According to [1], the US had net Greenhouse Gas emissions of 6558.3 MMT CO2 Eq, of which transportation CO2 energy emissions took 1817.2 MMT CO2 Eq, of which 58% was due to light-duty vehicles. That means 16% of total emissions are due to light-duty vehicles, not 12% as you say, and only 1% off the 17% listed above.
So then I pose the question again, which set of emissions would you cut? The only other categories that pollute more is industry or electric generation. Which would you cut and why would you rather cut that than the emissions from cars?
[1]: https://www.epa.gov/sites/default/files/2021-04/documents/us..., Section 2-3 Trends, Pg 94.