The highway that refuses to start at mile zero

Science · Published 20 October 2026 · Updated 21 October 2026 Swipe more like this →

In short: Most highways begin their journey at milepost zero, but Arizona's Interstate 17 defies this convention. This north-south artery begins at milepost 194, a quirk of history where the road inherited its numbering from a predecessor rather than resetting the clock. It is a physical artifact of old surveying logic.

Interstate 17 (I-17) serves as a primary north-south corridor in Arizona, connecting Phoenix to Flagstaff. While it functions as a modern high-speed freeway, its numbering system is a holdover from a bygone era of state route management. The freeway's milepost markers were taken from State Route 69, a road it substituted for in the stretch connecting Cordes Junction and Phoenix. Because the old system dictated that branching routes continue the milepost numbering of their original host, I-17 began at milepost 194 instead of starting over at zero.

This legacy numbering is tied to the original intersection of SR 69 and US 89, which was marked at milepost 201.6. Even as the federal government established the Interstate Highway System in 1956 and designated the route as I-17, the existing mileposting for SR 69 was retained. This creates a unique geographical anomaly where the highway's numerical identity is anchored to a road that no longer exists in its original form.

Beyond its numbering quirks, I-17 is a feat of engineering and history. This path mirrors the 1878 inaugural stagecoach trail traversing the Black Canyon. The highway climbs significantly, gaining over one mile in elevation between Phoenix at 1,117 feet and Flagstaff at 7,000 feet. To manage the heavy weekend traffic between Phoenix and the Grand Canyon, the state recently implemented 'flex lanes' between Anthem and Sunset Point, using six different safety systems to allocate lanes based on real-time demand.

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