I find it best to always assume someone in a position to do me harm, will, and therefore carry a loud whistle everywhere. I like the Storm brand. About $6 but well worth it. You can blow longer and louder than you can yell. Deters predators, too -- two or four-legged. Lighter and more compact than an air-horn. Or you can simply throw a brick at their windshield to get their attention. If lost in the woods and you hear someone yelling you don't know if they're calling someone else. If you hear 3 shorts, 3 longs, and 3 shorts you know that's an SOS.