Back in the day, before we had Satnav apps, we had maps, and even today the Major arteries are yellow, or orange (Depending on the Navigator) the back roads are usually just blue lines. Confusing when you got to a bridge, then try to look up the name of the street, only to find that it's a straight concrete culvert in the middle of Arizona (The valley of the Spun is covered in them) instead of a short-cut between neighborhoods.