Do you have a source on the average annual external cost for cars? There are a quarter million cars in the US, so billions could be anywhere from $4/car/year to $4,000/car/year (unlikely as that would be a third of all state/federal/local income tax.)

Carbon offsets cost about $100/year or less.

Auto crashes supposedly cost a trillion a year in direct costs and lost time, but those costs are mostly absorbed by insurance, employers and individuals, so don't factor into the $2k/year figure.

Interested to see corroborating or contradicting data.