are you allowed to overtake on the inside when...

[joej23]

... the left lane of the motorway becomes a filter lane off at the junction? Theres a section of the motorway I regularly use and sometimes the exit lane is empty and the other two lanes quite full. Is overtaking on the left legal as all the lorries and slow moving vehicles move into the middle lane.
Also some lorries will use the outside lane at this point to overtake slower lorries in the middle lane. Are they allowed to do this?

[Sciroccology]

Yes and yes.

[LeoSayer]

It's not really overtaking anyway, as your lane is just moving faster than the other lanes.

[tigger1]

Think once they 'split' (change in line markings) you treat them as two parallel roads - two separate roads once it's a solid white line

[Eliminator]

I understood that "undertaking" was not in itself illegal. If done badly then DWDCA or DD in extreme circumstances.

So, moving to the left to pass something when you could have moved right could be DWDCA, but travelling in lane 1 of a DC at 68mph and coming up behind a single vehicle in lane 2 doing 60mph then just cruising past might not be a problem.

[EU_Foreigner]

Interesting. What is the view in heavy traffic situations where say the left lane is quite empty but in the far distance there is a truck driving slow. Would it be ok then to drive on the left lane at say 50 (undertaking cars in the middle lane doing 45) , then to merge back into the middle lane when you get to the truck?

[Sciroccology]

tigger1 said:

Think once they 'split' (change in line markings) you treat them as two parallel roads - two separate roads once it's a solid white line

Exactly. The best example I can think of is where the M11 joins/leaves the M25 near Epping. In both directions, the inside lane of the M25 splits off around a mile away from the actual junction, so in effect, the inside lane becomes the M11 and as such is a separate road. I'd like to see what reaction you'd get if you didn't overtake slower-moving traffic that stays on the M25.

To fully answer the OP's second point, HGVs are not allowed in the outside lane of a three-lane motorway; however where the motorway slims to two lanes, the outside lane becomes fair game for trucks.

[Andrew D]

EU_Foreigner said:

Would it be ok then to drive on the left lane at say 50 (undertaking cars in the middle lane doing 45) , then to merge back into the middle lane when you get to the truck?

No. Highway Code clause 242.

To my mind your scenario constitutes "moving to a lane on your left to overtake".

[tigger1]

Andrew D said:

EU_Foreigner said:

Would it be ok then to drive on the left lane at say 50 (undertaking cars in the middle lane doing 45) , then to merge back into the middle lane when you get to the truck?

No. Highway Code clause 242.

To my mind your scenario constitutes "moving to a lane on your left to overtake".

Not moving to left if you are already in lane 1. I think the 'undertaking' bit is only ever really an issue if you close on somebody in lane>1, and then move left to pass them. If you move left a reasonable way in advance, and then continue at a legal speed in lane 1(or 2) then you'd be (just about) ok.

Problem really comes if you pass on the left at speed in excess of the posted limit, at which point you're somewhere between careless and dangerous.