Last weekend we drove through Kansas City and Oklahoma City during parts of a visit to see our son. He is a pilot in training at Vance Air Force Base, Enid, OK. Most of the driving was done on the Dwight D. Eisenhower System of Interstate and Defense Highways. That link provides a large amount of information on the system begun on June 29, 1956. On that date, President Eisenhower signed the Federal-Aid Highway Act of 1956. This post is about one aspect of the interstate system, namely how the roads are numbered.
Numbering in General
East-west roads are even numbered. North-south roads are odd numbered. The even-odd scheme applies to the general route of the roads and allows for some deviation in places. The one and two digit numbers start small in the south and west. They progress larger for the roads toward the north and east. Here is an example of a few chosen roads.
Numbering Near Big Cities
Large urban areas pose some different challenges. I constructed a map for a fictitious location called Big City. Interstate highway I-60 passes through as an east-west route. There are two suburbs a few miles to the northeast and northwest. Notice there are two types of connections to I-60. There are loops and spurs.
A spur connects to the main artery at one end such as the roads 160 and 360. The first digit of a spur is an odd number. The next two denote the parent highway. In this case it is 60. A loop connects to the main artery at both ends such as 260 and 460. The first digit of a loop is an even number. The next two denote the parent highway.
As with all systems and schemes, there are some exceptions and oddities. The Federal Highway System has them, too. I invite you to read the link in Wikipedia for some comments about them. Enjoy your next road trip.