Good deal. I take it you have a pretty small window to cover if a single stone can do the job, that or it is at a pretty steep angle. Did you make the slight incline yourself, or was that already a natural feature of the terrain?
What the heck does a 1.25ton boulder cost to have delivered and 'installed'?
Plant some trees around it, in 5-8 years the trees will be big enough also to stop or sufficiently slow down a car.
Not sure how people could have a moral objection to a tree, but then again, you are probably dealing with a drunk who would think you put that tree there just to damage his car! I personally have no sympathy for any drunk who hits it. 9 houses and 3 have been hit by a drunk or distracted person, you are very well to be concerned and applauded to be proactive to protect your family. Thankfully though it sounds like no one was hurt on your street yet from the wrecklessness (pun/spelling typo intended!) of others.