Ahh transition lenses. I've seen a couple of people in Army wearing them, and I must say, it is a really smart investment.

A smart investment for the company that made those lenses, I mean.

Transition lenses changes its darkness with the amount of UV light entering the eye. Actually, it is theoretically possible to increase the opacity of the lenses to visible light without increasing the opacity to UV. In fact, increasing the opacity to visible light actually makes your pupils dilate, increasing the amount of UV light and enters your eye. Of course, as long as the increase in UV opacity overshadows the dilation of the pupil, there is no harm done. Additionally, it reduces glare, which is kinda the whole point of sunglasses.

However, transition lenses also signal to everyone around you how much UV light their eyes are getting. Even when it is overcast, transition lenses still get discernably tinted.

This would remind everyone to protect their precious eyes from UV, so they would buy sunglasses, and preferably transition lenses.