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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
Maximum speed limits (which apply to rural interstate highways) now vary from state to state, ranging from 60mph in Hawaii, 65 mph in many of the north-east states and Oregon, 70mph in the south-east states and west coast, and 75mph in the mid-west states.