I-17 has the unusual distinction of starting at milepost 194 instead of at zero. This is apparantly 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.