Settlements tend to be near water like rivers, lakes or oceans and the larger the body of water, the more likely there are people. A small stream is likely to join another at some point. This is why going downstream so most(or merely going down if there is no stream) is the safest bet if there are no other clues. Even if you're not actually on a mountain, it's more likely that streams of water will join each other than that they will split. So you should follow the trail in thatthe direction of the stream going down as much as possible. Some situational awareness will probably come into play as well, it’s unlikely you will have absolutely no idea of the surrounding settlements. If you know you’re near an ocean where this stream might end up, or near a river that this stream might join, that would make it even easier. If there
As an added note, of course this is no stream like the one described in the quotenot exact science. This is about making a choice in the question, I would still head downhill if there were no further cluesa situation where you have two options and one of them is more likely to get you safe.