It appears to me that the time has come for the Forest Service to take the initiative to specifically designate trails for horseback riders and bikers. If not, a fat lawsuit against them might forthcoming.
In my opinion, as a retired law enforcement official and horseback rider, who has dealt with numerous bicycle groups in the past, most mountain bikers ride the trails for one thing and that is to have a "fun time." There is always the minority that will not conform to common sense and ride with due caution. Unfortunately, it appears these few have caused a lot of pain and suffering to many horseback riders.
When encountering a biker that is going as fast as he/she can down a mountain trail, the horseback rider, riding a 1,000-pound animal, usually has limited escape routes to safely pursue. On the other hand, the biker usually has many due to his/her small footprint.
The biker has the responsibility to ride with due caution and at a reasonable speed dependant on the conditions as to not place anyone in jeopardy, including horseback riders. He or she has the mental capacity to do so. Not with horses. They are constantly looking around for that lion that will eat them and will do anything to escape the situation, including bolting away because of being startled by a biker racing down the trail for a "fun ride."
Therefore, it is imperative that horseback riders and bikers be separated on our trails before a horseback rider is killed.