Do you pick up horseshoe nails in your car tires? I'd get flats every few months in Lancaster County, PA.
So far no horse shoe nails ! After my wife shoes the horses we run a big magnet around to pick up the nail points clipped off ! But as I do the city building inspections I get 3 or 4 nails a year, most likely from being on sites. When we were doing the downtown street and utility work several years ago we found a lot of horseshoes, metal buggy parts, etc.buried as deep as 5'. The town is from about 1840 or so.