The Complex History of the U.S. Interstate Highway System

Begun in 1956, the U.S. Interstate Highway System is responsible for today's trucking industry, suburbs, gas stations, motels and the "road trip".
Marcia Wendorf

In 1919, a young Army Lieutenant Colonel named Dwight D. Eisenhower took part in the Motor Transport Corps convoy that drove 3,251 miles (5,232 km) between Washington D.C. and Oakland California. It took them 62 days to complete.

1919 Motor Transport Corps Convoy
1919 Motor Transport Corps Convoy Source: U.S. Army

During World War II, when Eisenhower was Supreme Commander of Allied Forces in Europe, he got a good look at Germany's Reichsautobahn system, which was an early implementation of Germany's modern Autobahn network. Eisenhower correctly deduced that a national highway system was a necessary component for a national defense.


After he became U.S. President in January 1953, Eisenhower appointed General Lucius D. Clay to investigate an interstate highway system. Clay stated that, "It was evident we needed better highways. We needed them for safety, to accommodate more automobiles. We needed them for defense purposes, if that should ever be necessary. And we needed them for the economy. Not just as a public works measure, but for future growth."

Clay came up with a 10-year, $100 billion plan to build 64,000 km (40,000 miles) of divided highways that would link all of America's cities having a population of 50,000 or greater.

With the passage of the Federal Aid Highway Act of 1956, construction got underway of the Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate Highway System. Each Interstate Highway was required to be a controlled-access highway with at least four lanes, and no at-grade crossings. Controlled-access highways have on and off ramps and are designed for high-speed traffic.

A map was created called the General Location of National System of Interstate Highways Map, which laid out what became the Interstate Highway System.

General Location of National System of Interstate Highways Map
General Location of National System of Interstate Highways Map Source: Secretary of Commerce/Wikimedia Commons

 Interstate Highway System milestones

Missouri was first state off the block when on August 13, 1956, work began in St. Charles County on US-40, which is now named I-70. On September 26, 1956, Kansas kicked off its portion of I-70. Additional milestones include:
* October 17, 1974 - Nebraska becomes the first state to complete all its Interstate Highways when the final piece of I-80 is dedicated
* October 12, 1979 - I-5 is dedicated near Stockton, California, linking Canada to Mexico, and making it the first contiguous freeway connecting the North American countries
* August 22, 1986 - the last section of I-80 is completed in Salt Lake City, Utah, making it the first coast-to-coast highway, going from San Francisco, California, to Teaneck, New Jersey; it is the world's first contiguous freeway to span the Atlantic and Pacific Oceans
* August 10, 1990 - the final section of the southerly, coast-to-coast I-10 is completed in Phoenix, Arizona, linking Santa Monica, California, to Jacksonville, Florida
* September 12, 1991 - the final section of the northerly, coast-to-coast I-90 is completed near Wallace, Idaho, linking Seattle, Washington to Boston, Massachusetts
* October 14, 1992 - the Interstate Highway System is considered finished with the completion of I-70 through Glenwood Canyon in Colorado; it is considered to be an engineering marvel with a 12-mile (19 km) span containing 40 bridges and numerous tunnels.

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Flies in the ointment

Up until 2018, there were actually two discontinuities in the Interstate Highway System: I-95 in New Jersey, and I-70 in Pennsylvania. On September 22, 2018, the Pennsylvania Turnpike/Interstate 95 Interchange Project which filled the gap in I-95 was completed. The gap in I-70 is more complicated.

Travelers going both east and west on I-70 near Breezewood, Pennsylvania must exit the freeway and go down a stretch of US-30 which has a number of roadside services. Those merchants have opposed the completion of I-70, fearing a loss of business.

A North-American highway system

In 1966, the Interstate Highway System was designated as part of the Pan-American Highway System, linking Canada, the U.S. and Mexico. Spurred by the North American Free Trade Agreement (NAFTA), plans are to have I-69 connect Tamaulipas, Mexico to Ontario, Canada. I-11, which will bridge the interstate gap between Phoenix, Arizona and Las Vegas, Nevada, will then become part of the CANAMEX Corridor linking Sonora, Mexico and Alberta, Canada.

IHS numbering system

The Interstate Highway System utilizes a numbering system whereby primary roads have one- or two-digit numbers, and shorter routes have three-digit numbers, with the last two digits matching the parent route. For example, I-294 is a loop where both ends connect to I-94, and I-787 is a short spur that attaches to I-87.

