Oh, that's an easy one.. Basically it's called "route continuity". Over time some old highways, or more commonly parts of old highways, get abandoned and physically drop off the system. This might be because an entire new highway is built and it's close enough in some areas that the two highways, the old one that's being abandoned, and the new one can be combined into a single highway for some distance. But people who start their trip out of town on the still existing part of the old highway find it a lot easier to just follow the signs for that old highway number even where it "no longer exists". So they stick it's number up on the stretch of new highway that has replaced it. And the old highway might still exist 100 miles farther on and separate from that new highway. So you could have old highway 50 and new highway 70 with parts of new highway 70 also signed as highway 50 so that highway 50 is still a "continuously signed route". It's a lot easier to explain with a drawing on a piece of paper. I've seen places where one physical highway carries route designations for 3 or more routes I-10/US60/US70/SR93.