CA-23

[TheStranger]

I wonder if the AASHTO definition of the route includes the bridge (I know legislatively it isn't such). 

Of course it does. The only gaps are ferries and Yellowstone (and US 2).

I wonder why Yellowstone gets the "special treatment" as AASHTO route gap, yet other parks allow routes to continue officially through them.

In any case, while it's easy to suggest them as a final authority for numbering, the US 377 saga in Oklahoma makes this a little bit more of a gray area.

[xonhulu]

I wonder why Yellowstone gets the "special treatment" as AASHTO route gap, yet other parks allow routes to continue officially through them.

This has been discussed a number of times on here, but a new thought just occurred to me: it's possibly because there are no other examples of US Highways crossing a park where the park predates the establishment of the US Route System in 1926. 

Here's a list of national parks established before 1926 (I realize I'm leaving out a little history on some of these):

Yellowstone NP - 1872
Sequoia NP - 1890
Yosemite NP - 1890.
Kings Canyon  NP - 1890
Mount Rainier  NP - 1899
Crater Lake NP - 1902
Wind Cave NP - 1903
Mesa Verde NP - 1906
Petrified Forest NP  - 1906 as NM, redesignated NP 1962
Lassen Volcanic NP - 1916
Zion NP - 1919
Glacier NP - 1910
Rocky Mountain NP - 1915
Hawaii Volcanoes NP, Haleakala NP - 1916
Denali NP - 1917
Grand Canyon NP - 1919
Hot Springs NP 1921
Bryce Canyon NP - 1924

Of these, only Rocky Mountain NP shares the distinction with Yellowstone of having a US Highway cross it, but US 34 didn't get extended through the park until 1939, and US 36 didn't enter the park until 1978.  And while there are a few US 34/36 directions in the park itself, the routes really aren't signed in there. 

Petrified Forest also had a US Highway (66) crossing it, but I think the Painted Desert section it crossed was added to the park later.  As for the rest, none have US Highways crossing them - that is, enter on one side, exit on another.  So Yellowstone is pretty unique in that regard.