What Keith Brown is either forgetting or he's too stupid to realise that it's not only the speed of vehicles but the number and type. When you've got town centres that look like this
then obviously speed limits are not the only answer. The road in that video is the A7, and if you're going from Carlisle/west of England to Edinburgh/east of Scotland you'd either pick that road or the A702 through Biggar. As well as pushing straight through every town they come across they're also single carriageways, so lorries should technically be limited to 40mph but I don't think I've ever seen one in my life that actually sticks to that.
If you want to avoid driving through these small towns where people live work and play than you could go up to Glasgow then across to Edinburgh and have motorway the whole way, but who in their right mind would choose to cover two sides of the triangle? The A1 in the east also takes a meandering detour around the coast so if you want the direct road south you've got to go through Jedburgh or Coldstream. It's a pattern repeated everywhere. Let me repeat that, the only roads from the South to our capital city look like the one in the video. Keith Brown has got a much larger problem than simple speed limits can solve.
If it was up to me we'd start a programme of building bypasses and filtered permeability in towns and villages on major through routes. However that costs a lot more and is more political than putting up a few non-enforced sign posts.