Primary Interstate Highways are the major interstate highways of the United States and have a one or two-digit route number. Even (0, 2, 4, 6, or 8) route numbers are given to east/west routes, with the smaller numbered routes in the south (I-10) and bigger numbered routes in the north (I-90). Similarly, odd (1, 3, 5, 7, or 9) route numbers are given to north/south routes, with the smaller numbered routes in the west (I-5) and the bigger numbered routes in the east (I-95). Routes that end in a 0 or a 5 are major routes in the system. These Interstates generally go from coast to coast or from the bottom to the top of the U.S. For example, Interstate 5 goes from the Mexican Border at San Diego, California to the Canadian Border at Blaine, Washington. Likewise, Interstate 10, the southernmost cross-country Interstate Highway, goes from Santa Monica, California on the west coast to Jacksonville, Florida on the east coast. The longest Interstate Highway in the United States is Interstate 90.
There are five sets of numbers that are used more than once throughout the system; the highways whose numbers are used more than once are separated by big distances to avoid confusion. Below, these are made separate from each other by West and East.
Three-digit Interstates are spur or loop Interstates that usually are in big cities and areas outside of cities. The last two digits of a three-digit Interstate are always the number of the route it is related to. If the first digit is odd (1, 3, 5, 7, or 9), the Interstate is usually a spur route while if the first digit is even (2, 4, 6, 8, or 0), the Interstate is usually a loop route.
The Interstate Highways in the island of Oʻahu, Hawaii are signed with the normal Interstate shield, with the letter "H-" added before the number. They are fully controlled-access routes built to the same standards as the Interstate routes in the main part of the United States. These are marked and considered interstates.
The Federal Highway Administration gives money to four routes in Alaska and three routes in Puerto Rico under the same program as the rest of the Interstate Highway System. However, these routes do not have to meet the same standards as the Interstate routes in the main part of the United States. Note that these are not marked and considered as interstates, but state and territory highways.
Highways on the Interstate System in Alaska and Puerto Rico shall be designed in accordance with such geometric and construction standards as are adequate for current and probable future traffic demands and the needs of the locality of the highway.
Like Alaska, Puerto Rico signs its Interstates as territorial routes, and the numbers are not the same as their official Interstate designations. Many of the territory's routes are freeway-standard toll roads.