This route links the two largest cities in Scotland and provides connections to other towns in the Forth/Clyde Valley, Including the New Town of Livingston. At its western end there are connections with the M74 leading south to the A74 and Carlisle and thence to the English motorway system via the M6. The route at present is of motorway standard between Newbridge and Newhouse but between Newhouse and Edinburgh and between Newbridge and Glasgow is all-purpose dual carriageway road. Consideration has been given to upgrading these sections to motorway standard at the Glasgow end and the Edinburgh end.
Poor soil conditions along the route meant that a great deal of excavation and importation of fill was required during construction. In some places, such as the Harthill-Newhouse section, there were dangers of subsidence from old mine workings not far below the level of the road and those workings had to be filled; pulverised fuel ash was used for this purpose.
The Newbridge-Dechmont section involved not only extensive excavation but also the diversion of the Union Canal and the River Almond. Tarmac (now Carrilion Construction) took over the contract after the original contractor A.M. Carmichael went into liquidation.
Contract details
| Section | Engineer | Contractor |
| Newbridge – Edinburgh City Bypass | W A Fairhurst | Balfour Beatty |
| Newbridge - Dechmont | Sir Alexander Gibb | AMC & Tarmac (now Carrilion Construction) |
| Dechmont - Whitburn | Sir Alexander Gibb | Whatlings |
| Harthill Bypass | Sir Alexander Gibb | Whatlings |
| West of Harthill - Newhouse | Lanark County Council | Whatlings |
