Many of us who have livestock have been talking about Maggie the cattle dog (see http://www.sacbee.com/2011/07/06/3752292/injured-cattle-dog-maggie-goes.html#storylink=misearch for the story). Most of us agree that we don't know if we'd be able to show as much restraint as Maggie's owner, John Reader. Personally, I think it would probably be a good thing that I don't carry a firearm - I'm not sure what I'd do if somebody tried to run over my livestock, my friends and family and my dog.
I was relating this story to a fellow stockman from Utah last weekend. He told me that he had a friend who rode a huge (nearly 17 hands) black horse when he drove cattle to the mountains. If a motorist showed any inclination towards driving through the herd, his friend would stop his horse in the middle of the road and point at the offending driver. Invariably, the driver stopped and waited.
Recently, I've been riding my mule more often. She's almost the same size - nearly 17 hands. I'm probably not an imposing person to most, but when you're sitting 5 feet 8 inches off the ground, even the shortest person is somewhat intimidating! My goal is to get Frisbee (my mule) comfortable enough with cars to be able to stop traffic when we're moving sheep!