Northbound M5 at Jun 4a goes to 1 lane-why

[Steviesam]

Been using the northbound M5 recently and at junction 4A, the 4 lanes split.

2 for the M42 and 2 for M5.

But the outside lane of the 2 for the M5 is filtered into 1, causing some considerable tailbacks, only to immediately return to 2, then 3 lanes. No roadworks are there.

Anyone know why?

[Steviesam]

I see, that does make sense.

I use the M5 then M42, so doesnt really affect me, but I just wondered.