[jrootabega]

I agree that clumps usually end up blocking passing lanes, but they're also bad when in any lane(s).

[greentxt]

A clump == blocking the passing lane. They are definitionally the same.

[taurknaut]

I think about a large cluster of cars, say 6-12 cars, going roughly the same speed. Nothing about that entails blocking multiple lanes, certainly not densely enough to preclude someone from passing.