I didn't think this was true, but according to the EPA in 2021 it is
~~Personal transport~~ road vehicles are the largest category of transportation which is the largest source of greenhouse emissions, accounting for roughly 15% of total us emissions.
The largest sources of transportation-related greenhouse gas emissions include passenger cars, medium- and heavy-duty trucks, and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans. These sources account for over half of the emissions from the transportation sector. The remaining greenhouse gas emissions from the transportation sector come from other modes of transportation, including commercial aircraft, ships, boats, and trains, as well as pipelines and lubricants.
https://www.epa.gov/ghgemissions/sources-greenhouse-gas-emissions#transportation
There is nuance!
Heavy duty trucks is broad, and includes semi trucks, buses, tractors, etc
I'd be curious to see those commercial vehicles broken out, and the question answered for how much impact personal transport has on us emissions
https://fleetnetamerica.com/blog/post/classifying-medium-and-heavy-duty-trucks