A new BMW 3 series and a pickup truck are tested using three sets of tires.
One brand new (10/32 of tread), one with 4/32 of tread remaining, and one with 2/32 of tread remaining. Oh, and by the way if you have less than 2/32 remaining you won't pass inspection.
Test was done to see stopping distance on wet roads. The results are frankly a bit scary:
For reference here are some "penny tests" you can do to figure out what your tire depth is:
Excuse me while I shop for new tires! (well done tirerack sales team....well done...)