(Note from Willis in regard to leading horses: I can't tell you how many people have ended up in the hospital doing just that. The horse doesn't consider that the human can't see laterally as he can and won't notice him coming if he startles. The average horseperson isn't going to understand what's going on in this blind spot and probably won't "scritch" at the appropriate moment. Especially with mustangs, a reaction to something can be instantaneous and when you can't see the horse, you will not notice him telegraphing his anxiety.
In the safety program we teach leading the horse within the handler's peripheral vision. This usually amounts to the horse's head being roughly even with the handler's shoulder on a slightly loose lead so he can move his head. If he swings away, the handler can check his head. If he swings in, the handler can simply raise his/her hand by the horse's eye and wave him off. If the horse bolts forward, the handler can simply give the horse a tug on the nose to shift his balance and swing his rear end out, avoiding a stepped on foot or a pop from a kick.
Gates and bottlenecks are particularly dangerous, accounting for many hospital visits as the horse has nowhere to go if spooked forward but over the handler. At narrow gates, the handler can stop the horse, step through and around the gate post, then call the horse through the gate. To cross arroyos or other critically narrow passes, the handler can stop the horse, attach a long lead or lariat, then draw the horse through or across from a safe position.)