In the early 1990s, Wichita leaders helped to convince the US DoT of the importance of an east-west interstate through the city, and the proposed route (I-66) was included among the "High Priority Corridors" of ISTEA legislation. However, states to the west of Kansas offered no support, and some interests even expressed opposition. To the east, most states could not justify the cost or generate enough interest, so eventually the idea was dropped. Wichita would not be getting a new interstate after all, and it was back to the drawing board for KDoT. The agency had three main goals:
That last goal presented a challenge, because I-44 just misses the southeast corner of Kansas. So the connection with the Interstate System had to be made in another state, namely Missouri. It was probably for this reason that KDoT requested a US route number for their corridor (rather than simply using a state route designation). AASHTO not only granted their request, but reportedly they also let KDoT choose the number from a list of available options. Thus the US 400 designation was born in 1994.
An (essentially) intra-state US route
Way back in 1937, AASHO (as it was known back then) adopted a policy stating that "No new U. S. route located wholly in one State shall be established." 57 years later, when US 400 was originally commissioned, it went only as far west as Garden City. So the route was entirely in Kansas, except for its easternmost segment, which went a fraction of a mile into Missouri. Thus, while US 400 technically complied with the letter of the law, it certainly did not observe the intent.
Long overlaps with other US routes
At its commissioning, US 400's total length was 398 miles, but none of it was new construction. Rather, the entire route was along existing highways, and 270 miles (or 68%) of its distance was concurrent with other US routes. Two years later, the US 400 designation was extended 86 miles further west to its current terminus in Granada. So then US 400 was 483 miles long, spanning the entire length of Kansas, and ending about 15 miles into Colorado. But because the entirety of its extension was along US 50, that increased its redundant distance to 356 miles, or an astounding 74%.
At the time of US 400's inception, a 30-mile stretch was concurrent with US 160. But about six years later, the routing of US 160 between Pittsburg and Independence was changed, eliminating most of its overlap with US 400 (more on this below, under "Collateral Damage"). In addition, a few changes have been made to the routing of US 400 itself:
If we zoom in a little closer and check out the western two-thirds of US 400, we observe that along the 321-mile stretch between Granada CO and Leon KS, US 400 has only two short standalone segments, totaling only 37 miles. In other words, only 12% of western US 400 is solo pavement; the remaining 88% is piggybacked with other US routes. With that much redundancy, one might well question why there was ever a need for US 400.
Referenced above was the c. 2000 reroute of US 160. This was an irksome move, because that realignment increased the length of travel along US 160 by seven miles. Apparently KDoT decided the 30-mile overlap with US 400 was somehow wasteful, and they figured they could put the US 160 designation to better use by running it through the former K-96 corridor. But now US 160 jumps around so much that it resembles the line on a habitual liar's polygraph. Nobody would have a reason to follow US 160 between Pittsburg and Independence anymore. The reroute made absolutely no sense, and no one would have even considered it if US 400 had not been run along US 160's former alignment.
The number "400" is not compliant with US route numbering principles. It implies that the route is either a branch of US highway Zero (which does not exist), or else a sibling route to US 100, US 200, or US 300 (none of which exist either). Making it even worse is the fact that this was not just some misguided idea that Kansas came up with. No, AASHTO actually offered this number to KDoT. In other words: the organization that is ostensibly the guardian of the integrity of the US highway system gave its blessing to this completely nonsensical number.
In light of all these disqualifications according to its own policies, why was AASHTO willing to overlook all of that, and instead proceed with approving a US highway designation for this route? Perhaps it was viewed as a sort of consolation prize to Wichita, to make up for their failed interstate bid. But whatever the reason, it should not have been enough to justify the implementation of a route that violated so many of the expectations for a US highway.
Let's face it: US 400 is a Kansas intra-state route. True, it does cross into Missouri... but only for a fraction of a mile, and only to connect with I-44. The US 400 designation was never intended to provide any benefit to Missouri. To begin with, the entire Missouri segment of the route already held a US highway designation (US 166). And clearly KDoT was not interested in providing a connection between Wichita and a Missouri population center, namely Joplin or Springfield. Rather, they wanted an all-Kansas corridor with a connection to the interstate system. That is the only way to explain why the route takes such a drastic turn to the south, through the extreme southeast corner of Kansas, instead of providing a more direct connection with one of those Missouri cities.
It is also true that US 400 enters Colorado. But originally that was not the case. And, like Missouri, the entire Colorado segment is concurrent with another US route (US 50). The most likely scenario in 1996 was that KDoT decided they wanted to extend their corridor all the way to their western border, but they assumed AASHTO would not have allowed a US route to terminate at a state line*, so they asked CDoT to jointly apply for a western extension of US 400. Colorado agreed simply for the sake of being a good neighbor, but clearly CDoT has no use for the US 400 designation, as they terminate the route at the first US highway junction it encounters.
*this may have been an erroneous assumption, considering all of the other violations that AASHTO was willing to overlook with regard to US 400 designation.
So, to reiterate: US 400 is essentially a Kansas intra-state highway. There is no reason to begrudge KDoT's right to unify their southern tier with a single highway designation. But it did not have to be a US route designation. In fact, it most certainly should not have been a US route. For an intra-state corridor, the proper designation is a state highway. KDoT could have chosen any designation they wanted; they did not need to involve AASHTO nor invoke the US highway system. Let's say they decided on the number "123" for their state highway. They could have ended it at the Colorado line, and no one would have blinked an eye.
To summarize, there are many things wrong with the US 400 designation, and it was not a worthy addition to the US highway system. It should be decommissioned and replaced with a Kansas state highway designation.