This great video made by my buddies at Friends of Capitol Forest (staring Olympia ultra runner Kris Ryding and the Race Director for the Capitol Peak Ultras, John Pearch) details how to handle runner/horse, runner/mountain bike, and mountain bike/horse interactions on the trails and at trailheads. This is film describes and walks through the key elements of effective communication and personal actions that result in safe and enjoyable mulit-use trail experiences.
Viewing this video will be mandatory for all participants in my races where the trails and or trailheads used for the race are shared with horse and mountain bike riders--so you might as well watch it now and make sure you're totally prepared to handle these possible scenarios the next time whether it's at a race or just a training run. enjoy!