I found an industry paper discussing the affects of tire tread depth and traction on wet roads. Although it is from 2014, it makes an interesting read. https://www.rubbernews.com/assets/PDF/RN96660106.PDF
Guys and Gals, the article is 16 pages long. IMO you have two choices: 1. Wait until SRT-Tom summarizes the article for us 2. Do what I did, just go to page 14 "Conclusions" and read the 4 points they listed. Thanks for posting SRT-Tom. I strongly agree that under 4/32 presents wet weather traction problems. I usually change out at 5/32 tread depth.
I especially like the Michelin device of having tunnels in the rubber that only turn into grooves as the tire wears down to that level. That's even better than the ever-widening grooves.