I-17 has the unusual distinction of starting at milepost 194 instead of at zero. This is due to a holdover from Arizona's old system of marking mileposts on highways that did not cross into another state. I-17 inherited the mileage from AZ 69 which it replaced, and the original mileage for that highway began at the intersection with US 89 at milepost 201. When I-17 was constructed, the original mileage for AZ 69 was kept, starting back a few miles since the new freeway began a few miles before the old highway.
There has been some talk of extending I-17 northward from Flagstaff to follow US 89 to its intersection with I-70 in Utah, completing an interstate grade connection between Phoenix and Salt Lake City to the north. While the idea has merit in that both cities are rapidly growing and current traffic between them generally is routed to the west through Las Vegas, the proposal would carry significant costs and environmental impacts and has not been seriously considered by either state.
I-17 and I-19 could and should hook up (via I-10 between Phoenix and Tucson), become one longer interstate and be renamed I-21.
Interstate17 is the main north-south freeway in Arizona, connecting Phoenix with Flagstaff.
Interstate17 is a special case: Its exit numbers are actually those of the former longer routing of Arizona69, which branched from U.S. 89 at Mile 201, since Interstate17 and Arizona69 were paired early.
Southbound Interstate 17/Arizona 89A at the cloverleaf off-ramp for Interstate40 east/Exit 340A.
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