I started riding alone on Australian bush trails at age 14. This is before mobile phones (and just after microwave ovens
...soon I can be a historical relic). My father rode with me before that but then my parents decided I was good to go alone, and I rode out a lot and got hours away from home. My parents knew the general direction / riding plan and the time I expected to be back. If necessary they would follow hoofprints in the sand, but that never eventuated because I never actually fell off on a trail riding alone, still haven't, although you get close sometimes when a kangaroo jumps out of thick bush unexpectedly. (I seemed to reserve falling off for rides on our own property. Usually the horse was surprised by the loss of rider and stopped long enough for me to dive for the reins and get back on, but I did walk back to the homestead a few times.) Most of my rides were and are at a good clip. I found horses actually startle less when you keep them trotting or cantering along. In summer I sometimes did trails bareback.
These days I ride trails accompanied by my dog Jess, who is a Kelpie and runs like a rocket for long distances (Kelpies working sheep often do 30-60km of running in a day's work). The dog notices any wildlife well before us and the act of her noticing stops the horse from startling compared to riding without the dog - also the dog is excited by wildlife rather than scared and so the horse seems to pick up on that and use it as a "don't worry" cue. The dog is a speed freak and if we fall back to a walk she often tries to egg the horse on to a faster pace by gambolling around his front end and yapping excitedly. The dog and horse enjoy racing each other at least once in a ride.
I ride in a Spanish Snaffle, which is actually a mild curb bit with a port mouth, because it makes a spooking episode so much simpler to manage than a snaffle. You can encourage the horse to tuck his face down instead of poking it up and this means you can nip any idea of running off in the bud quite easily and gently. This really reduces injury risks to rider and horse.
Also I carry a multi-tool on the saddle in case my horse gets tangled in stray wire. It hasn't happened but it's good to have a plan in case.
When I ride alone I don't cross roads with regular traffic so if my horse does ever go home without me I don't have to worry about it getting hit by a vehicle.