I've learned the lesson that even on days when I know I'll leave work in mid afternoon I should still bring some lights with me: the other day an unexpected long conversation with someone resulted me leaving an hour later, about 4:30pm, and it already began getting dark. By the time I got home, it was dark. When I was only few minutes from getting home I suddenly remembered I had spoke lights on my wheels that I never used, so I turned them on .