A12 (Northern Ireland)

Two miles of dual carriageway in central Belfast somehow manage to form the most important link in Northern Ireland's road network.

Belfast doesn't have much of an outer ring road and, as a result, anyone heading in to the city, out of the city, around the city or who wishes to get from any location on one side to any location on the other, will need to use the A12 Westlink.

This little bit of urban highway forms the entirety of the A12 in Northern Ireland and links the M1, M2 and M3. It is built on land reserved for a proposed inner ring motorway, but it was finally completed in the early 1980s to a much lower standard with several roundabouts and traffic signals. Its upgrade and widening, to form what is effectively an urban motorway, was finished in early 2009.

The Westlink is home to something commonplace in other parts of the world, but unique in the UK: a Single Point Urban Interchange, or SPUI. Divis Street Interchange was built in the 1980s as a conventional diamond junction, but received an upgrade that would be familiar in the US, Canada and parts of Europe but is so far largely unknown here: right turns are offset across the central bridge, meaning the whole junction can run extremely efficiently with just a few signal stages.

The one remaining snag is the York Street interchange at its northern end, which remains a flat signalised junction underneath the M2 and M3. Plans are afoot to improve this, and while the initial plan would have left at least one set of traffic signals on the Westlink, the Northern Ireland Roads Service is now hinting that it may have found a way to make the whole interchange free-flowing between the motorways and the A12.

When this is done, who knows: in as pioneering and optimistic a place as Northern Ireland the A12 might even turn blue and become a motorway itself. Stranger things have happened.