Then it is very clear. I am a major fan of ProPublica, but I feel like they were too biased here and were trying too hard to write a good story.
straight road, no reason to go suddenly slower, the sign is completely irrelevant. drivers are not robots, if they get a good reason to slow down, they will. (eg. Waze telling them a speedtrap or patrol car is there ... or some actual traffic reason.)
There is a village ahead, that's plenty of reason to slow down.
The two pairs of signs are another reason.
The end of the divided highway is further reason.
sparse traffic, flat terrain, good visibility, right of way ... all things that incentivize drivers to not take the signs seriously.