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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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 most built up areas, towns and villages, etc., the speed limit is 30mph. Effectively this speed limit is in force wherever there are lamp posts (with a few exceptions determined by the spacing of the lamp posts). Where the UK speed limit exceeds 30mph, there will be 'repeater' signs reminding you of the higher limit.