The tire tread depth gauge has no margin of error. It is universally agreed that the minimum tread a tire needs to remain safe is 1/16 of an inch of tread. It is at this point that the treadware bars will appear as perpendicular bars across your tires, indicating that it is past time to replace the tire. Unlike many car systems where you can haggle over whether it will fail today or tomorrow, when the tire’s tread depth gauge shows, a tire shop is legally bound to replace the tire. There is no court of appeals in this. The tire has to be changed.
For more info, see 5 Point Car Safety Checklist