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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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.
In Texas the speed limit for urban districts is 30mph. The speed limit for alleys and public roads that are adjacent to public beaches is 15mph. Notably, the 85mph speed limit on rural highways in Texas is the highest legal speed limit in the United States.