Good question - I'll see what my Dad says about it. Having to demolish virtually all of Shiney Row might have caused some opposition, I guess. As I remember, though the roundabout at East Herrington looks big on the photo, I can't see how they would have "plumbed in" another dual-carriageway from Washington.
Incidentally to the west of the original roundabout in question you'll see another one. This has been added only in the last year or two, and the spur coming off it to the WSW parallel to the railway line, will, according to my uncle, eventually sweep generally southward between Houghton and Fence Houses, to come out at the centre or SE corner of this