I realize the pre-existing German autobahn provided evidence that people could handle such a system, but was there any question of whether the system could successfully transfer to another culture?
Superhighways didn't just spring up out of nowhere, as a completely new network, in the 1950s. The Bronx River Parkway opened in 1908. Incremental improvements from the 1910s had created superhighways and limited-access high-speed parkways in several metro areas during the 1920s, with short stretches of urban freeways open by 1935. Both the Pennsylvania Turnpike and Southern California's first freeway opened to traffic in 1940. Though geometric standards for things like length of merging lanes evolved, the new highways quickly proved both safer and more efficient than ordinary roads.