The Interstate Highway numbering system was established in 1957. The Interstate route marker is the familiar red, white, and blue shield with the word “Interstate,” the state name, and the route number.
Major Interstate routes have one or two-digit numbers. Routes running north and south have odd numbers, such as I-29. The lowest numbers are in the west, such as I-5 along the west coast. East and west routes have even numbers, such as I-70. Their lowest numbers are in the south, such as I-10 along the southern border. Major route numbers usually go through urban areas along the major traffic stream with the shortest, most direct line of travel.
Interstate Connecting Routes
Connector interstate routes are full or partial beltways around or in a city that connect two sections of a major interstate route. They use a three-digit number beginning with an even number. The second two digits denote the major interstate route. For instance, I-470 in Topeka is a partial connector around southern Topeka which connects I-70 on the west with I-70 on the east. I-435, a connector route for I-35, completely rings the Kansas City metropolitan area in Kansas and Missouri.
If there is more than one connecting route along a major route in any given state, the first number is different for each one. For example, to avoid duplication, I-470 in Topeka is different than I-670 in Kansas City.
Interstate Spur Routes
Interstate spur routes branch from a primary interstate route. They use three-digit numbers beginning with an odd number. The remaining two digits denote the primary interstate route. I-135 and I-335 are the only two spur routes in Kansas; both branch from I-35.
I-135 and I-335 are examples of multiple spur routes in a state being given different first digits to avoid duplication.
Mile Marker and Exit Numbers
The major interstate mile marker numbers begin at the western or southern borders of each state. For connecting and spur routes, the mile marker numbers begin at the western or southern starting points of the routes. In Kansas, exits (interchanges) are generally numbered according to the nearest mile marker number. So, Exit 1 on I-70 is located about one mile east of the Kansas/Colorado border. Exit 4 on I-35 is about four miles north of the Kansas/Oklahoma border. There are 276 interchanges on the Kansas Interstate System.
The numbering exception in Kansas is on portions of the Kansas Turnpike. The turnpike carries three different Interstate route numbers between the Kansas/Oklahoma border and the Kansas City area. The mile marker numbers for I-35 are used along the entire route to avoid confusing turnpike travelers. So, Exit 197 on the Turnpike is between Topeka and Lawrence, even though that section carries I-70 and the location is more than 370 miles east of the Colorado border.