The M74 was built in the short term, and wasn't initially planned to go all the way to England. It was initially opened as the Hamilton by-pass (current junctions 4-8). It was then extended South to J12 in the '80s. In the early '90s, the Tory government built the new A74(M) to the Cumberland Gap, and they had the sense to build it with 3 lanes. Sadly, we've not yet widened the road between junction 6 and junction 12. Like so many things, it's a bit of a historical anomaly. Forward planning all those years ago would have seen it built wider, but given that the Edinburgh-Glasgow M8 was (and still is) only two lanes, it's easy to see why they didn't foresee the need or demand.
You'll be pleased to know that the M74 completion will be three lanes in each direction, and there will be some further widening on the existing road between Junction 4-6 as part of the M74/M73/M8 network improvements. I don't think there's a scheme in the pipeline to widen the road, and although it is a bit of a pain I don't think it's a particularly serious problem at the moment. Certainly not as bad as the M8. I think you'd struggle to warrant widening almost 17 miles of rural motorway at this moment in time.
You'll be pleased to know that the M74 completion will be three lanes in each direction, and there will be some further widening on the existing road between Junction 4-6 as part of the M74/M73/M8 network improvements. I don't think there's a scheme in the pipeline to widen the road, and although it is a bit of a pain I don't think it's a particularly serious problem at the moment. Certainly not as bad as the M8. I think you'd struggle to warrant widening almost 17 miles of rural motorway at this moment in time.