> Presumably in the vast majority of urban areas, 50’ would be more than enough to avoid any and all obstructions. This would definitely not be enough in, say,

> Presumably in the vast majority of urban areas, 50’ would be more than enough to avoid any and all obstructions. This would definitely not be enough in, say, NYC. Grand Central Station is 150 feet below street level, for example. The Oculus is about 80 feet below street level. Things are deeper than they might first appear!