I found a reference at http://remotefootprints.org/ http://remotefootprints.org/ linked from BBC, but it looks like the site was hacked by a Bitcoin/crypto scam

I found a reference at http://remotefootprints.org/ http://remotefootprints.org/ linked from BBC, but it looks like the site was hacked by a Bitcoin/crypto scammer. Anyway, they claim that in USA rhat 22miles is the farthest you can get from a road of any kind that supports a vehicle, and still be in "developable" terrain. That would exclude mountains, lakes, floodplains, and maybe or maybe not forests (because they could be cut down?)