That's not true everywhere. My wife's parents live in rural Iowa on a highway that has a 45 mph speed limit. Their home is part of a small cluster of homes and their mailbox is across the road, similar to the circumstances in the OP story.
45 is a speed zone if the normal speed is 60 or 70. I just got done traveling on US 395 through most of California, and we would typically speed zone down to 45, then 40, then 35....around houses. I’m sure there are actually well defined rules about this in California.
Regardless, living on a highway is not prime real estate.
Sounds like CA and IA work differently. 45 is the speed limit for that highway because it's windy. On the straighter roads, the limit is usually 50-55. However, there are still no speed zones (as you call them) for small clusters of houses.
The whole west works different I think. I mean, who in their right mind would build their house right next to the highway (without a fairly long highway) if they didn't have to? As long as you have the land, you would build it 50-100 meters back!
The only time I see otherwise is if mountains or rivers are involved, or there is density (in which case it is sure to be zoned!).
There are many types of highways. You might even call these rural roads just roads, even if they share the default speed limit for the area (80 km/h here).
With very low traffic density, you might build the house closer to the road (maybe 10m off, separated with trees) for just that reason – to save land. (Not all rural houses are farms.)
It's common in the midwest and becoming an issue when towns expand and traffic picks up. Mailboxes on the opposite side of a county road should be illegal. I'm surprised actually that they are not.
