin Cities metro and the urbanized portions of Sherburne and Wright counties. Some cities have only a portion of the city area in the MPO boundary. Due to data availability, the entirety of each city is included in the estimate.

    The 7-county metro population is compiled from the 2010 and 2020 decennial census counts and Met Council intercensal population estimates for 2011-2019, calibrated to 2020 census counts, and Met Council 2021 population estimates.

    The urbanized areas of Sherburne and Wright counties are compiled from the 2010 and 2020 census counts and intermediary American Community Survey (ACS) 5-year estimates.↩︎

  3. VMT estimates for 2015 from MnDOT are unavailable.↩︎