Glossary
The terms, defined.
A short reference for the highway and numbering terms used across this site.
- Mainline (primary) route
- A one or two-digit Interstate that forms part of the core national grid.
- Auxiliary route
- A three-digit Interstate that branches from a two-digit parent as a loop, bypass, or spur.
- Parent route
- The two-digit Interstate named by the last two digits of a three-digit auxiliary route.
- Loop or beltway
- An auxiliary route that leaves its parent and rejoins it at both ends, usually circling a city. Typically an even first digit.
- Spur
- An auxiliary route that connects to its parent at one end only, usually reaching into a downtown or a port. Typically an odd first digit.
- Axis
- The general direction a mainline runs: north-south for odd numbers, east-west for even numbers.
- Controlled access
- A highway that traffic can enter or leave only at designed interchanges, with no driveways or cross streets.
- Terminus
- The point where a route begins or ends, often at a junction with another highway or an international border.
- Concurrency
- A stretch where two route numbers share the same physical roadway, such as I-20 and I-59 across Alabama.
- Non-contiguous route
- A federally recognized Interstate outside the connected lower-48 network, in Hawaii, Alaska, or Puerto Rico.
- Shield
- The red, white, and blue marker that identifies an Interstate route on signs.
- Suffix route
- A mainline split into lettered branches through a metro area, such as I-35E and I-35W.