Final answer:
The maximum length allowed for a single vehicle is regulated by state law to ensure safety.
Step-by-step explanation:
The greatest length allowed by state law for a single vehicle varies by state. In general, state laws regulate the maximum length of a vehicle to ensure safety and minimize the risk of accidents. For example, in California, the maximum length for a single vehicle is 40 feet. However, it's important to note that these regulations can vary, and it's always best to consult the specific laws of the state you're in for the most accurate information.