Boy is this one a tough one!! I had an Arab mare that was a runaway horse and often bolted until I found the perfect combination for her likes in a bit. Luckily we worked through that without either of us getting hurt.
And I have a "dead broke" mini gelding that once ran away with me in the cart due to a foreign object getting into his eye! But thanks to my Arab mare, I understood this gelding better and was able to get him under control. Even the best trained horses can bolt if they get startled, and one should always realize that this is a possibility with ANY horse at any time!
What did I do? Well, first, I remained calm. This is the biggest issue that most people forget to do. Hard to do yes, and maybe almost impossible, but try your best as that may make the biggest difference in the outcome! Keep in mind that most people who panic tend to curl up in a ball, "throw the reigns away" along with forgetting many of the basics that help you like counter balancing, voice cues, and even whip cues, ect. During a runaway situation is when you need to remember the basics the most.
Next, remember that once a horse bolts like this, they are NOT listening to the bit in any way! They most likely have clamped their teeth onto that bit and removed pressure from the mouth. So pulling steady on the reigns will not help you.
I also don't recommend anyone trying to run a horse into an object. Most horses will run straight at that object and then at the last second before they hit the object they will suddenly turn very sharply. If in a cart, you will tip over when that horse makes that effort to avoid that object.
With any runaway horse, there is an order of doing things to get your horse back into control. First and most important is to NOT try to stop the horse. Yep, that is right! Let your horse run if it has already bolted! Your first goal is to try to gain left and right control. In other words, get your steering back into control. If you can gain control of going left and right, that will help you avoid obsticles that can cause you to crash. Once you can get that control, you can then work on circling your horse until it stops on it's own. What you want to do is put your horse into a very large circle and then spiral down the size as much as you can, but keeping the circle large enough that you won't tip the cart. (Of course that is if you have the room to do so!) Now if you find your circle is as small as you dare go with the cart, then spiral that circle up larger and then back down smaller. This change in size causes the horse to focus more on you than on running.
Again, your goal is to get control over steering. Your horse will tire it's self out and will stop eventually on it's own, so see if you can guide that horse safely until it tires out. Keeping your horse and yourself from getting injured is your ultimate goal, and that starts with controling where your horse goes.
Now, some other things that people should know that will help and that you want to do long before your horse bolts. First, with minis, many people don't teach them to canter in the cart as the shows don't require this gate. But, all minis should know how this feels with their carts!!! Your mini should be able to canter in the cart and not be upset by the feel of the cart bouncing to a different gate rythum. If you teach this in the arena during training, then if your horse should bolt, it won't be spooked by the difference in the way your cart bounces on the harness. This is definately a plus as you don't need to add that stress to your horse when it isn't thinking clearly.
Next, many people don't do enough circles with their horses during normal training! There are many benefits to doing spiral exersizes, but one of the biggest benefits is that by doing circles and spirals, you build a mental "comfort zone" for the horse to work in when it gets stressed out. This comfort zone starts building as soon as you begin to teach your horse to lunge and should be routinly used through out training. Any time your horse acts up, or starts to get spooky when you are driving or riding, go right back to that circle until they calm down and think things through. Build that comfort zone where the horse understands that circling is a very safe activity and an activity that is so understood that it is like second nature to them. So do lots and lots of circular work as a foundation for everything else so that when your horse has an issue, you have a way of trying to get your horse back into it's mental comfort zone.
And finally, a general safety tip that many of us forget about over time... NEVER EVER ride alone! This is a rule that is taught to ALL kids in 4-h and pony club, or at least it was when I was the age to participate in those clubs! This also should go for carts, especially if you are driving down the road! Make sure you have another human being go with you that can help if you get hurt. And if you should have to go by yourself, at least carry a cell phone! Did you know that most cell phones will be able to call 911 even if you don't have a cell phone policy??? That is why the police ask for old phones to be donated to them so that they can give them out to people who are in fear of their lives.
I hope the lessons that I learned dealing with a runaway horse will help others. They are hard lessons to learn as one is always panicked when it happens, but creating a mental game plan before it happens will definately aid in getting one through the situation.
And I certainly hope they find those kids that did that and they are sued for causing the accident! I once had a similar thing happen when I was riding my Arab mare where some fellow highschool kids pulled up along side of us and then gunned their car to make my horse spook. She took off, and due to having a well thought out plan in my head before it happened, I was able to circle her in my neighbor's yard (my aunts place) and get her back into control until I got her stopped. Boy was I hot and those kids certainly got an earful as on rural roads here in Idaho, livestock has the right away and they could have been in big trouble if they were turned in to the law enforcement... To make matters worse, they had horses of their own! But I had witnesses and well, my boyfreind (now my hubby) certainly let them know that they better not ever threaten my wellfare like that again.
Laughs.
Thoughts and prayers going to your friend for a quick recovery!