Following up to Adrian Tupper

No, the blame is still on the undertaking driver for driving carelessly.

that's what I was trying to say.

If someone is doing 50mph in a motorway fast lane then it takes seconds
to undertake and pull in front into the huge gap that by definition must
exist in front of the slow driver.

that's not the sort of situation I'm talking about, there's no
huge gap, there isn't even a small gap:-

either there's a lot of traffic in L3 because a few people are in
L2 when they could be in L1 or
there is a long queue in L3 overtaking a vehicle in L2.

L3 will typically be doing about 70

The prat, undertakes in L1 or 2 and then *forces* his way into L3
when blocked by traffic in L1 or 2 from further undertaking. He
either thinks he has some right to get to the "front" by any
means or falsely believes all the other people in L3 don't in
fact want to go faster (actually they are waiting to overtake).
As the motorway is probably in the same condition ahead he
probably only makes a few hundred yards progress over other cars
and may be shut out by drivers like me[1], when they get a chance
and make no progress at all

1] by driving in "bursts" from beside one L2 car to the next L2
car, thus preventing undertaking unless L1 is empty (which it
often is as many drivers seem to just sit in the middle lane, the
original cause of this behaviour).
--
Mike Reid
Walk-eat-photos UK "http://www.fellwalk.co.uk" -- you can email us@ this site
Walk-eat-photos Spain "http://www.fell-walker.co.uk" -- dontuse@ all, it's a spamtrap