Ummmmm... What color is the sun on your native planet?
Speed Limits are set by morons - always have been, always will.
The Traffic engineers try to use a rationale like they're setting them at "the 85th Percentile of the average traffic flow on that stretch of road" or other nonsense, but plain and simple they're guessing. Drivers will drive at a speed they feel is safe for the conditions at the moment, and each driver has a different idea of what constitutes 'safe' (some of which are nonsensical or ignore certain hazards in their estimate), which creates an instant conflict.
Too low in many places, too high in others - you have to know to look out for blind intersections, people pulling out (or worse, backing out) of blind driveways without looking, people pulling out from curb parking without looking - on both sides, which covers the U-Turn from a standing start at the curb.
We have a stretch of secondary road that was posted for 30 MPH for several decades, to get around the defacto residential limit of 25 MPH when unposted. Then they put in killer speed-bumps you have to slow to 10 MPH to clear without damage - and left the 30 MPH signs posted...
Then you get to the open highways and the restricted-access freeways and turnpikes, where there is no cross-traffic, they're fenced off so no stray animals, broad shoulders, proper grading and drainage... And some moron in the Legislature decides that we have to save fuel, so we'll set the limits artificially low to force you to.
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