Over the years, a number of improvements have been made by the Route 9 Committee, a partnership of local government officials and business interests in the Baileyville-Calais area.

The leg from Bangor to Calais is often referred to as "The Airline," commonly thought to be because of its shorter route than the older U.S. 1.

Although anecdotes about unexploded ordnance deriving from bombers flying from the former Dow Air Force Base (now Bangor International Airport) using a number of towers and landmarks along "The Airline", in fact the term goes back to the early 1850s as a contrast to the shoreline route.

The "Whale's Back", a two-mile stretch built atop an esker in Hancock County was one of the most notable features of the highway until it was rebuilt.

[6] The state went forward with the project not as an I-395 extension but as a 5.5-mile (9 km) super two rerouting of SR 9.