The M74 starts at the M8/M73 jcn a few miles from Glasgow. It ends at the A74 near Gretna, a mile or so from the end of the M6. Small sections are still being built, so occasionally one is diverted back onto the A74 for the duration. It is almost complete now, and provides an excellent route to Scotland via the western side of England.
The M74 is an immensely important route, mainly as there are only really two fast ways to get to Scotland from England; the eastern side (which chiefly involves the A1, but most of this past Morpeth is single carriageway) and the western side M6, M74. Long distance freight is moved this way, as it is so much quicker.
As most of the M74 is situated in the Southern Uplands, the choice is either to climb (which is no good) or wind around the hills. Therefore, the M74 has lots of bends, far more than most motorways. The scenery is magnificent however, as you circle the high peaks.
Most of the M74 is either built on top of, or directly next to the A74. Bits of the A74 still exist, and if you check your atlas, you will discover there is nearly always a little B road winding around the motorway. Eventually, all the A74 will disappear, as the fashion is to renumber the once-main route down to B road grade, to encourage use of the motorway.
As the M74 connects mid Scotland (Glasgow, Edinburgh) with the north west of England (Carlisle, Penrith, Lake District) there are some interesting sights both at the top and the bottom. The M6 takes traffic to the midlands and eventually to London, and the M8, and later the M9 and M90 takes traffic up to the north of Scotland.
The M74 has the honour of being the scene of my most frightening incident ever. Back when I was delivering cars, I got paired up with another driver, let's just call him John. I drove from our base up to the services on the M74, Annan. There we had coffee, and he woke up and decided to do the remaining driving to Edinburgh. When we got back on to the M74, I watched in terror as the speedo climbed up past 90, past 100, on and on (meanwhile I am white-knuckled gripping the seat) up past 110, 120 and settled at about 135 MPH; I had never heard a 2 litre engine scream before - but it might have been me. Bearing in mind the road conditions were appalling. There was an extremely heavy cross wind, and the concrete surface was water logged. In the end I had to beg him to slow down. He slowed back to about 100, and funnily enough it felt loads slower. We made it from Annan to Edinburgh in 30 minutes. Check that on a route plan. I firmly believed that day would be my last. When at last we dropped the new car at Edinburgh, I insisted on driving up to Perth to get the next car. We had a coffee there, then he took off in the Laguna from Edinburgh and I trundled down in a little diesel Magane back down to Yorkshire. I arrived a good few hours after him. Now, I am the first to agree that the 70mph speed limit is a bit too slow, but 130? This was a good few years back now, and I am still alive. It was a close thing though.