It's raining in Denver. Has been for over 24 hours now.
Point being, it's gray and cloudy and wet outside, and visibility is low.
On the way home from driving my partner to work just now, I stopped at an intersection near my house. I had a stop sign and cross traffic did not. I looked each way twice before proceeding, and one more each way as I entered the intersection.
And as I started to clear the intersection, I saw a cyclist pass close by, right behind me, looking at me disapprovingly. I apparently had come close to cutting her off.
I am hyper-aware of cyclists, so I was surprised that I had carefully surveyed the area and had flat out, straight up not seen this one.
She was on an upright bike, probably a cruiser, wrapped up in a large black wool coat, covering herself with a gray umbrella. (If you imagine riding a bike in the rain, you'll realize that you need to hold the umbrella more or less in front of you—blocking your face and covering your head—in order to utilize it.)
No lights. Perfect ninja.
Even with her dark attire, and even hiding behind an umbrella, I might have seen her had she had lights on her bike.
Which is why my bike lights and my automobile lights are on, day and night. There's no reason to ever not have your lights on. All they do is make you more well seen. All they do is increase your chances of not getting hit.