The regs.
As far as the Department of Transportation and or the National Highway Traffic Safety Administration is concerned, 2/32" or less tread depth measured anywhere on the contact surface of a tire is considered to be unsafe. If you take a penny and insert it in the tread (between the tread and the carcass) and the tread is greater than the distance fron the edge to the top of Lincoln's head, you are good to go. The front tire, however, has to be 4/32" or greater.
I don't know what depth the molded in treadwear indicators are but I presume they are 2/32".