We have a cat who likes to roam, including around the busy street by our house. Where a previous cat was hit and killed. So we got him one of these as a collar. We turn it on when he goes out at night - he only stays out for an hour or two, then returns to sleep.
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When a light is on him, like headlights or a flashlight, the collar glows brilliantly. And when the LEDs are blinking, he looks like a low-flying UFO. Hilarious. The battery should last a month or so, then we'll either replace it (CR2032) or just rely on the absurdly reflective material.
The downside is that there is no release mechanism if he gets the collar snagged on something. So we just adjusted it loosely so he can get his head out if need be. These bands sell for $6 in stores, cheaper than an actual cat collar. You can write on the surface w/ a Sharpie. Should work for a dog too.