I have seen these in a bike shop:
http://www.reelight.com/

In theory it's the best idea ever.

But I wonder if it works just as well in practice.

They are not cheap: around $55 for the regular style and almost $95 for those that stay on for at least two minutes when you stop moving.

I am partly worried because I find them pretty low on the bike. A good "backup" to always have on the bike, but how safe on its own?

Anyway, if someone has tried them or knows someone who has, it would be great.

Thanks!