The classic cycler's triangle from the '70s, and a perfect product. It shouts "Watchit, Fella!" to approaching motorists,  and works nearly equally well day or night. The center part is orange,  which helps the daytime visibility.
It straps on instantly with a 3/4-inch belt closed with a quick-release buckle. The center is vented, so you won't get a triangle-shaped patch of acne beneath it. It weighs nothing and costs just $10.

Many of us here are reluctant to wear super bright bike clothing, and this comes in handy when we want to be seen. You need it in your arsenal, my friend. It also straps onto a big saddlebag, basket, or whatever you like.

Now available in a mini size for $7.  The mini has a couple velcro strips sewn on instead of a waist strap.