Great find, Michael! Although further viewing leads me to believe that this road remains in New Mexico the whole time.
Not to get too off-topic, but US 340, which runs through Virginia, into West Virginia and then back into Virginia for only several hundred feet is thought to be the shortest. This location is near the MD/VA/WV tripoint.
http://www.topozone.com/map.asp?lat=39.32360922847994&lon=-77.706354276978&s=25&symshow=n

Also up for consideration is US 161 in the Four Corners area (NM/CO/AZ/UT quadpoint).
http://www.topozone.com/map.asp?lat=36.996859594201005&lon=-109.03193193499461&s=25&symshow=n

for the very shortest such consider http://www.topozone.com/map.asp?z=13&n=4041216&e=67516 8&s=25 at the northwest corner of the texas panhandle where both rumor & fence lines indicate texas gets barely nipped by the eastbound lane & not at all by the westbound lane of us56&64 depending on which side of the vehicle one is sitting in the travel distance might be measured in inches rather than feet