As far as I understand it's the green portion of this map.
View attachment 271932
So all the traffic for the west portion of Ferndale Rd plus a few other streets off it, accesses it from that one point at its west (Clapham) end.
That's in theory though assuming all the filters are working.
Collateral Dama 's view I think is that this setup generates too much excess traffic even with the filters working. That seems surprising to me and I am inclined to think that if all the filters were working properly you would not be seeing a problem.
The graphs posted above are from before the filter halfway along Ferndale was introduced, I think. If I've got it right, prior to that the 'blue' and 'green' areas were not separated.