@Butrflynet,
Not necessarily.
For centuries the Rules of War governing hostilities between
a nation or person engaged in war or conflict consisted of a body of unwritten law.
Yes some are codified, but most are not.
Furthermore, the Supreme Court has ruled that the due process clause of the Fifth and Fourteenth Amendments to the U.S. Constitution protects the right to privacy even though the word privacy is not mentioned in the written text of the Constitution.
So I think there is a unwritten law for left lanes in highway!
The Cop just forgot it.... but hey, she didn't ticket me, so I forgive her