The biggest problem is with riding without lights at night or around dusk or dawn.

I usually use 2 rear lights in case one fails (battery dies, light fails, etc). That sounds like overkill at first, until you realize that if the back light goes out - you won't know, because you can't see it. You'll be biking along and have no idea that you longer have a light in the back.

I'd say lights are the #1 thing.