Between 1961-63, Arizona finished paving its segment of what is now mostly US 160 (from US 89 west of Tuba City to the NM state line east of Teec Nos Pos, where it connected with NM 504 to Shiprock). This route was initially signposted as AZ 64 (the designation was extended eastward from Cameron). A few other roads were also paved, and were numbered as branches of AZ 64. AZ 264 began at Tuba City and headed east to New Mexico (as it still does). AZ 364 went northeast from Teec Nos Pos (along modern US 160) to Four Corners. The road heading north from Kayenta to Utah (which had just been paved a couple years earlier) was given the AZ 464 designation:
Offically, on its west end, US 164 overlapped with US 89 southward, ending in Flagstaff; and on its east end, it overlapped with US 666 northward, terminating in Cortez:
Just a few years later (in Oct. 1969), AZ, CO, NM, and UT (or at least some of those states) proposed to AASHO that the US route designations in the Four Corners area should be shuffled thus:
It is possible that the inappropriateness of the number 163 was considered, but in the absence of any more logical odd number, US 163 was deemed to be as good as any. After all, the proposed route did not connect with any north-south US routes. It had a junction with US 6-50 on the north, US 666 in the middle, and US 160 on the south. So was a better numbering option even available? One suggestion would have been to commission US 289, branching off from US 89 near Tuba City. From there it would run through Kayenta and Monticello, ending at Crescent. US 160 would end at its junction with US 289 in Kayenta, and US 666 could have been extended north to Monticello as planned.
The next year, Utah and Wyoming actually sought an extension of US 163 to the north, essentially along what is now US 191, ending at the south entrance to Yellowstone. This would have eliminated the US 187 designation in Wyoming, but AASHO denied the proposal in both 1971 and 1972.
In Arizona, the former AZ 64 (which by then had probably been changed to AZ 504) was the road which continued west to Teec Nos Pos and the junction with US 160. So at that time US 64 was only 54 miles away from reaching Teec Nos Pos. (Note: it was 1973 when AASHO became AASHTO.)
In 1981 a major change was made to the US 191 designation: it was extended south through Wyoming and Utah, ending in Chambers AZ.* The Utah segment of this extension included a 136-mile stretch along US 163 (from Crescent Jct. to Bluff). Rather than dual-signing that segment, reportedly by 1985 Utah had removed all US 163 signs from it... although it was more than two decades before they formally asked AASHTO to decommission that part of US 163. So, even though US 163 signage ended at Bluff, AASHTO records for the next 20+ years indicated that US 163 still continued all the way up to Crescent. *(In 1992, US 191 was extended south from Chambers, replacing the former US 666 all the way to Douglas. Then in 2003 the remainder of US 666 through NM, CO, and UT was replaced with the US 491 designation. Both of these were good changes which improved route continuity, reduced sign theft, and replaced an even number on a north-south route with a more-appropriate odd number.)
In 1989 the US 550 designation was truncated at Farmington*, and instead US 64 was extended west via former US 550. But US 64 went 27 miles beyond US 550's historic terminus in Shiprock, ending at its junction with US 160 at Teec Nos Pos (the arrangement that still exists today). *(In 1998 New Mexico obtained approval to truncate the US 550 designation at Aztec, and instead extend it down to Bernalillo, which remains the current arrangement).
Catching up to the present day
Since the 1920s, several paved highways have been added in northern Arizona and southern Utah, greatly improving accessibility in the area. During the development years, some route numbering decisions were made which later needed to be changed. For example, the highway between Crescent Junction and Monticello has carried four different US route designations: US 450, US 160, US 163, and US 191. Similarly, the Monticello-Cortez segment has been US 450, US 160, US 666, and US 491. The result of these changes is that many of the early numbering problems have been fixed:
Proposed changes to the US 64 and US 160 designations
It was a good move to unify the highway between Farmington and Teec Nos Pos under the US 64 designation. Unfortunately, the proposal did not go far enough: the US 160 designation should have been truncated at Teec Nos Pos, and US 64 should have replaced it to the west. Even if US 64 went only as far as Tuba City and ended at US 89 (where US 160 ends currently), it would create several positive improvements:
Additional benefits under that scenario:
Proposed change to the US 163 designation
As noted previously: if the US 64 designation were to be extended through Kayenta, then that would set the stage to renumber US 163 as a branch route of US 64, specifically "US 164". Benefits:
Over the decades, highway projects have gradually improved access to the Four Corners region of the Colorado Plateau. Historically, the completion of these highways sometimes necessitated new US route designations and/or changes to existing designations. That process is not quite complete: some of the US highway designations in the area still need to be changed. This article highlights the specific segments that should be redesignated, and explains the rationale for the proposed changes.