The minimum tread depth for semi-trailer tires is typically 4/32 of an inch for steer tires and 2/32 of an inch for other tires, including drive and trailer tires. Maintaining proper tread depth is crucial for ensuring adequate traction, especially in wet or slippery conditions, and for maintaining safe handling of the trailer. Regularly checking and maintaining the correct tread depth helps prevent accidents and extends the lifespan of the tires. If the tread depth falls below the minimum requirement, it's essential to replace the tires promptly to ensure safety and compliance with regulations.