OK so I have solid running gelding that has one issue that I just can't figure out.  Leaving the 3rd barrel and heading out he finds a great gear however he wont stop!!!!!!!!!!!! Actually have had him carry me across a parking lot or 2.  He has regular teeth work, been using a 3 piece combo bit with short to long shank for leverage. Works great at home and slow work will sit down when asked just not running out.  There is no medical issues the vets have found to explain.  Been working with this guy for over 5 years now.  Getting to point of being dangerous.  Dont wont to give up on him yet!  Thnks

I had one just like him, turned out he just loved to run. Would smoke a set and clear the parking lot. Only medical issue I can think of is some horses will run off when they bleed in theor lungs. Maybe worth a look? 

My off the track gelding did this. It always puzzled me because he was very soft mouthed and responsive with a great stop, but when that racehorse mentality kicked in on the way home he would take me clear across the parking lot. I was talking to a good friend about it and he replied "Have you see-sawed his a** yet?"

      It's not pretty.  But it took once, or maybe twice and I haven't had a problem since. Firm pressure back and forth on the reins. Every now and then he needs a gentle reminder,  but it helped tremendously. 

I echo the other suggestions. For medical, (I've never had one) but I know some will "run off" if they are a bleeder because it makes them feel like they are choking usually by the time they are leaving the 3rd barrel.

Also, have you tried putting something real strong in his mouth bit-wise? Just to see if he would listen to it? I like to use "softer" bits myself but I'm also not afraid to bit one up if I need to get a point across.

I have had him scoped (Twice) once the next day after a run off and another time just to recheck and nothing. No evidence of bleeding, tears, broken blood vessels etc.  Have used lasiks on  hime a couple times because he also tends to hold peeing while at a race then will go when gets home and no change.

I have bitted him up before (rutledge roper) and works for that ride and next like you did nothing.  Not good with running him with something that strong.

What happens if theres a closed gate? If it were me, I'd try to find a race to exhibition that had a closed gate and do that for a little while.  I'd probably put a setup with a lot more Whoa to it on him. Gotta have those breaks working at all times! Of course after ruling out any pain issues.... 

If you're sure he has no medical issues and is broke properly and has been taught to stop.  I would put a easy stop, or quick stop on him and teach him to stop after his run.   Now that beind said, if  you've never ridden in one you have to be careful and it needs to fit him  properly.  Stop is a big deal to me.  I expect mine to stop during a run, if asked to do so. 

So I had one start doing this as well. She did it once and I kept anticipating it over and over. I have to mentally prepare to ASK her to stop. Relax my legs and seat, sit down on my pockets and say WHOA out loud. I guess I was subconsciously riding her through it by anticipating her not stopping. She slows and stops easily now. 

ETA: she stops like a dream in slow work and that was never a problem. Only after a run were we having this issue. 

What about actually practicing running and stopping/slowing at home? You wouldn't even necessarily have to do it with the barrels present but you could ask him to scoot a bit around the perimeter of the arena, and then ask him to come back to you. Rinse and repeat.

IMO and experience a Rutledge Roper LOOKS a lot harder than it actually is. I actually used to it help stiffen up a mare that was noodle-bendy but sensitive in the mouth. 

I would try a different bit.

If you are absolutly certain there is no medical reason- I'd be thinking about how you ask him to stop.

My stepmother had a horse start running off with her coming out of the pen because she was so abrupt about how she asked him to stop. She gave him zero indication it was time to stop unitl she hauled on the reins. He started running off out of self defense because he knew there was going to be sudden pain. I started riding him and at first eased him out of the pen not running full speed. I'd set up, say woah and then pick up the reins. I wouldn't give a direct solid pull, but instead kept the bit alive in his mouth (and thus gave him nothing to pull against) with pressure on the right, left, right,  left reins. Only getting rough if I ABSOLUTLY had to.  When he figured out the jerk his head off wasn't comming, he started stopping with no problem.

If I get one in one that sure enough thinks it can run off with me- I'll put something on it that it can't run off with. I'm of the mindset to back one off and work on the problem instead of continuing to run it. There is no need to continue to teach one that they can do it. I'll give up as many barrel races as I need to tune and fix the problem. I've also rigged one up before where I had two bridles on them. A bit for running the barrel race and a bit /hack/quick stop  that would stop them. But I always try to go back to basics and figure out the root of the problem.

I guess my question would be how you are asking for the stop.  I all the time see horses running off with the jockey hauling back on their mouth with steady pressure, which just gives them something to brace against and keep on running.  I would start with sitting down, saying whoa and repeatedly bumping to get his attention. Successive bumps becoming harder.   If that was unsuccessful than move to alternate rein pressure.

Teaching your horse to stop is part of the process of getting them REALLY broke. I am not talking about a sliding stop. My horses' favorite word is whoa. 

Mine too. Both my horses respond to my voice.. I use to not be able to say anything, even clearing my throat theyd plant lol! That was a bit too much... now I've got them to were I can say "easy" that means slow down or rate or settle down, when i say whoa, its butt in the ground and they usually take a step or 2 back ... but I'll admit I haven't ran my horse on the pattern we are just high loping at the moment so I'm really hoping alllll my slow work and slow patterning sticks with him when I finally ask for speed.