Since the 40's the Detroit area has basicaly filled in between the towns to create Metro Detroit.  That's about a 70x70 square mile of the best farming ground in Michigan.  It was probably 15-20% of that in the 40's.  She's all concrete now.  That's 3.1 million including roads and all.  Then you lose a thousand acres around 100,000 cities nation wide and that's 100 million, but probably not all farm land. That's only 2000 towns per state.  These are just random numbers out of my random head.