“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.”
You can use the same logic to also argue that finding a parking spot will be easier. And if more people cycle there is more demand for separated bicycle lanes, which means drivers don’t need to share the lane anymore with others.
Also, if the car park is smaller due to fewer people driving, it means it will be easier to remember where you parked your car, and you won’t have to walk as far to the destination. It also means you can fit more stuff in the same space, so you won’t have to drive as far to get to the places you want to go, saving you time and fuel!
These arguments are won and lost on the phrasing.