The CO2 emissions are easily estimated and not even close. A container ship can burn 225 tons of bunker fuel per day at normal cruising speed.[1] At a generous 300 days in transit per year (they spend time in port loading / unloading / waiting), that's 67,500 tons of fuel per year. It's a massive number, but a drop in the bucket relative to the 913 million tons of oil consumed by Americans every year, the majority of which is spent on motor vehicle fuels.[2]
Even assigning just 50% of US oil consumption to cars, you're comparing about 450 million tons consumed by American drivers versus about 1 million tons for 15 large cargo ships.
[1] https://transportgeography.org/?page_id=5955 [2] https://www.statista.com/statistics/264825/oil-consumption-i...