Highways in the United States
The Interstate Highway System spans roughly 48,756 miles and connects nearly every major US city. Below: how the numbering system works and a few historically significant routes.
How Interstate Numbering Works
Odd-numbered interstates (like I-5 or I-95) generally run north–south; even-numbered ones (like I-10 or I-90) run east–west. Three-digit interstates are spurs or loops connecting to their two-digit parent route — I-405 branches from I-5, for example.
Before the interstate system, the US Numbered Highway System — created in 1926 — handled long-distance travel. Route 66, decommissioned in 1985 after being superseded by interstates, remains the most culturally iconic of these older routes.
Planning a Drive?
See our California, Texas, and New York road trip guides for specific route recommendations and timing.
Highways in the United States — Quick Facts
- Total Interstate Mileage
- ~48,756 miles
- System Established
- 1956 (Federal-Aid Highway Act)
- Longest Interstate
- I-90 (Seattle, WA to Boston, MA)
- Numbering Convention
- Odd = N–S, Even = E–W
- Historic Predecessor
- US Route 66 (decommissioned 1985)