It is the shortest motorway in Northern Ireland, and one of the busiest, carrying 60,000 vehicles per day as of 2005.

The M3 was originally planned in 1956 as the Eastern Approach, named the M3 the following year, which would run from east Belfast to Bangor.

The plan was extended to include an orbital Belfast Urban Motorway, close to the city centre, in 1964.

[2] Due to a combination of financial cutbacks and public opposition construction of the M3 never took place and the Belfast Urban Motorway was downgraded to the A12 Westlink dual-carriageway and only partially completed.

[3] Traffic had to make do with crossing the River Lagan on the Queen's Bridge and using the A2 to Bangor.