We've all seen it. You're driving down a country road, kicking up dust as you straddle the well-worn tracks in the gravel, and just as you start to pick up some speed, you have to slow down for a Y curve or a T intersection. But after you jog over a mile or so, the road goes right back to a straight gravel stretch, for another 20 miles or so.
Turns out, those jogs are called "grid corrections," and they have to do with the curvature of the earth and Thomas Jefferson.
As Jason Kottke writes in his blog, these roadway phenomena are increasingly more common the further west you go across the United States. That's because, in 1795, Thomas Jefferson implemented a way of surveying the territories west of the Appalachians, dividing the land into square mile plots. A grid was laid over the land, making it easier to survey, sell, and map.
Our modern-day roads mimic those survey lines, creating that patchwork grid you might see from an airplane window or on a drone feed as you fly over the Midwest. While the east-west roads plot out nicely, the curvature of the earth means that the north-south roads can't be straight. Rather than build roads at a slight curve, these roads adjust every 24 miles, to account for the subtle curve of the earth. The result are those pesky jogs that prevent you from going to fast on a back road ... and probably kept me out of more than one ditch as a kid.
Check out this mesmerizing video that explains grid corrections.