PENNINGTON, N.J.—The past few years have been thick with promises of shiny new infrastructure and the revival of American greatness. Funny, then, that so little has been made of a quiet victory for U.S. infrastructure due later this year. By September 2018, one of the country’s most famous civil-engineering projects will finally complete construction, six decades after work on it began. Interstate 95, the country’s most used highway, will finally run as one continuous road between Miami and Maine by the late summer. The interstate’s infamous “gap” on the Pennsylvania and New Jersey border will be closed, turning I-95 into...