The M80 takes Glasgow traffic north. It gets traffic off the M8, then collects the entire load of the M73, and carries it north-east to Cumbernauld. The it splits, with the M80 proper taking traffic to the M9, which in turn leads to Perth, Aberdeen and the Highlands. The other fork, the M876, takes traffic over the Kincardine Bridge to Fife and the M90.
To say it performs such a vital strategic function, the M80 was unfinished for an awfully long time. The first section, from Haggs to the M9, opened in 1974, but it took a further 18 years for the next part to open. The Stepps Bypass arrived in 1992, beginning on the M8 and running through the Glasgow suburbs. In between, from Stepps to Haggs, was the old A80 - a dual carriageway, yes, but plagued with traffic lights, local access points and the hideously congested Auchenkilns Roundabout. Part way along, the M73 merged in, adding considerably to the problems. Meanwhile the M80 remained in two parts, rather improbably finishing at junction 3 and resuming some considerable distance later at junction 4 - as though the idea of providing any sort of junction for the M73 or the large town of Cumbernauld had never been considered.
The 12-mile section of A80 saw some upgrades in the intervening time, most notably the grade-separation of Auchenkilns in the early 2000s. The proposals to finish the M80 continued to evolve, varying between an on-line upgrade of the A80 and a completely new route up the Kelvin valley to the north. Eventually approval was given for a modest upgrade scheme, and work was carried out to improve the A80 (but not widen it, except to add hard shoulders) to motorway standards. A new bypass of Moodiesburn was built, and junctions were improved. The original section was given new junction numbers. After a 37 year wait, the M80 was finally completed in 2011.
With thanks to Fraser for information in this section.