Statistically, truth to tell, not very often. But that doesn't mean they are not effective. Most people don't carry, or use them.

However, a signal mirror easily has a range of around twenty miles, much greater than a whistle. And a mirror flash is difficult to overlook, particularly if the signaler is persistent. Most of the use occurs at sea, where a mirror is often included in life raft survival packs.

Theoretically, you can send Morse code with a mirror. We had one case where a party was sending SOS mirror flashes. The people who noticed his flashes only saw random flashes, but that was enough to call SAR and check out the scene -just about the same scenario that you often see with whistles.

I have used a mirror to indicate to a helicopter about to take off easily eleven miles away, our precise location in the mountains above Tucson. That makes for an easy, routine, no sweat mission, which is a good thing.

An actual signal mirror is versatile and can be used for other purposes. Conversely, many items can serve as mirrors, particularly mirror equipped compasses.

I have spent a lot of time in the desert Southwest, where th sun shines a lot. I dare say they are not quite so useful in less sunny areas, but I really do find them useful and worth carrying.

Have we (me) ever drifted this thread!