“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.”
That is a good way to do it, but requires the bus lanes to be properly arranged in the first place.
It’s useless to give priority to a vehicle that is packed behind other vehicles.