Final answer:
To operate a heavy vehicle with metal tires on the highway, one must obtain a special permit from the Texas Department of Transportation for safety and road preservation purposes.
Step-by-step explanation:
To operate on the highway a motor vehicle or trailer having metal tires and weighing 5,000 pounds or more, you must get a special permit from the Texas Department of Transportation. This is mainly for safety concerns and to ensure that heavy and potentially damaging vehicles don't damage the roadway or pose a safety risk to other users. Vehicles with metal tires can cause significant wear and damage to the road surface, especially if they are carrying heavy loads. The permit allows for regulation and monitoring to prevent such issues.