There is a distinction between sending a distress signal and making noise. The problem with shouting is that it frequently is not perceived as a distress signal. Whistles are probably superior in this regard. The book solution is the rule of threes - three shouts, blasts, or whatever, repeated regularly. I am not sure that many folks will pick up on that either. Just because people might be in the area doesn't mean they will respond meaningfully.

I am a real fan of signal mirrors and fires. They are odd enough that they are going to attract attention. There are situations where fires should not be kindled, even for emergency signaling, but they have the benefit of putting out smoke and providing heat. Mirrors only work in the daytime, but they work extremely well in the right conditions. A lot of lights have a strobe function, even some with SOS, and they could be useful, as well.