I've read it and Im confused that in autocross people don't set their tire pressure higher than a few psi since they would love maximum braking to improve their times.
It is measuring traction but "gradual braking" is not used to stop as fast as you can(neither is immidiate braking which would induce a skid). Looks like barry the tire expert isn't fully convinced either.
Barry's Tire Tech
"Dry Braking & Wet Braking:
Both of these tests were conducted using a traction trailer – pulled behind a truck. A
brake is gradually applied to the test position on the trailer until the tire locks up and instrumentation on the axle measures the peak torque and then the stabilized slide value. For the wet portion of the test, water is sprayed on the ground in front of the tire at a prescribed rate. This generates a consistent water depth. This is a common way to conduct traction testing.
These results are a surprise to me. Given the results of the wear test, I would have expected the traction values to peak at some pressure and then drop.
Perhaps it is the nature of the braking test and perhaps a different set of values would have been obtained if a test had been conducted on a circular skid pad using a vehicle – but they didn’t do that, so we don’t know."