I remember reading of a couple incidents, years ago in the LA Times, where drivers went off a road and were trapped for days. I recall them saying they could see sections of the road in the distance, and helicopters and aircraft flying overhead, but they had no way to signal them. A mirror would have helped unless the angle of the sun precluded them from getting a good reflection. These incidents happened years before cell phones were in common use. <br><br>If I recall correctly, a horn wouldn't have been much help given the din of traffic, especially since no one was searching for these drivers. A mirror is one option, but one of the best if sunlight is accessable to the trapped occupant. A horn, though less useful in the particular situation in the TImes stories, might be useful in other scenarios. <br><br>I always keep road flares in my car, which might be useful in some scenarios, if there's no risk of me starting a fire that could trap and kill me. <br>