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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
Most highways in the U.S. top out at 75 mph, and there are no longer any roads in the U.S. with no speed limit like Germany's autobahn. Some highways in rural West Texas and Utah have 80 mph speed limits. The Texas Legislature last year approved 85 mph limits for some new stretches of road.