Interstate-Guide: Interstate 17

http://www.interstate-guide.com/i-017.html

Between Phoenix and the Arizona 69 split (Exit 262), the freeway replaces old Arizona 69. This is significant because the exit numbering scheme still follows the original mileage of that route. According to the Misc.Transport.Road FAQ (Marc Fannin) based on comments from Richard C. Moeur, "Until the mid-1980s Arizona used a special system for its mileage -- any route which did not enter Arizona from another state used the mileage of the route where its southern or western end was for its point of origin rather than zero (e.g.: If Route 2's southern end was at Route 1 and Route 1's mileage at the Route 1/Route 2 intersection is Mile 57, then Route 2's lowest-numbered milemarker would be Mile 57). Interstate 17 is a special case: Its exit numbers are actually those of the former longer routing of Arizona 69, which branched from U.S. 89 at Mile 201, since Interstate 17 and Arizona 69 were paired early. Arizona 69 has been truncated since the exits of Interstate 17 were numbered (see Arizona 69 (Alan Hamilton)). The exit numbers on Interstate 17 are not related to those of Interstate 19."
---SPSmith