I've raised a tone to patients before but never yelled. I yelled at a driver on an MVA today because as I was walking to the rig to get a backboard and collar, he was speeding towards me and didn't stop until I stood there with my hand out, only stopping (and may I add this was a slammed stop, not a gentle coast) about 5 feet from me. I signaled for him to roll down his window, walked to the drivers side and proceeded to yell: "WHEN I TELL YOU TO STOP YOU STOP. DO I MAKE MYSELF CLEAR?" He tried to speak to back his case but I just yelled "DO I MAKE MYSELF CLEAR" one more time and he quieted down. Sure scared the heck out of me how fast he was coming though.