A. Stop, and then select a very low gear
B. Position in the left gutter of the side road
C. Check that the central reservation is wide enough
D. Check there is enough room for vehicles behind you
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Rule 173 of the Highway Code states:
Dual carriageways. When crossing or turning right, first assess whether the central reservation is deep enough to protect the full length of your vehicle.
Before emerging right onto a dual carriageway make sure that the central reservation is wide enough to protect your vehicle. If it's not, you should treat it as one road and check that it's clear in both directions before pulling out. Otherwise, you could obstruct part of the carriageway and cause a hazard, both for yourself and other road users.