when does filtering become DWDC?

[Grahamm]

I searched high and low for a definition on filtering when I was doing the IAM training and couldn't find anything anywhere.

The advice I was given when I did my Advanced Test was "20 delta 15" ie you shouldn't filter if the traffic is doing more than 20mph and you shouldn't filter at more than 15mph faster than the traffic is going.

This is covered in the IAM's "How to Be a Better Rider" book.