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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
The UK dual carriageway speed limit is 70 mph (112 km/h) maximum. UK dual carriageways that have the national speed limit of 70 mph do not have speed limit signs. Where dual carriageways have a restricted speed limit such as 50 mph in an urban area for example, speed limit signs will be located alongside the road.