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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
Washington, D.C. and the U.S. Virgin Islands have a maximum speed limit of 55 mph (89 km/h), and Guam and the Samoa have speed limits of 45 mph (72 km/h). Unusual for a state east of the Mississippi, much of I-95 in Maine north of Bangor allows up to 75 mph (121 km/h).