Netherland has a lot of polders, which are very flat, and particularly the largest polders (Flevoland, Wieringermeer, Haarlemmermeer) are very recent (last 200 years) and have a lot of perfectly straight roads. It's a shame the Afsluitdijk got divided between 4 pixels instead of getting its own pixel, or you might have had one pixel of 100% there.

Also, we're pretty boring and organized people. You should see how perfectly the trees in our "forests" are lined up. Also straight.