[snogglethorpe]

Hmm, I suppose it depends on how you defined "urban," and the particular area (etc), but a quick google generates a lot of results that show the U.S. generally passing the 50%-urban threshold around 1910-1920...

E.g. the graph here seems typical: http://blogs.census.gov/2012/04/04/how-do-we-measure-urban-a...