Gaps in US highways

The initial reason I envisioned this page was because there were a couple factoids that needed to be addressed: 1.) US routes do not exist within the boundaries of Yellowstone National Park, and 2.) for that reason some US highways consist of two non-contiguous segments.  But then of course there are other US routes with gaps that have nothing to do with Yellowstone.  So ultimately I decided to create a page which covers all of the US routes that have various types of discontinuities.  Some of these discontinuities have been acknowledged by AASHTO, while others have not, so they can be classified into two broad categories: official gaps and unofficial gaps.

That is a term that is often used in the road enthusiast community, as in "US 89 has an implied route through Yellowstone".  I have no objections to that term, as long as we can agree on exactly what is "implied".  Since there were (and are) US routes with the same number on opposite sides of the Park, it is obvious there was an implication that drivers on one of those US highways could pass through Yellowstone and then resume travel on that same route.  However, no specific route through the Park has ever been implied.  For example: is US 89 traffic supposed to use the west Grand Loop or the east Grand Loop?  That is a question that cannot be answered, because either option would work (as would any hybrid option using some combination of both sides).  So it is up to each individual driver, and NPS has never made any attempt to influence that decision by defining the route of US 89 through Yellowstone.

So already by 1935, US 89 consisted of two separate sections.  Ever since then, if someone wants to know the mileage of "US 89", the answer is actually somewhat complicated.  One option would be to simply add the mileage of US 89[n] and US 89[s].  That is the approach AASHTO typically took, back in the days when they published route logs.  On their most recent (1989) route log cover page, they provided a figure of 1837 miles for US 89.  But they also pointed out those numbers included any associated bannered routes, and when we examine the 1989 route log details, we find that US 89's total included US 89[n] + US 89[s] + two US 89 alternate routes.  So, obviously not an accurate reflection of end-to-end mileage.  When we subtract out the alternate routes, AASHTO's 1989 measurement of mainline US 89 would have been 1661 miles.  However, that does not account for the mileage one would have to travel through Yellowstone in order to bridge the gap between the north and south segments of US 89.  But on the other hand, since US 89 does not exist in Yellowstone, any number which includes mileage in Yellowstone would also produce an inaccurate total.  And as if all that is not enough, US 89 has since been truncated from Nogales to Flagstaff.  So, taking all of that into account, as of 2020 our own mileage breakdown was as follows:

• US 89[n] (all in MT): 399.7 miles

• ​US 89[s] (WY through AZ): 839.3 miles

• Both segments of mainline US 89 combined (which is a somewhat meaningless total because they are not connected): 1239.0 miles

• Total mileage along US 89 between Flagstaff AZ and Piegan MT, including the shortest connection through Yellowstone (which is misleading because that segment is not a part of US 89): 1333.8 miles​

In the original 1926 list of US routes, there was only one US 422: the one running between Cleveland and Ebensburg PA.  But Pennsylvania did not begin signposting their routes until 1928 because they had numerous objections to AASHO's plan, and they threatened to leave their US routes unmarked unless AASHO acquiesced to their demands.  One of their requests was to re-number the segment of US 120 between Philadelphia and Reading as US 422.  AASHO immediately recognized that this would create two non-contiguous segments of US 422, and in June 1927 they questioned Pennsylvania about it:

It should be acknowledged that in AASHTO's latest route log, there was one additional highway that was listed as having two separate sections: US 10.  This was because of its ferry segment across Lake Michigan.  It seems odd that US 10 was considered to have two segments, whereas US 9 (which also has a ferry segment) was listed as a single route.  Regardless, the point is moot now, because a 2015 AASHTO action made the Ludington-Manitowoc ferry an official part of US Route 10.  So US 10 no longer has an official gap in its route.  However, I do consider both US 9 and US 10 to have unofficial gaps; I discuss those more later on this page.

Above I mentioned that US 191 is the one exception to the statement that no US routes go through Yellowstone (this blog post has more info).  To be clear: US 191 does have a gap through the Park, between West Yellowstone and the south entrance (as noted above).  However, the Montana segment of the highway between West Yellowstone and Big Sky is all considered by AASHTO to be part of US 191.  That includes a 20.2 mile segment that crosses inside the northwest corner of Yellowstone (although there is no entrance gate/fee station on either end).  US 191 is reportedly even signposted through that Yellowstone segment (which includes a 5.9 mile stretch that passes in and out of Wyoming).

So it is not entirely accurate to say that no US routes exist in Yellowstone.  Moreover, it is absolutely incorrect to assume that the National Park Service has a broad policy that disallows US route designations within any of its holdings.  For example, US 34 and US 36 both run through Rocky Mountain NP; US 33 and US 211 both run through Shenandoah NP.  So it is up to individual Parks to decide whether or not to signpost US routes within their boundaries.  However, there is another National Park that is similar to Yellowstone in terms of how it handles US routes, and that brings us to another subcategory...

...or, in this next example involving US 441, it would be more accurate (but also more verbose) to say "Gaps made official when AASHO acknowledged that they do not have the authority to designate US routes within National Parks".

Until the early 1950s, US 441 ran only between Florida and Georgia.  But in 1951 GA, NC, and TN asked AASHO to extend the route north through Great Smoky Mountains National Park.  Each of those state DOTs cited a desire to make travel to the Park easier for its many visitors:

By "unofficial" I mean the gaps in these routes were neither created by AASHTO nor are they recognized by AASHTO.  Nevertheless, it would be absurd to act as if they do not exist.  I have classified these unofficial gaps into two subcategories: "defacto gaps" (which physically cannot be traversed) and "perceived gaps" (which are not technically gaps but can easily be perceived as such), and under each of those headings are additional subclassifications.  Let us begin with real, physical gaps:

US 34 and US 212 both go over high-altitude mountain passes that receive heavy snowfall.  These road segments are not cleared of snow during the winter months, so they are closed to traffic seasonally.  And winter lasts a long time in the Rockies; in fact, on an annual basis these sections are actually closed to traffic for a longer period of time than they are open.  The red segments shown on the maps below are closed annually, which has the effect of dividing their respective US routes into two disconnected sections:

Motorists also perceive a gap in a US route in places where no US highway shields are posted on the road to reassure them, or to direct them to make a turn, or to take an exit.  There are innumerable examples of missing signs, and in most cases the lack of signage is unintentional on the part of the state DOT.  Signs get damaged, or they fade to illegibility, or they get stolen.  Or sometimes the sign maker (either the DOT sign shop or a private contractor) omits a US route shield simply due to oversight or human error.

However, there are some DOTs that intentionally do not sign some of their US routes.  Most often this is done in places where the US highway joins with an Interstate highway, but only the Interstate route is signposted.  From AASHTO's perspective the US route exists along with the Interstate, but from a driver's perspective there is nothing to inform them that the US route exists along that segment.  The reason commonly given for this type of signage policy is to reduce expense, driver confusion, and "sign clutter".  Arkansas is known for this practice, as are some other states, but it is probably Colorado that is most notorious for it.  As just one example: between north Denver and the town of Byers, US 36 traffic ostensibly follows a 43-mile segment along Interstates 70 and 270.  However, there is no signage directing US 36 traffic onto those Interstates, nor is there a single reference to US 36 anywhere along that entire stretch.  And many other US routes that enter Colorado have a similar signage gap where they join an Interstate, including US 6, US 24, US 40, US 50, US 85, US 87, US 287.