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 In 1995 Congress repealed the national speed limit of 55 and it is now up to states again to determine the limits.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 NSW there are two default speed limits, default urban speed limit for roads in built-up areas (ie where there are buildings next to the road or where there is street lighting) and a default speed limit for all other roads. 1  Default urban speed limit is 50 km/h. 2  Default speed limit for all other roads is 100 km/h.