“Giving people more viable alternatives to driving means more people will choose not to drive, so there will be fewer cars on the road, reducing traffic for drivers.”
Concise, easy to understand, and accurate. I have used it at least a dozen times and it is remarkable how well it works.
Also—
“A bus is about twice as long as a car so it only needs to have four to six passengers on board to be more efficient than two cars.”
But that’s fine, no need to convince them to use mass transit.
The approach is to improve infrastructure - good buses, frequent routes, dedicated bus lanes, trains to feed from the suburbs, subway, etc.
Make it more convenient to use, and people will start using it. But you need to stop designing everything around cars, like every single store can’t be a cube in the middle of a huge parking lot…