Just for the hell of it, I decided to make a list of all known places where the US Interstate Highway System's numbering system for 2-digit routes is violated.

The 2-digit numbering system is based on the principle that:

• north-south highways are numbered in increasing order as you go from west (I-5 in California) to east ( I-97 in Maryland)
• east-west highways are numbered in increasing order as you go from south (I-4 in Florida) to north ( I-96 in Michigan)

This standard is violated for a number of perfectly good reasons:

• Some interstate highways are diagonal rather than strictly east-west or north-south, especially those that follow the southeastern US coastline or the Appalachian Mountains
• Some highways were extended beyond their originally planned terminus
• Some highways were renumbered in the 1970s, when the authorities decided to get rid of numbers like "15W" and "80S"
• Some highways were relocated from their original planned alignment

When you think about it, it's actually surprising how _few_ times the standard is violated.

Anyway, here's the list, as taken from my 1993 AAA Road Atlas. Additions and corrections are welcome.

East-West Interstates:

• I-20: See I-26
• I-24: Crosses I-40 at Nashville, Tennessee. The section from Nashville southeast to Chattanooga is south of I-40. Reason: I-24 is a diagonal Interstate.
• I-26: Crosses I-20 at Columbia, South Carolina. The section from Columbia southeast to Charleston is south of I-20. Reason: I-26 is a diagonal Interstate.
• I-40: See I-24, I-44
• I-44: Crosses I-40 at Oklahoma City. The Bailey Turnpike section, from Oklahoma City southwest to Wichita Falls, Texas, is south of I-40. Reason: I-44 is diagonal, and the Bailey Turnpike was added to I-44 after the original numbering plan was designed.
• I-70: See I-74
• I-74: Crosses I-70 just west of Indianapolis. The section from Indianapolis southeast to Cincinnati is south of I-70. Reason: I-74 is a (somewhat) diagonal Interstate. (Note: If I-74 is extended into the Carolinas as proposed, the "violating" section will be much longer.)
• I-82: All of this highway, from Ellensburg, Washington to Nowhere, Oregon (where it meets I-84), is north of I-84. Reason: I-84 used to be I-80N; when it was changed, I-82 wasn't.
• I-84: see I-82
• I-90: Splits from I-94 in Chicago, then crosses it in Gary, Indiana. The section from the Dan Ryan Expressway to Gary is north of I-94. There is no violation west or east of this small section. Reason: No good reason that I can see. In fact, I-90 and I-94 were originally numbered the opposite way here, and there was no violation then!
• I-94: See I-90, I-96
• I-96: Crosses I-94 in Detroit, and continues for another mile southeast to I-75.

North-South Interstates:

• I-59: Crosses I-65 at Birmingham, Alabama. The section from Birmingham northeast to Chattanooga is east of I-65. Reason: I-59 is a diagonal Interstate.
• I-65: See I-59
• I-69: Crosses I-75 at Flint, Michigan, and continues due east to the Canadian border at Port Huron. Reason: this section was added after the original numbering plan.
• I-71: Joins I-75 a few miles south of Cincinnati, and splits from it in Cincinnati. The section from Cincinnati to Cleveland -- which is nearly all of I-71 -- is east of I-75. Reason: I-71 is diagonal. But the numbering is still a bit puzzling, given the relative length and importance of the non-violating and violating sections.
• I-75: See I-69, I-71, I-85
• I-77: see I-81
• I-81 : Joins, then splits from, I-77 near Wytheville, Virginia. For a few miles you are actually going simultaneously north on I-77 and south on I-81. The section of I-81 southwest of this point is west of I-77. Reason: I-81 is a diagonal Interstate that follows the Appalachian Mountain range. Much of it, including this section, is largely east-west.
• I-85: Crosses I-75 in Atlanta. Crosses I-77 in Charlotte, North Carolina. The section from Montgomery to Atlanta is west of both I-75 and I-77, and the section from Atlanta to Charlotte is west of I-77. Reason: I-85 is a diagonal Interstate, and much of it is largely east-west.
• I-87: Crosses I-95 in the Bronx, New York City, and continues south for another 3 or so miles. (Where does I-87 officially end?)
• I-89: Crosses I-91 at White River Junction, Vermont. The section from White River Junction southeast to Concord, New Hampshire, is east of I-91. Reason: I-89 is a diagonal Interstate.
• I-91: See I-89
• I-93: Crosses I-95 at Reading, Mass, north of Boston. I-95 detours counterclockwise around Boston on Mass. Route 128, while I-93 runs straight through the heart of the city, then turns clockwise on Route 128 until it meets I-95 again in Canton. Reason: I-95 was originally intended to go through Boston, not around it. In this plan, I-93 and I-95 would have joined in Charlestown, just across the Charles River from downtown Boston, and I-93 would not have continued south of this point.
• I-95 : See I-87, I-93