After the initial interstate routes were designated, additional corridors became eligible for interstate highway funding. Since the initial routes had used all the available numbers ending with “0” and “5,” an alternate numbering system was developed using even and odd numbers, as follows:

The number selected would generally fall in the normal sequence between two existing interstate routes. For example, IH-27 is located between IH-25 (New Mexico, Colorado, Wyoming, etc.) and IH-35 (Texas, Oklahoma, Kansas, etc.).