Ethics is two things. First, ethics refers to well-founded standards of right and wrong that prescribe what humans ought to do, usually in terms of rights, obligations, benefits to society, fairness, or specific virtues.
If you mean the average speed limit, it's probably 65 MPH. In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits. About 30 states have limits of 70 MPH or higher. A few Western states have no speed limits on certain stretches of highway.
If registered, a low-speed vehicle may be operated upon a highway where the posted speed limit is 35 miles per hour or less. A person shall not operate a low-speed vehicle upon a highway where the posted speed limit is greater than 35 miles per hour, except to cross such a highway at an intersection.