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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
A properly set speed limit will be within 3 miles per hour (Â±) of this observed speed. The posted speed limit will then be rounded to the nearest 5 miles per hour. Research has shown that the 85th percentile speed is the speed where accident involvement is the lowest. Reducing the speed limit below what is warranted can actually be detrimental to safety.