Final answer:
The maximum allowable length of a truck tractor and semi-trailer combination is determined by state regulations in the US, with federal law setting the maximum length at 65 feet.
Step-by-step explanation:
When operated in a truck tractor and semi-trailer combination, the maximum allowable length is determined by individual state regulations in the United States. However, federal law sets the maximum length at 65 feet for the combination of a truck tractor and semi-trailer. Some states, such as California, have more restrictive regulations that set the maximum length at 53 feet.