Either mount it on a piece of 6" iron pipe that is mounted in, and filled with, concrete or if you want it to be mounted on wood, get an 8' 6x6, bury it 4' in the ground, on a 6" concrete pad, surrounded on all sides by 4" of concrete. Either one of these, when hit by a car, will stop the car.
Or even an entire state plow truck. The 6" concrete-filled iron pipe anyway. Don't ask me how I know that.