Directions
Even Numbers travel east-west (I-4, I-8, I-10, I-12, etc.)
Odd Numbers travel north-south (I-5, I-15, I-17, I-19, etc.)
Significance
Routes ending in "0" are major east-west routes (I-10, I-20, I-40, I-70, I-80, I-90)
Routes ending in "5" are major north-south routes (I-5, I-15, I-25, I-35, I-55, I-65, I-75, I-85, I-95)
Orientation
Lower numbered routes are generally located in the south and west
Higher numbered routes are generally located in the north and east
As a result, the Interstate Highway system forms a grid with numerical designations increasing gradually from low to high - both from west to east and south to north.
Interstate Highways were intentionally numbered so that they would not conflict with the preexisting U.S. Numbered System; in fact, the intent was that no Interstate Highway and U.S. Highway would share the same number within the same state.
I-24 and U.S. 24 currently both exist in Illinois.
Current plans for extensions to I-49, I-69, and I-74 will result in those routes meeting and intersecting their U.S. Highway counterparts.
Interstate 50 and Interstate 60 were not assigned so as to avoid conflicts with U.S. 50 and U.S. 60 in the central part of the country.
Some Interstate Highways are Unsigned, including a variety of spur and loop routes that are often superfluous or confusing designations in addition to the primary route number.
Loop routes and through routes that generally connect to an Interstate highway at either end have an even first digit.:
Full 360-degree beltways include I-270 OH, I-275 KY/OH/IN, I-285 GA, I-410 TX, I-465 IN, I-495 MD/VA, I-610 TX, I-695 MD .
I-275 in KY, IN, and OH is the only three-digit Interstate currently to serve three states.
I-495 (Capital Beltway) serves Virginia and Maryland, but a portion of the route passes through the District of Columbia while crossing the Potomac River at the Woodrow Wilson Bridge.
I-271 and I-480 merge together briefly east of Cleveland, marking the only location where two three-digit Interstate routes merge together.
The following routes have an even first digit but do not end at another Interstate highway on one end: I-264 VA, I-295 NY, I-478 NY, I-495 NY, I-678 NY, I-635 TX, I-878 NY.
The following routes are examples of intercity routes, acting in a regional capacity: I-280 CA, I-476 PA, I-495 MA, I-680 CA, I-684 NY/CT
I-238 in CA acts as a spur of I-80, connecting I-880 and I-580 in the San Francisco Bay Area. At the time it was commissioned, no more I-x80 route numbers were available for use, so the designation of adjacent CA 238 was used to number I-238.
Some even-prefixed three-digit routes serve states that are not served by their parents, including I-275 OH/KY/IN and I-287 NJ/NY.
Spur routes and city routes that may or may not connect to an Interstate highway at one end have an odd first digit:
I-585 in SC does not connect to its parent (I-85) currently; it begins at Business Loop I-85 and ends in downtown Spartanburg.
The following routes are examples of intercity routes, acting in a regional capacity: I-380 PA, I-385 SC, I-505 CA.
Some odd-prefixed three-digit routes serve states that are not served by their parents, including I-129 NE/IA and I-535 MN/WI.
Some Interstates have state route extensions, including I-15 in San Diego (continues as CA 15), I-265 in New Albany (continues as IN 265), I-381 in Bristol (continues as VA 381), I-481 in Syracuse (continues as NY 481), I-690 in Syracuse (continues as NY 690), and I-794 in Milwaukee (continues as WI 794/Lake Parkway).
Some proposed future Interstate corridors have state route designations for now, including TN 840 near Nashville and CA 905 in San Diego.
Still other state routes have numbers that may make it look like a future Interstate corridor but are not, including VT 191 in Newport and IL 394 near Chicago Heights.
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