IHS numbering system
IHS numbering system Source: FHWA/Wikimedia Commons

Major arteries that span long distances are assigned numbers that are divisible by five. East-west highways are even-numbered, while north-south highways are odd-numbered.

Odd and even IHS numbering system
Odd and even IHS numbering system Source: Devinhorn/Wikimedia Commons

Even-numbered routes increase going from south to north, and odd-numbered routes increase when going from west to east. For example, north-south I-5 runs between Canada and Mexico along the West Coast, while I-95, which spans between Canada and Miami, Florida, runs along the East Coast.

West–east arteries include I-10, which spans between Santa Monica, California, and Jacksonville, Florida, and I-90 which runs between Seattle, Washington, and Boston, Massachusetts. There are no "I-50" and "I-60" because other U.S. highways currently use those numbers.

Alaska, Hawaii and Puerto Rico

The Interstate Highway System extends to Alaska, Hawaii, and Puerto Rico. In Hawaii, the interstates are all located on the Island of Oahu, and they all have the prefix "H". For example, there are H-1, H-2, H-3 and H-201. Besides connecting cities such as Honolulu, these interstates also connect several military and naval bases.

Interstates in Alaska and Puerto Rico are numbered sequentially in the order of their funding, and they have the prefixes "A" and "PR", respectively.

Mile markers and exit numbers

For one- or two-digit interstates, mile marker numbering begins at their southern- or western-most points. If an interstate originates within a state, then mile marker numbering starts at the southern or western state line.

For three-digit interstates that have an even first number and form a complete circular bypass around a city, their mile markers are numbered in a clockwise direction, beginning just west of the interstate that bisects the circle. For example, mile marker 1 of I-465, a route around Indianapolis, is just west of its junction with I-65.

Exit numbers of interstates are either sequential, or else distance-based, so that the exit number is the same as the nearest mile marker. For locations having multiple exits within the same mile, they are assigned letter suffixes.

Business routes

Business loops or spurs are routes that intersect an interstate and go through a city's central business district. A city may have more than one business loop. Business loop signs are green shields which differ from the regular Interstate Highway System's red and blue shields.

IHS regular and Business Loop shields
IHS regular and Business Loop shields Source: Wikimedia Commons

The red, white and blue shield signs are trademarked, and two-digit shields are 36 inches (91 cm) square, while three-digit shields are 45 inches (110 cm) square.

Speed demons

Currently, speed limits are set by the individual states, but between 1974 to 1986, the maximum speed limit on any highway in the U.S. was 55 miles per hour (90 km/h), which was the law at the time. Today, speed limits are lower in Northeastern and coastal states, and higher in inland states west of the Mississippi River.

The speed limit is 50 mph (80 km/h) in New York City and the District of Columbia, while you can do 80 mph (130 km/h) on I-10 and I-20 in rural western Texas, I-80 in Nevada between Fernley and Winnemuca, and portions of I-15, I-70, I-80, and I-84 in Utah. As someone who lives near one of these areas, I can tell you that we take our ability to do 80 mph very seriously. Some interstates in Idaho, Montana, South Dakota and Wyoming will also let you do 80 mph.

Road trips

The Interstate Highway System's impact on the U.S. was enormous. It caused a sharp decline in both railway shipping and passenger traffic, while at the same time, the trucking industry expanded. This caused a drop in the cost of shipping goods. 

The Interstate Highway System is responsible for the explosive grown of suburbs during the late 1950s and 1960s. The new roadways linked suburban homes to jobs located in the cities.

The IHS also is also responsible for the "road trip", where entire families packed into the car and hit the road. This, in turn, led to the creation of visitor attractions, service stations, motels, and restaurants.

The Interstate Highway System opened up the Sun Belt of the U.S. to both vacationers and new residents. It also led to the creation of Southern-based corporations, such as Walmart and FedEx.

U.S. Sun Belt
U.S. Sun Belt Source: Derfel73/Wikimedia Commons

The Interstate Highway System has been blamed for the decline of cities not on the highway's grid, and for the decay of urban centers.

IHS today

IHS today
IHS today Source: SPUI/Wikimedia Commons

Today, the Interstate Highway System is comprised of 68,000 km (42,00-miles) of roadway. It was initially estimated to cost $25 billion and take 12 years to complete. In actuality, it ended up costing $114 billion (in 2006 dollars $425 billion) and took 35 years to complete.