Northern Ireland's most common motorway interchange, these were constructed in a great many places. Today you can see examples at:

M1 j1 (Broadway)

M1 j2 (Stockman's Lane)
M1 j6 (Saintfield Road, Lisburn)

M1 j9 (Moira)

M1 j10 (Lurgan)

M1 j12 (Birches)

M1 j14 (Tamnamore)
M1 j15 (Dungannon)

M2 j1 (Fortwilliam)

M2 j4 (Sandyknowes)

M2 j6 (Rathbeg, Antrim)

M2 j10 (Larne Road, Ballymena)

M12 j2 (Carn, Portadown)

M22 j1 (Antrim)

M22 j2 (Randalstown)

Variants

M1 j11 / M12: The junction of the M2 and M12 is a modified roundabout where two of the sliproads cross the motorway to meet the roundabout which is set off to one side. It is, however, functionally identical to a roundabout.

Partial Cloverleaf / Folded Diamond

The partial cloverleaf ('parclo') interchange is not as common as the roundabout, but it crops up in a few places. They are ideal for junctions where most of the traffic comes to/from one direction on the minor road. Essentially the slip roads leave in pairs. You can see perfect examples at:

M1 j13 (Verner's Corner)

M2 j5 (Templepatrick)

Variants

M1 j7 (Sprucefield): Not a true parclo, this junction was (until it was modified in 2003) really a "trumpet" where one pair of slip roads left in parallel, but the rest of the junction was a roundabout interchange. After junction 8 was built, the west-facing slips were both closed.

With thanks to Chris Marshall for a correction to this.

Dumbbell

A cheap way of constructing a roundabout interchange that requires only one bridge. A recent develpoment in Northern Ireland, the two currently in existence were both opened after 2000 and are at:

M1 j8 (Blaris, Lisburn)

M2 j7 (Crosskennan, Antrim)

Note that the short section of each roundabout which is closest to the motorway is redundant as no movements require use of it – teardrops would perform just as well. Dumbbells are much more popular in the Republic of Ireland.

Diamond

The diamond interchange is like the dumbbell except that the roads simply join the through road/roads at either a priority junction or traffic lights. There are two examples in Northern Ireland:

M2 j1A (the Westlink/M3 junction) where the sliproads join local roads under a series of flyovers.

M3 j1 (Middlepath Street) just a few hundred metres from the previous junction and where the sliproads also join the local road network.

Variants

M1 j3 (Black's Road): is a partial diamond with east facing slips only.

M2 j11 (Ballymena): This is essentially a diamond interchange except that a fifth sliproad has been added at one side so that traffic going out of town towards Belfast does not have turn turn right across the traffic. See map.

Greencastle-style

I have named this the "Greencastle" style after the Greencastle interchange on the M2 because I can't find a standard interchange type to identify it. There are actually two of these in Northern Ireland, both on the M2. Essentially it creates a free-flow junction since the road beneath the motorway splits into two diverging lanes which act rather like a large roundabout onto which the slip roads join. However, since the offslips join from the inside it is not a true roundabout. There never seems to be bad congestion at them, even though they can be hair-raising to use! You can see them at:

M2 j2 (Greencastle). The diagram on the left is based on this interchange.

Variants

M2 j12 (Ballymena). This is an incomplete version of the "Greencastle" design which was left unfinished when the M2 project was cancelled in 1975.

Free-flow merge