A study to analys vehicle behaviour demonstrated a couple of surprising things.
Firstl in general, car drivers make assumptions of the risk of passing a given cyclist based on how they look. So a cyclist in hi-viz with helmet will be passed closer than a cyclist in normal clothes, as the hi-viz helmeted one is seen as being more professional and so more likely to behave in a particular way. Which means that the work they do with the helmet and hi-viz is undone.
Secondly, white van drivers are the worst, passing more closely more frequently than any other group.
Thirdly, red van drivers are universally the best group giving the most time and waiting for the safest time.
This third point was quite a confusion and general outlier in the whole "van" category. Then it was suggested at a conference that the reason for this is that red vans are most commonly Royal Mail vans and most Royal Mail posties have done routes on bikes.