With the helmet industry being "self regulated" I wonder what percentage of snell rated helmets would fail as do DOT rated? It'd be interesting to know which manufacturers fail the random testing and how often.

Snell certification is not "self-regulated" like DOT. Snell does the initial testing AND continues to randomly select stock from warehouses intended for retail sale.

Since Snell M2015 helmets tend to be higher end my guess is failure under random testing is limited.

I'm also going to look into the M2020 standard a little more... it might not actually be better than M2015. The European standards, ECE, apparently are written in a way that it's hard to get a helmet built that meets ECE and Snell standards... and many countries require ECE-rated helmets. Haven't looked into it much but I am sure anything with an ECE or Snell rating is going to offer much better reliable protection than one with just the DOT rating.

I believe Snell certified helmets are the only helmets allowed for racing and track days. DOT only helmets are not allowed. Nuf said.