This article suggests the figure is 18 miles for the US. https://www.nytimes.com/interactive/2015/12/24/upshot/24up-family.html https://www.nytimes.com/interac

This article suggests the figure is 18 miles for the US. https://www.nytimes.com/interactive/2015/12/24/upshot/24up-family.html https://www.nytimes.com/interactive/2015/12/24/upshot/24up-f... It's from 2015 but I wouldn't expect the number to be too different today.