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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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 in the United States vary depending on jurisdiction, with 75 to 80 mph (120 to 130 km/h) common in the Western United States and 65 to 75 mph (100 to 120 km/h) common in the Eastern United States. States may also set special speed limits for trucks and night travel along with minimum speed limits.