The division of Liverpool and Birkenhead into separate urban areas is a statistical anomaly due to the fact that the Mersey is over 200m wide. In reality they function as a single urban entity and this should probably be acknowledged in the article. Anyone who doubts this should travel to Birkenhead and examine the (small and very local) scale of its commercial/retail district. By contrast, Leeds, Bradford, Huddersfield, Halifax and Wakefield are separated by many miles of sporadic development and exist as proudly independent settlements, as any examination of their commercial/retail districts would confirm. They might satisfy the strict definition of a contiguous urban area, but they generally don't function as one.