Good brakes.
After seeing a bus stopped, no car should be speeding down beside it, let alone a truck with a longer braking distance. This is why it always baffles me how cars zoom by stopped right-lane traffic. You never know when someone will decide to jump out and make you have a very bad day. The only people who shouldn't be expecting anything while going gung-ho down their clear lane are inexperienced teenage drivers. There's no excuse for anyone else.
Part of that though will also be dependent upon the current conditions and situation. If there is slow/stopped traffic in one lane on a highway but traffic can flow in the other lane(s), traffic shouldn't come to a crawl. Slower from the speed limit, that may be called for, but sometimes it's just ridiculous what I see. (note, I'm talking divided or multi-lane highway, especially interstates and limited-access freeways) In the kind of situation I'm describing, sudden evasive maneuvers should only ever be expected for vehicles. Bodies running out into traffic in such a scenario is literally one of the last things you should ever be concerned about, and while utterly unfortunate it would never be a driver's fault (I'm talking "la di da cruising past some cars in the right lane on the highway OMG bodies on my windshield!" - obviously if there is time to effectively maneuver to evade, take it, and this doesn't cover distracted driving obviously).
Now, on a straight country two-lane road (one lane per direction), oncoming traffic absolutely should slow while coming upon a stopped school bus. Honestly it should stop in my book, that's the rules in this here country and rightly so, but if the codified rules in another country don't tell you to stop then stopping would be the last thing you ought to do (drivers will not expect someone stopping for no reason). I'd still say slow with caution would be most appropriate.
In this specific scenario though, it looks like the truck wasn't going all the fast at all. It looked like the car that immediately preceded it was perhaps slowing while giving extra room. It doesn't look like a great distance covered during stopping, and no tire marks. It must have had a light or no load, and it couldn't have been traveling all that fast, not to stop in the distance it did.