If I’m approaching the top of a downhill run, I’ll click back a bit so when the downhill bit starts, the engine brake cuts in, and I don’t get any “Overspeed” running down there.
So… I’m pottering downhill around 50-55 and then some other artic whizzes past me, chuffing his brakes away, thinking that this approach is better than just letting it run down on the engine brake.
I’ll often get Stobars, Arla, TNT and Tippers come up alongside for a slow overtake. The moment their nose gets level with my arse - I’ll click off five, and let them pass me in a decent time frame. The bother comes when someone running at exactly the same speed some distance behind me ALSO pulls out for a slow overtake - not realizing that I’m back up to 56 the moment the first one is decently enough past me. FFS. Where’s “reading the road” in all of this? You seem to have these “Visor and Blinker” drivers who are not aware of anything but the vehicle directly in front of them. 
Ecoroll on the Actros - seems very much like rolling out of gear to me. I was taught that “you don’t coast, because you’ve got less control over the vehicle”. Something to do with the gears snatching at the road torque should you be trundling along downhill, and then try to re-gain control… Good way to Jackknife? The same could be said of braking only the cab with a big load on behind running downhill…