The rule and measure of acts defined by natural law is unwritten. Natural law is something internal, tied closely to our conscience. It is our participation in God's eternal law, and it guides us in making decisions.
Natural law is concerned primarily with universal principles.
But, it is when we are faced with a specific choice of action that our conscience guided by natural law tells us the correct choice. Human law, on the other hand, is concerned only with particular situations. To drive 35 miles per hour on a specific road might be within the law, but 35 might be an excessive speed in a school zone or dangerously slow on a freeway. Attempting to buy alcohol in one state at the age of 20 might be illegal, but in another state or country, it might be permissible according to the written law.