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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
Speed limits have traditionally been the responsibility of the states. In the mid-1970s, however, Congress established a national maximum speed limit by withholding highway funds from states that maintained speed limits greater than 55 mph. The requirement was loosened for rural interstates in 1987 and completely repealed in 1995.