US 89 Nogales

One of our US 89 "puzzles" is "Precisely where did US 89 begin at Nogales?"  Was it right at the exact international boundary?  Or was it on the north side of the buffer zone that existed for may years along the border there?  In some of the photos below you can clearly see the buffer zone to which we refer.  Chances are pretty good that it started right at the  international line.  However, in the interest of historical accuracy, we hope someday to determine once and for all precisely where US 89 started there.

https://youtu.be/NVJ0HvKting