While
riding through the cloud over the Chilterns yesterday (in daylight), I was somewhat nervous about drivers approaching at speed from behind, and had to opportunity to observe a variety of audaxy cyclists passing in both directions in foggy conditions. I started paying attention to what actually seemed to make people more visible from a distance.
Lights:The brighter the better. Standard Brompton rear lights were useless (in that the light disappeared while the profile of the rider was still clearly visible), which I'd extrapolate to the usual B&M-style StVZO designs, and most things you'd consider appropriate for riding in a group at night. Obnoxiously bright COB rears and those intense flashy things you tend to see on TT bikes were best.
Obnoxious battery front lights of the type popular with roadies (Lezyne and similar) worked well, particularly the constant-light-with-bright-pulses effect. StVZO dynamo fronts (presumably higher-end ones, given the demographic) were better than I expected.
Flashing was only really useful in as much that it allowed higher brightness - there was relatively little car traffic, so it wasn't a situation where a single light would be lost in the clutter.
Reflectors:They did nothing.
Clothing:This was really interesting. Fluro yellow was good, but so was solid black. Patterns which broke up the profile of a human amongst patchy light filtering through trees were bad. Provis-style 100%-reflective was terrible, as it wasn't dark enough for reflected light from my front light to be visible, and it just blended into the greyness.
As a recumbent rider, I was aware that I lacked the profile of a human from behind. I turned my Radbot up to Super Essex Disco Frenzy mode and hoped for the best. I think this kind of situation is a strong argument for having a truly obnoxious rear light, and rarely using it.