Do want to note, since this has not been mentioned but is important: this is far from the only instance where there is a mismatch between a signed traffic route and the underlying SR number. Poke around the
Type 10 maps and you'll spot a bunch.
Often, these mismatches are because traffic route designations were moved around but the underlying SR numbers were not changed. For example, in center city Philly: PA 611 south of city hall is part of SR 0291. And PA 3 west of city hall is a one way pair of SR 3037 (JFK) and 3010 (Market) while SR 0003 applies to Chestnut and Walnut, a couple blocks south and not part of PA 3.
So, the section of Mercer Drive between US 62 and US 62Bus being SR 3035 does
not, by definition, mean it is not also PA 418.
And at any rate, signage is the final arbiter, at least so long as signage is consistent. If it's properly signed as PA 418, it is PA 418 for our purposes regardless of what it is on paper.