The previous context was, "Why on earth would there be a 40mph road next to a school"
The surrounding context was a comment about how the flashing light WASN'T on and how the speed limit was, per the sign, only enforced when the flashing light is on.
So, "why would it be 40mph?" because it's a stroad - a main thoroughfare through a town that's like maybe 5 or more lanes wide (including both directions and the 'turning lane'.
There's value in asking the question why there's such a wild variance in speeds between "normal" speed and "school open/close" hours, and that's a useful question to ask and likely comes down to the car-centric politics that filled the United States for well over 50 years, and will take a very long time (and a lot of political clout) to fix.
Depending on the size of the town, that may also, literally, be "the only road" that makes sense for that school to be on, so you have conflicting interests of a major, high-speed arterial road as well as the safety of children during particular hours.
And to add *even more* to that, if you're going 40 and expect the speed limit to be 40 and everyone around you also expects the speed limit to be 40 (because that's the posted speed limit, and the sign isn't flashing), then you going 20 creates a 20mph differential between you and the rest of the traffic around you and now you're *going* to cause an accident.