Effect of helmets and other impressions of the bike rider on driver behaviour?
Seems there are studies showing both:

2008 Walker study - "Cars get closer to cyclists wearing helmets"


https://www.scientificamerican.com/article/strange-but-true-helmets-attract-cars-to-cyclists/
http://news.bbc.co.uk/2/hi/uk_news/england/somerset/5334208.stm
https://www.sciencedirect.com/science/article/pii/S0001457506001540



  when Walker wore a helmet drivers typically drove an average of 3.35 inches closer to his bike than when his noggin wasn't covered.


However in https://helmets.org/walkerstudy.htm  in 2013  this was refuted.  Also https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3783373/


  After re-analysis of Walker's data (2006), helmet wearing is not associated with close motor vehicle passing. The results, however, highlight other more important factors that may inform effective bicycle safety strategies. 




I don't have my own data, merely subjective anectodes about how much room I receive when riding a tandem, or towing a 2-wheel trailer, or the two times I trialled a recumbent and had cars crossing the road to go around me. I'm NOT that wide.

Locally, milk tanker drivers are awesome at passing with extreme room. They don't slow down from their permitted maximum of 90 km/h but they will give me a full lane of passing space where possible, which is 5-6 metres.