Mr. Hugh Creegan:

The reason for this is that there are more things to deal with in an urban area. Generally, a road in a greenfield site is quite easy to deal with and we can be very specific. In an urban area with shops, trees and everything else, if we write down absolutely explicitly thou shalt not go below 1 m, we might then find it is 1 mm less than this. We need flexibility in the document. This is why there is a little more flexibility.