The southern end of I-17 begins where 24th Street goes under The Maricopa Freeway near the far southwest corner of Sky Harbor Airport. Technically, I-17's POB (Point Of Beginning) is Milepost 193.89 but there is no sign marking the highway's actual POB. The sign at I-17's POB is actually Milepost 150 for I-10.
"I-17 has the unusual distinction of starting at approximately milepost 193.89 instead of at milepost zero. This is a holdover from Arizona's old system of marking mileposts, where a branching route would continue the milepost numbering of its original host, instead of starting over at zero. Interstate 17 inherited its milepost locations from SR 69, which the freeway replaced between Phoenix and Cordes Junction. SR 69's mileposting was such that it coincided with US 89's mileposting, which was 201.6 where the two routes intersected. When I-17 was constructed, the existing mileposting for SR 69 was retained."
(Source 1)
We were able to learn of I-17's precise southern POB by studying an ADOT Right-Of-Way document. the key information is shown above. It appears from the ADOT document that the west edge of the 24th Street alignment is actually the true POB for I-17 but it may actually be the centerline of 24th Street. We are attempting to clarify that very minor fine point.
(Source 2)
Above is the very nondescript spot on 24th Street where I-17 begins on the overpass deck above. We visited this site at 8:30 AM, January 4, 2017.
For anyone wishing to visit I-17's southern POB, we recommend parking in the Best Western motel lot a short distance south of The Maricopa Freeway. (See "A" in above photo.) There is a safe sidewalk on the west side of 24th Street. Proceed under the Maricopa and then climb the north embankment of the freeway. (See "B" in above photo.) Stay well outside the guardrail for your safety.
The I-17 sign above appears to be the first actual I-17 marker. The location is actually rather picturesque. It is adjacent to the landing pattern of one of the 3000-acre Sky Harbor Airport's busy runways. The site also affords a nice distant view of downtown Phoenix and a stand of nearby mature palm trees add to the interstate highway ambiance. Abundant urban photo ops abound.
Our next project will be to visit and document the northern end of I-17 in Flagstaff.
