A dual carriageway is two separate one-way roads with a physical divider.

Is there a physical barrier preventing a vehicle from crossing directly into the opposite direction?
✅ Yes → Map as dual carriageway
❌ No → Keep as single carriageway

✅ Examples of physical dividers include:

❌ Not a dual carriageway

This section of Gladstone Benaraby Road (in orange) is currently mapped as a single-carriageway. We can see that there is physical division of the roads at the junction between Kirkwood Road (in yellow in the screenshots above).

First, disconnect the four dark grey nodes connected to Gladstone Benaraby Road, this includes the slip roads, and Kirkwood Road. We are modifying the road geometry to allow for the presence of a dual-carriageway at this junction.

Tip: right-click the grey node to select it. It will glow red when selected

The connected nodes will now appear as white node, representing their disconnection from other road segments.

Note: save this edit now will cause routing issues, these errors can be found in the bottom right of the editing view as an orange warning sign

Realign the pre-mapped road into a suitable alignment. This example realigns the road onto the south-bound carriageway

Split the road where physical division begins, this is because we need to set the one-way restrictions for both the north-bound and south-bound carriageways. There’s no set in stone rule about where to split with a painted median such as the case below. Map where vehicles begin to divert from the joined carriageway

Map the second carriageway. I had to realign the slip lanes make way for the newly mapped carriageway

Typically, you can copy all the tags from the pre-existing road and paste them over to the new mapped road. However, keep a note if there are differences in road properties such as lane count and turning lanes. This example does not have these tags so we can copy all the tags over. Note the presence of the relations/road route as this is a coded highway, more can be found here (relations link)

You will need to add the one way restricted onto the road. The arrows will show you the direction in which traffic is required to travel.

We can now re-align Kirkwood Road and the slip lanes to connect with the correct carriageways. Ensure when connecting roads onto the carriageway that they connect with both carriageways (with the grey nodes).

Note: A warning will appear; you can click connect the features to quickly connect the carriageway

Do the same for the slip lanes, ensure that they connect at the correct locations along the carriageway (not traversing over physical barriers)

Ensure there are no warnings relating to the edits you’ve made. You can save this edit and add a changeset comment such as “Added dual carriageway alignment”

The final changeset for this example can be found here: Changeset: 179373787 | OpenStreetMap

More can be found here Dual carriageway - OpenStreetMap Wiki