Increasingly heavy rail traffic throughout the turn of the century snarled street traffic and created a dangerous at-grade crossing. In 1908 the county erected an overpass near the spot of James' train robbery. By the early 1920s increasingly heavy traffic required a larger more substantial replacement overpass. In 1923 the Iowa State Highway Commission (ISHC) designed an immense concrete viaduct for the crossing and in May of that year, the Adair County Board of Supervisors awarded a contract to build the Adair Viaduct to the Federal Bridge Company for $42,263.00. The Des Moines contractors began excavating for the concrete substructure soon thereafter; by June 1924 the bridge was ceremoniously opened to traffic, "with the usual accompaniment of music, speeches, and motion pictures," the highway commission reported. After the hubbub of the dedication ceremony died down, the Adair Viaduct functioned as a regionally imported railroad overpass. It remains in use today in essentially unaltered condition.
The Adair Viaduct crossing the Rock Island rail line provided an important entrance to the town from the south. Jesse James notwithstanding, the true significance of this handsomely proportioned structure is technological and aesthetic. Despite an oft-stated preference for concrete for highway bridges, ISHC designed steel trusses for its medium- and long-span structures. As a result, most of Iowa's concrete arches feature relatively short spans and filled spandrel configurations. Less than ten open spandrel arches have been identified by the state historic bridge inventory, of which the Adair structure is a distinguished example. The viaduct is also noteworthy for its aesthetic handling. ISHC rarely embellished its bridges with any architectural treatment, eschewing aesthetics for functionality. With its decorative guardrails and flanking towers, the Adair Viaduct thus marks a rare foray for the state agency into bridge aesthetics. As a regionally important crossing, and a well-preserved example of an uncommon structural type in Iowa - and a site for local lore - the Adair Viaduct is both historically and technologically significant among the state's highway spans. [adapted from Fraser 1992]