Recent research has shown that the nominal legal
minimum tread depth requirement of 2/32 of an
inch (1.6 mm) for passenger car tires is inadequate
for wet-road traction at highway speeds. Significant
reduction in available friction occurs when tread
depths fall below 4/32 of an inch (3.2 mm), even for
minimally-wet roads, and well before hydroplaning
begins. No rational basis in highway safety has
been found for the present requirement.