Why are there two sets of Navigational rules of the road?

Answer 1

As far as the US is concerned, the first set of rules (International) was set forth by the International Maritime Organization (IMO) in the 1972 Convention for Preventing Collisions at Sea (72 COLREGS). All countries ratifying this treaty are expected to comply by these rules.

Prior to the 72COLREGS being published, the US had an old set of Inland rules that were outdated and rarely updated or standardized. When the 72COLREGS was enacted the US also updated their Inland rules. Using a lot of the same rules as the International ones. In some rules there are slight differences and in some rules there are major differences. For those weekend boaters and professorial mariners, you should refresh your knowledge from time to time.

Also there are two sets to cover two different types of operating area's.