Complex sentence: The Northern Irish authorities have no plans to replace the A1 route (currently a mixture of single and dual carriageway) with motorway, although the upgrading of the remaining single carriageway stretches to dual carriageway is underway, as well as plans for several grade separated junctions.
Split into simpler sentences: The Northern Irish authorities have no plans to replace the A1 route (currently a mixture of single and dual carriageway) with motorway. Although the upgrading of the remaining single carriageway stretches to dual carriageway is underway, as well as plans for several grade separated junctions.