It seems to me like there's logic to Pennsylvania's two-digit, even-numbered state routes. If the route number is X8, you're in western Pennsylvania. If the number is X6, you're in central-western Pennsylvania. If the number is X4, you're in central-eastern Pennsylvania. If the number is X2, you're in eastern Pennsylvania. As for X0, Pennsylvania has a law against state route numbers that are duplicates of Interstate or U.S. route numbers, which means that 10, 50, 60 and (technically) 100 are the only ones free. PA 10 and PA 100 serve the Philadelphia metropolitan area, and PA 50 and PA 60 serve the Pittsburgh metropolitan area.
Here are the counties that each route serves:
X8
PA 8 - Allegheny, Butler, Venango, Crawford, Erie.
PA 18 - Greene, Washington, Beaver, Lawrence, Mercer, Crawford, Erie.
PA 28 - Allegheny, Butler, Armstrong, Clarion, Jefferson.
PA 38 - Butler, Venango, Clarion.
PA 48 - Allegheny.
PA 58 - Mercer, Butler, Armstrong, Clarion.
PA 68 - Beaver, Butler, Armstrong, Clarion.
PA 88 - Fayette, Greene, Washington, Allegheny.
PA 98 - Crawford, Erie.
X6
PA 16 - Fulton, Franklin, Adams.
PA 26 - Fulton, Bedford, Huntingdon, Centre.
PA 36 - Bedford, Blair, Cambria, Clearfield, Jefferson, Clarion, Forest, Venango.
PA 46 - Cameron, McKean.
PA 56 - Allegheny, Westmoreland, Armstrong, Indiana, Cambria, Somerset, Bedford.
PA 66 - Westmoreland, Armstrong, Clarion, Forest, Elk, McKean.
PA 86 - Crawford.
PA 96 - Bedford.
X4
PA 14 - Lycoming, Tioga, Bradford.
PA 24 - York.
PA 34 - Adams, Cumberland, Perry.
PA 44 - Columbia, Montour, Northumberland, Union, Lycoming, Clinton, Potter, McKean.
PA 54 - Lycoming, Northumberland, Montour, Columbia, Schuylkill, Carbon.
PA 64 - Centre, Clinton.
PA 74 - York, Cumberland, Perry, Juniata.
PA 94 - York, Adams, Cumberland.
X2
PA 12 - Berks.
PA 32 - Bucks.
PA 42 - Columbia, Lycoming, Sullivan.
PA 52 - Chester.
PA 72 - Lancaster, Lebanon.
PA 82 - Chester.
PA 92 - Luzerne, Wyoming, Susquehanna.
X0
PA 10 - Chester, Lancaster, Berks.
PA 50 - Washington, Allegheny.
PA 60 - Allegheny.
PA 100 - Chester, Montgomery, Berks, Lehigh.
The evening numbering seems pretty consistent across the state, with maybe one or two outliers. One thing I don't see, though, is any consistency with two-digit, odd-numbered state routes. Maybe I'm missing something, though.
http://www.m-plex.com/roads/numbering.html
Interesting. I've seen this sort of thing with some county route systems, although they generally have the most significant digit indicate where you are, but here it's the least significant digit.