That's it! All it took is one sentence - relative velocity Delta V= V1-V2, plain and simple. I was thinking about vector sum of forces but that's completely different story. Now it all makes sense. Thanks.
Two cars approaching each other with speed 100km/h each. Relative to one of the cars the road moves with the speed of 100km/h (car pulls it toward itself), the other car moves on the moving road, vectors add - relative velocity 200 km/h just the way it should be. Relative to the road two cars...