The US Interstate System might seem confusing at first, but it’s designed to make driving between states convenient. Let’s take a closer look.
Officially called the Dwight D. Eisenhower National System of Interstate and Defense Highways, it’s the highway network that spans the contiguous US, with routes in Alaska, Hawaii and Puerto Rico.
It consists of restricted freeways that meet national standards for interstate highways and use the same signage.
The Interstate was built for the following reasons:
encourage economic development
make highways safer
decongest heavily trafficked routes
reduce automobile-related lawsuits
The Interstate makes it easier for the military to transport troops and supplies overland. It also plays a major role in evacuations during disasters.
Image credit: History
Construction began in 1956 and officially ended in 1992. Kansas, Missouri, and Pennsylvania each claim that they laid down the first part of the Interstate.
At first glance, the numbering scheme for the Interstate might seem confusing.
However, it’s actually quite intuitive.
Image credit: U.S Department of Transportation
Main routes get one- or two-digit numbers.
Odd-numbered routes go north and south. The number increases as you move northward.
Even-numbered routes go east and west. The number rises as you drive eastward.
Parallel or split branches have the main route’s number plus a letter for the cardinal direction.
Ex: I-35E and I-35W.
Connecting routes and urban circumferential beltways add an even number prefix.
Ex. I-240 in TN.
Supplemental radial and spur routes add odd number prefixes.
Ex: I-105 in CA.
Image credit: Wikimedia
Meets the AASHTO’s* Standard for Interstate Highways or
Secured a waiver from the Federal Highway Administration.
*American Association of State Highway and Transportation Officials
Image Credit: Stanford University
Controlled access on and off the highway
Minimum speed of 70 mph (50 mph in mountains and cities)
At least two lanes both ways
Lanes are at least 12 feet wide
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