“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.”
Most people I have found care relatively little about the topic. They drive and think in terms of driving only because that is the context they have been exposed to their entire lives, and there’s not really their fault.
If someone really is that deep into the rabbit hole then nothing you say will change their mind, so don’t waste your time.