I would suggest that once one lane has slowed by a reasonable amount, undertaking is fine. Blasting up the inside at 90 probably not as acceptable, but once the lane starts to stack, a cautious move will be fine.
When the M2 was widened it just ended up with random lane huggers in each of the lanes, and many just moving along at a speed they consider fine and not moving over for others.
It is quicker to spend most of the journey in the inside lane, overtaking the lorries now and then than move over to the other 3 lanes.
here's a sketch..
yes there is signage telling you where the single.lanes.go to but no signage about whether you can undertake.
ive been in a situation where im on the right hand side doing 60mph..i need to go to.left side lane... there is a car in front doing about 45-50ph on the left side who goes to the right side. car still doing 44-50 once on the right aide...does i slow down to less than 45-50 as im now on the left lane.. i find sometimea drivers change lane early..like a mile before the road splits especially when its a busy road..
Considering you say that both of you are switching lanes, I would adapt and align with slowest moving traffic. Keep the speeds the same so it is much easier to keep it flowing and open the zip in a safe manner.
Personally I wouldn’t speed up to national limit unless others are as well, or unless the actual split has taken place.
so all.the cars on both lanes behind the driver who has switched from.the left to.right lane still.doing 45-50 would need to slow.down to 45-50...if the driver switches lane a mile before the split it means i.have to slow down for that mile before the.split?
Usually the road markings will change to the give way lines(shorter and more frequent white lines)
If another car has taken up position to turn/leave to the right, and you are in the left-hand lane you may drive at higher speed than the other car and pass to his left.
If you change your mind about the direction you wish to take, and can change lane safely, you may well enter the right-hand lane ahead of that other driver data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7.
Don't do it though if it's queued such that you'd have to 'force' your way in.
I misunderstood the original question, sorry.I would say that once the left/right arrows appear then the dual carriageway "ends".If the queue extends further back down the dual carriageway, then treat it as slower moving traffic.
I don’t think you can undertake unless all the lanes are in slow moving traffic jam or the right hand car is indicating to turn right.
This looks like an official website;
Q891: Can I overtake on the left (undertake)?
So until this sign you would be illegally undertaking.
https://www.avforums.com/attachments/upload_2019-6-3_19-13-56-png.1158179/