1. Primary Interstates: These are the major east-west or north-south routes in the Interstate System. They are numbered in even numbers (e.g., I-2, I-4, I-6, etc.). North-south primary routes with even numbers are located to the west of those with odd numbers. East-west primary routes with even numbers are located to the north of those with odd numbers.
2. Auxiliary Interstates: These are routes that connect to the primary routes or provide bypasses around urban areas. They are numbered in odd numbers (e.g., I-1, I-3, I-5, etc.).
3. Suffixes: Some interstates have suffixes, such as A, B, or C. These suffixes are used to differentiate between multiple routes that share the same number. For example, I-405 in California is a spur route of I-5.
4. Three-Digit Interstates: Three-digit interstates are genellikle loops or spurs of higher-numbered interstates. The first digit of a three-digit interstate indicates the region of the country where the route is located (e.g., I-105 in California, I-295 in the Mid-Atlantic region, I-495 in the Northeast region, etc.). The second and third digits indicate the specific route or routes that the three-digit interstate connects to.
As the Interstate System has grown over time, some exceptions and deviations from these numbering rules have been made. However, the general principles outlined above still play a significant role in determining how interstate highways are numbered.
