I'm not saying you're wrong, but look at it from the other car's perspective. They're gaining on the truck at 40 KPH, which is fairly quick, and they check their mirror and see nothing so they go for the pass. Before they get around the truck you've shown up gaining on them at 130 KPH. Before they can accelerate out of the way you have to slow down to avoid rear-ending them.
No one is at fault for this - it's just the nature of a road system that allows such diverse speeds. In the US on roads which allow high speeds, typically greater than 60mph, there is also a minimum speed not more than 30mph below the maximum. If you don't have a maximum speed designated for the road it's much harder to manage a minimum speed.
One thought I've had is to have speed ranges defined per lane. It works best on roads with 3 or more lanes. Lane 1 would be 50-100 KPH, lane 2 would be 80 - 150 KPH, lane 3 would be 130 - 200 KPH, lane 4 would be 180 - unlimited KPH.