Good grief, 15 lumens isn't even enough to look for a real flashlight in my bag.
If I was absolutely forced to, I'd maybe ride with 100 lumens up front, but I'd have to reduce speed. I consider 200 to be bare minimum, 400 to be comfortable.
You can get 400 lumens for < $25 these days, including rechargeable battery and charger.
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Work: the 8 hours that separates bike rides.