For my job I frequently visit construction sites. Flat tires are pretty frequent.I'm out in the built-already burbs. But I found out the hard way when my next-door neighbor got his roof replaced. Those roofing nails are sharp, not only the point, but also the flat heads!
Roofing nails are the preferred poor man's caltrop for anarchists. The broad flat heads increase the chances of them sitting point up when scattered, and even if they land on their side, tires rolling over them can stand them up. For some time, they were the calling card of ecoterrorists in the Pacific Northwest.
In subsequent years kevlar tires have become increasingly popular with serious off roaders and have come down a bit in , and they offer some protection.