Christiana Briganti-Dunn is a civil engineer who oversees design-build projects for the Virginia Department of Transportation, but two years ago, she found herself holding a set of paint swatches up against a wall like a homeowner planning a kitchen renovation. The wall was at Chapman Mill, a five-story stone gristmill built in the 18th century near Broad Run, Va. Christiana’s job was to pick colors that would be painted on a set of bridge abutments on nearby Interstate 66. Everyone wanted the bridges to look pretty. “When we were meeting with the communities of Haymarket and western Prince William County,...