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As far as I know, the only primary three-digit U.S. Route is US 101, and since this template only includes the primary U.S. Routes and not the auxiliary U.S. Routes, shouldn't US 163, US 400, US 412, and US 425 be removed from this list? I am aware that these routes are the only three-digit routes without parents, however this alone doesn't make them primary routes. Using this logic, parentless Interstate 238 would count as a primary Interstate, but Wikipedia gladly does not count it as such.
Also, I checked through the edit history of this template and found that the reason these routes are included here is that this template used to be split into "primary routes" (one- and two-digit routes + US 101) and "parentless three-digit routes" (US 163, US 400, US 412, US 425). When the two sections were later combined, it made US 163, US 400, US 412, and US 425 appear as primary routes alongside the one- and two-digit routes + US 101. If there isn't any good reason to keep them, I propose that US 163, US 400, US 412, and US 425 be removed from this template. Azmjc02 (talk) 02:28, 21 June 2021 (UTC)[reply]
The comparison between Interstate and US Highway nomenclature doesn't work. The primary–auxiliary divide in the former system is denoted by the number of digits in the designation: primary Interstates have one or two digits, and auxiliaries have three. In the US Highway System, auxiliary highways are special routes, and primary highways are not. So US 141 is just as primary as US 41, but Bus. US 41 is an auxiliary.
We do need though to keep navbox templates easy or navigate, or they fail to work well. Filling them with hundreds more links will clutter the box, pollute the "What links here" lists for individual highways and prompt people to deploy the template to many more articles. Instead, because the child primary highways are linked from the parents, this navbox only links to the parent primary highways to keep the size of the box more manageable, bar the few exceptions where something that looks like a child lacks a parent from which it would be linked. Imzadi 1979 → 21:56, 28 June 2021 (UTC)[reply]