but yes, even in that case, the number "20" is very strange. is it 20 miles to the western end of Texas, and if so, is US-54 formally east-west in the state logs even though it is signed north-south? (I believe the other segment of US-54, in the panhandle, is signed east-west.)
Here is US 54 in the "state logs":
http://www.dot.state.tx.us/tpp/hwy/us/us0054.htmIt says nothing about "book direction." (TxDOT's
Sign Crew Field Book recommends that routes be signed by book direction--generally understood to be east-west for even-numbered routes and north-south for odd-numbered routes--but this rule is often ignored.) The certified mileage is given but this is total mileage for the two segments and does not refer to mileposting.
I note that I-10/US-54 is exit 22 on I-10, giving credence to my idea that it's 20 miles to the New Mexico line by some abstract measurement.
AIUI, the way the location reference system works in Texas (full details are given in the relevant manual) is that one point along a route or route segment is given a grid coordinate, and other mileposts are derived by counting up and down from the abscissa (in the case of east-west routes) or ordinate (in the case of north-south routes) of that grid coordinate, which functions as an "origin" for the route. So, in the case of SH 130, Exit 430 is not necessarily exactly 430 miles south of the north-edge-of-panhandle latitude; rather, it is X miles along the SH 130 alignment from the origin point, plus the distance of that origin point from the north-edge-of-panhandle latitude. This allows reference post numbers to correspond to mileage along the route when it does not run perfectly north-south or east-west.
In the case of US 54, 20 is far too low to correspond to the distance from the north-edge-of-panhandle latitude, and it seems to me just a smidgeon too high to correspond to the distance from the western-tip longitude. So my theory is that I-10's reference post at the center of the US 54 interchange has been assigned to the El Paso segment of US 54 as its origin point. Assuming that I-10's origin point is the abscissa of its crossing of the Texas/New Mexico border, the mileage would be approximately correct notwithstanding the fact that I-10 runs almost due south from New Mexico to downtown El Paso. (It is of course possible that "20" has just been arbitrarily added to the exit numbers; I am not sure how we could know for sure without actually getting in contact with TxDOT's El Paso district and asking them.)