I couldn't find some ready numbers, so I tried some estimations.
26% of all energy goes into transportation[1]. Automobiles are about 60% from that, so about 15% of total energy is spent moving automobiles[2].
1 https://www.eia.gov/totalenergy/data/browser/index.php?tbl=T...
2 https://www.europarl.europa.eu/news/en/headlines/society/201...
This does not include land use and agriculture emissions.
Looking at it another way, US has close to 300 million vehicles, and the average automobile emits 4.8 tonnes [3] of CO2 a year. It comes down to 1.5 gigatons of CO2 each year. Total US emissions are about 5 gigatons. This would make it a 30% part of CO2 emissions. It's higher than the number above because that was based on European values of 60% of all transportation emissions coming from automobiles. The US automobiles are bigger and consume more.
3. https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-t...
I think it's safe to say it's somewhere in the ballpark of 15-30% of all CO2 emissions are from automobiles.
I don't consider the manufacturing of the automobiles, because people will need moving regardless. It's either a battery powered car, plane, train etc. Those all need manufacturing and maintaining.
To max 0.5% sulfur in fuel from max 3.5% in maritime fuel:
https://www.imo.org/en/MediaCentre/HotTopics/Pages/Sulphur-2...
The time during pandemic lockdown where traffic levels plummeted and how quickly the air cleared pretty much proves that personal auto traffic is huge contributor to bad air quality.
The thing that people noticed during the 'rona was almost exclusively the latter... especially the noise and particulates. I could hear birds outside when the windows were open, the street-facing windows accumulated a lot less dust.
This is just a sad way to look at things. Changes have to start somewhere. They can start small, and then grow larger. People like you saying "too small, not effective" just need to sit down and be quiet, thank you very much.
The focus on "individual action" is a talking point that BP invented in the 70s (see https://mashable.com/feature/carbon-footprint-pr-campaign-sh...). I won't stop anyone from reducing their footprint, but we have tried this shit for decades now and it clearly hasn't worked a single bit.
We need to hold the big emitters of greenhouse gases accountable, they haven't been for about half a century. Remember the "ozone hole" and the CFC ban? That one worked, because the companies bringing CFCs into circulation were tackled instead of pushing the blame to consumers.