This resolves based on my own experience of how many cars I see driving around in whatever U.S. city I am living in (or have most recently lived in, on the off chance I'm not living in the U.S.).

I will go outside to the street and wait for a few cars to pass. I will try to look in the window or look at the model of the car to try to figure out if the car is driving itself. Once I feel confident about whether more or less than 20% of cars I see are self-driving, I will resolve this either YES or NO.

Nov 2, 5:45am: Will 20% of cars be self-driving by 2030? → Will at least 20% of cars be self-driving by 2030?

Nov 2, 2:58pm: Will at least 20% of cars be self-driving by 2030? → Will at least 20% of cars I see be self-driving by 2030?

In response to a question "Self-driving" means an SAE level 3,4, or 5. The self-driving system has to be engaged though - a car that is capable of SAE level 3 but which is actively being driven by a human won't count.