Reducing car size and mileage would be a good start - I don‘t have numbers at hand for the US, but in Germany, the individual mobility via cars produces about 20% of the entire CO2. Even a speed limit would already cut that by a few percent. Immediately, as in „the day after the law is in force“.

In 2021, cars (trucks not included) used 60 billion tons of fuel in Germany alone and rising https://www.umweltbundesamt.de/sites/default/files/medien/38... Shutting down the BASF steamcrackers would be equivalent to a reduction of that fuel consumption by 0.5% or so. So, a speed limit would be more effective and faster to implement.

But that’s a holy cow no one touches, neither here, nor in the US.