System history
How the Interstate system was built.
The Interstate Highway System is one of the largest public works projects in history. It reshaped how the country moves, where people live, and how goods reach markets. Here is the short version of how it came together.
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1919
A young officer crosses the country
Dwight Eisenhower joins an Army convoy that takes two months to drive coast to coast on poor roads, an experience he later credits for his interest in a national highway network.
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1944
The idea is authorized
The Federal-Aid Highway Act of 1944 authorizes a National System of Interstate Highways but provides no dedicated money to build it.
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1956
The system is funded
President Eisenhower signs the Federal-Aid Highway Act of 1956, creating the Highway Trust Fund and committing the federal government to pay 90 percent of construction costs. This is the true start of the modern system.
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1957
The numbering plan is set
AASHO, the predecessor of today's AASHTO, adopts the numbering grid and the red, white, and blue shield. Odd numbers run north-south, even numbers east-west, and the route map begins to take shape.
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1974
A national speed limit
In response to the oil crisis, Congress sets a national maximum speed limit of 55 miles per hour, tied to federal highway funding. It is fully repealed in 1995.
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1991
The original plan is declared complete
I-70 through Glenwood Canyon in Colorado is often cited as the completion of the original Interstate plan, decades after construction began.
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Today
Still growing
New corridors such as I-11, I-14, I-42, I-49, and parts of I-69 are still being designated and built, more than 65 years after the system began.
What makes a road an Interstate
From the start, Interstate highways had to meet a common set of engineering standards so the network would feel consistent across every state:
- Controlled access, with traffic entering and leaving only at interchanges.
- At least two lanes in each direction, separated by a median.
- Design speeds high enough for safe long-distance travel.
- Standardized signs, including the numbered shield and the green guide signs.