In simple terms a Dual Carriageway, but with 3 or more lanes in each direction with a physical centre divide.
(Therefor, technically not a Dual Carriageway, but don’t be picky!)
Can you legally use lane 3?
tricky one!! the highway code says you can’t use the outside lane on motorways but says nothing in the section about 3 lane dual carriageways!!
( although probably safer not to use it, car drivers won’t expect trucks in the third lane!!)
doesn't the definition of a dual carriageway mean - 2 seperate carriageways - one for each direction of traffic irrespective of the number of lanes :wink:Yes you can use lane 3 as there is no law stating that you can’t 
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Simple 
. Thank you and good bye.
…and as a DSA person you should have known the answer to that anyway without having to ask us ‘lesser mortals’ 
.
Like Rob says, there’s nothing tricky about it. You can use lane three of a dual carriageway.
There’s a pair of country lanes in North Norfolk signed up as a dual carriageway 
70? You’d be nuts to do more than 30…
You can even use lane 4 If like on the A2 at bluewater
In short all lanes are available for your use unless on a motorway