What is the minimum legal tread depth of the tire on a vehicle?

1/32 inch (according to driversed.com)

From the California Vehicle Code online: (1) One thirty-second (1/32) of an inch tread depth in any two adjacent grooves at any location of the tire, except as provided in paragraphs (2) and (3).

(2) Four thirty-second (4/32) of an inch tread depth at all points in all major grooves on a tire on the steering axle of any motor vehicle specified in Section 34500, and two thirty-second (2/32) of an inch tread depth at all points in all major grooves on all other tires on the axles of these vehicles.

(3) Six thirty-second (6/32) of an inch tread depth at all points in all major grooves on snow tires used in lieu of tire traction devices in posted tire traction device control areas.