I went riding with two friends recently. One was wearing a fluorescent yellow/orange vest, the other was wearing street clothes. It was about 11:00 a.m., with sunny skies. We were on rolling country roads, with forests on both sides.

I rode behind about 100 metres. When they rode in shadow, the person wearing street clothes was more or less invisible, while the one wearing the bright jacket was clearly visible.

The best answer might be bright lights AND high-visibility clothing.