- The Fourth Avenue Underpass is a significant part of Tucson's history and urban development - The original construction was in 1916 as a grade separation project to allow Fourth Avenue to pass under the Southern Pacific Railroad - It was Arizona's oldest underpass at the time and was listed on the National Register of Historic Places in 1988 - The underpass was instrumental in the development of Fourth Avenue as a commercial district, serving as a vital link between the University of Arizona, Fourth Avenue businesses, and downtown Tucson - The original underpass was demolished in 2008 as part of a $26 million reconstruction project - The current underpass, completed in 2009, was designed to incorporate modern features such as wider sidewalks, dedicated bicycle lanes, and accommodations for the Tucson Modern Streetcar - This reconstruction improved connectivity between Fourth Avenue and downtown, as well as enhance safety for various modes of transportation ... the project als...