S and I are religious about putting our lights on when it gets even slightly dark. In fact, last night we had her rear light moved as we realised her new rack had hidden it.
This means I get super cross when I see ninja-style cyclists (dark clothing, no lights, silent in their approach). You can't see them, traffic means you can't hear them and they're asking to be hit by someone/thing.
Last night my leggings purchase delayed us and it was really quite dark when we pedalled off. A ninja on a Brompton came past us both inches away, I saw him at the last minute. When we stopped at the next lights I remarked on it.
'Gosh, it makes me so cross. That guy was barely visible!'