I got this LED strap light, it is actually for cyclist to wear around their arms or ankles but it has an elastic strap that you can adjust the length. I tried it with a couple of dogs, fits quite well. There is two modes, constant and flashing. It is bright enough that I think even when it is the winter time and at night a driver should be able to see it from quite far away.
Only thing is that if the dog has long coat or think fur, the light strap can become obscured. Or if the dog is very big with a huge neck, this light might not fit. It does come with a quick release buckle so I guess a person could always buy two of them and clip them together to make it fit around a really big dog. That and get 360 degree light coverage.
It is water resistant and even using it in heavy rain there are no issues with it. The name of the product is "Planet Bike BRT LED Strap Light".
I give this a four out of five.