No no no, not the percentage of cars that sometimes carry a load, the percentage of cars on the road that are currently carrying such a load.

If you do that once a week, then you can use transit the other 90% of the time. If people use transit 90% of the time, then we can build smaller roads and de-prioritize cars. That's the argument here, that transit can dominate in co-existence with self-driving cars, not that we'd need to get rid of cars. And especially in the context of waymo there's no effect of "I'm already paying a ton of money to own and insure a car, I might as well use it every trip".

(And again, this is in moderately dense areas where transit works and you actually care about how many cars are on the road to begin with. And it doesn't have to be 90% in particular.)