Vehicle miles traveled (VMT) reflects the total distance traveled by cars and trucks during a given period. Prior to 2000, VMT in the US grew proportionally with real GDP, shown by the blue and red lines, respectively, in the FRED graph above. But this relationship appears to have weakened after 2000.
VMT statistics from the US Federal Highway Administration begin in 1970, and we give that year an index value of 100. Between then and 2005, VMT-per-capita also grew (shown by the green line), rising from about 450 monthly miles per person in 1970 to a high of about 840 monthly miles per person during 2003-2006. Since then, VMT-per-capita has stayed relatively stable, apart from a sharp decline during the COVID-19 pandemic. Over the same period, total VMT growth has declined, falling below real GDP growth.
So, how have vehicle emissions changed during this evolution in VMT? The purple line shows the growth of carbon emissions from the transportation sector relative to levels in 1970—again, with a starting index value of 100. Together, cars and trucks are responsible for the vast majority of carbon emission from the transportation sector—81% in 2021 (EPA, 2023a). While the share of electric vehicles is growing, most of the fleet is still combustion: For a conventional vehicle, emissions increase proportionally with driving volume. As a result, the growth in VMT been a main driver of the growth in transportation emissions since 1970.
However, transportation sector emissions have grown much more slowly than VMT growth alone might suggest, at a rate just slightly below that of VMT-per-capita. This has occurred, in part, because vehicle efficiency has improved. With technological innovations, conventional cars produced today tend to require less fuel and emit less carbon dioxide than cars of comparable weight and power produced five decades ago.
Average emissions efficiency of new cars produced is one way to measure changes in fleet efficiency over time: The average carbon dioxide (CO2) emissions per mile for a new car was almost 700 grams in 1975, the first year data on average new car efficiency is available. By 2021, average CO2 emissions per mile for a new car had fallen by about half, to 347 grams per mile. Most of those efficiency gains came during 1975-1985, when average new car emissions dropped sharply to just over 400 grams per mile, catalyzed in part by the consumer response to high gas prices. Emissions per mile declines are primarily due to these technological improvements in fuel efficiency.
Over the same period, the car market’s shift toward SUVs and trucks—which use more fuel per mile on average than smaller vehicle types—acted as a countervailing force, limiting the extent to which vehicle emissions fell (EPA, 2022).
To the extent electric cars become more common, vehicle emissions and VMT will continue to uncouple; and further increases in VMT may be less closely associated with transportation sector emissions (EPA, 2022).
Recent emission standards set by the EPA—including those set in 2023—focus specifically on the levels of CO2 emitted from vehicles (EPA, 2023b). Because there’s a lag between when standards for new car emissions are imposed and when those newer cars begin to dominate the fleet of vehicles overall, these newer standards do not yet show up on the FRED graph. But they may eventually lead to reductions in transportation emissions in the future.
How this graph was created: In FRED, search for and select the “Vehicle Miles Traveled” series. Use the “Edit Graph” panel to change the frequency of the series from monthly to annual using averaging as the aggregation method. With the “Add Line” tab, search for and select “Real Gross Domestic Product.” Use the “Add Line” tab again to search for and select “Vehicle Miles Traveled,” change the frequency from monthly to annual using averaging as the aggregation method. Within the same tab, search for and add the series “Population” and apply formula a/b. Use the “Add Line” tab again to search for and select “Transportation Carbon Dioxide Emissions, All Fuels for the United States.” Under the units option, select “Index” with date 1970-01-01 and click “Copy to all.” Use the date selector above your graph to choose 1970-01-01 as the start date.
Suggested by Marie Hogan and Mike Owyang.
