The United States has a complex national highway system that is roughly 160,000 miles long. For foreign visitors wishing to drive around, it is helpful to understand how the roads are named and numbered. You never know when this knowledge will come in handy.
 

Major Interstate Freeways

All major freeways that span across several states have one or two-digit numbers. Odd number means the freeway runs north-south, while even numbered freeways travel east and west. For example, interstate 5 (I-5) goes all the way from the Southern border in San Diego to the Northern border in Blaine, WA. I-10, on the other hand, connects the West Coast of San Diego and the East Coast of Jacksonville, FL.

 

Odd freeway numbers start from the West Coast, and increase toward east. I-5 is the first major north/South freeway, and it sits right by the Pacific Ocean. The next one is I-15, then I-25, and all the way to I-95 which is located along the East Coast.

 

Even freeway numbers start from south, and increase toward north. I-10 is the first interstate along the Southern border. Above I-10 is I-20, then I-30, and up until I-90.

 

Three-Digit Interstate Freeways

Three-digit highways connect to other major interstates. For example, 405 is a highway that is connected to I-5, while 215 is off I-15.

 

If the first digit is an odd number, the highway is likely to have only one end connected to a major highway. For example, 710 is a "spur" route that leaves I-10 and goes into the city of Long Beach.

 

If the first digit is an even number, the highway is usually connected to a major interstate at both ends. 405 splits from I-5 at one end (Lake Forest), but rejoins I-5 at the other end just north of Los Angeles. So they are often referred to as loops or "beltways."

 

U.S. Routes

Other than the Interstate Highway System mentioned above, there is also a grid of U.S. highways, more often referred to as U.S. routes.

 

Major U.S. routes are two-digit numbers. North-south routes are odd-numbered, and east-west even-numbered. However, what is different from the Interstate Highway System is that lowest odd numbers are in the east and highest numbers in the west. Similarly, the lowest even numbers are in the north and increase toward the south. For example, U.S. Route 1 runs north-south on the east coast, while Route 101 is located in California. Note that 101 is actually considered a two-digit highway.

 

Three-digit U.S. routes are "spur" highways, but unlike the interstate system, they may or may not have direct connection to their parent routes.

 

In general, U.S. routes are older roads and have largely been replaced by the newer and more functional Interstate Highway System. However, the grid of U.S. routes is still a major part of the country's transportation network, used by many local residents or as scenic drives.

 

Highway Mile Markers

You may have noticed those mile markers along interstates. They can help you estimate how far you have traveled into a state and how far away you are from your destination.

 

Mile markers always start at the state line in the south (for north-south highways), or in the west (for east-west highways). So if you enter Oregon from California, you will see mile 1 soon after you cross the state line, and then see increasing numbers as you travel north.

 

A rural stretch of I-5, with two lanes in each direction separated by a large grassy median, and with cross-traffic limited to overpasses and underpasses.

 

The Dwight D. Eisenhower National System of Interstate and Defense Highways (commonly known as the Interstate Highway System, Interstate Freeway System or the Interstate) is a network of freeways that forms a part of the National Highway System of the United States. The system is named for PresidentDwight D. Eisenhower, who championed its formation. Construction was authorized by the Federal Aid Highway Act of 1956, and the original portion was completed 35 years later. The network has since been extended, and as of 2010, it had a total length of 47,182 miles (75,932 km), making it the world's second longest after China's. As of 2010, about one-quarter of all vehicle miles driven in the country use the Interstate system.The cost of construction has been estimated at $425 billion (in 2006 dollars), making it the "largest public works program since the Pyramids".The system has contributed in shaping the United States into a world economic superpower and a highly industrialized nation.

 

The American national government's effort at constructing a national network of highways began on an ad hoc basis with the passage of the Federal Aid Road Act of 1916, which provided for $75 million over a five year period for matching funds to the states for the construction of and improvement of highways. While the nation's revenue needs associated with World War I intervened to prevent any premature expansion of this policy, by the 1921 expiration of the original legislation American troops had returned from Europe and a policy of so-called "Normalcy" had returned, despite an economic downturn.

 

As the landmark 1916 law expired, new legislation was moved forward—the Federal Highway Act of 1921. This new road construction initiative once again provided for federal matching funds for road construction and improvement, although now on the far more generous basis of $75 million annually. Moreover, rather than leaving road building decisions up to the states themselves, this new legislation for the first time sought to target these funds to the construction of a coherent national road grid of interconnected "primary highways."

 

The Bureau of Public Roads asked the Army to provide a list of roads it considered necessary for national defense. In 1922 General John J. Pershing, former head of the American Expeditionary Force in Europe during the war, complied by submitting a detailed network of 200,000 miles of interconnected primary highways—the so-called Pershing Map.
 

A boom in road construction followed throughout the decade of the 1920s, with such projects as the New York parkway system constructed as part of a new national highway system. As automobile traffic increased, planners saw a need for such an interconnected national system to supplement the existing, largely non-freeway, United States Numbered Highways system. By the late 1930s, planning had expanded to a system of new superhighways.

 

In 1938 President Franklin D. Roosevelt gave Thomas MacDonald, chief at the Bureau of Public Roads, a hand-drawn map of the United States marked with eight superhighway corridors for study. In 1939, Bureau of Public Roads Division of Information chief Herbert S. Fairbank wrote a report called Toll Roads and Free Roads, "the first formal description of what became the interstate highway system", and in 1944 the similarly themed Interregional Highways.

 

The Interstate Highway System gained a champion in President Dwight D. Eisenhower, who was influenced by his experiences as a young Army officer crossing the country in the 1919 Army Convoy on the Lincoln Highway, the first road across America. During World War II Eisenhower gained an appreciation of the German Autobahn network as a necessary component of a national defense system while he was serving as Supreme Commander of the Allied forces in Europe during World War II. He recognized that the proposed system would also provide key ground transport routes for military supplies and troop deployments in case of an emergency or foreign invasion.

 

A long standing myth that still has believers said that one mile in five (1.6 in 8.0 km) of the Interstate System was to be straight so they could be used as emergency landing strips for aircraft if the regular strips were bombed. It sounded good but has never been a requirement.Emergency landings, like one in Jacksonville, Florida, in December 2011 are not unusual.

 

The publication in 1955 of the General Location of National System of Interstate Highways, informally known as the Yellow Book, mapped out what became the Interstate System.[13] Assisting in the planning was Charles Erwin Wilson, who was still head of General Motors when President Eisenhower selected him as Secretary of Defense in January 1953.

 

Construction

The Interstate Highway System was authorized by the Federal Aid Highway Act of 1956[14]—popularly known as the National Interstate and Defense Highways Act of 1956—on June 29.

 

Three states have claimed the title of first Interstate Highway. Missouri claims that the first three contracts under the new program were signed in Missouri on August 2, 1956. The first contract signed was for U.S. 66. On August 13, 1956, Missouri awarded the first contract based on new Interstate Highway funding.

 

Kansas claims that it was the first to start paving after the act was signed. Preliminary construction had taken place before the act was signed, and paving started September 26, 1956. The state marked its portion of I 70 as the first project in the United States completed under the provisions of the new Federal-Aid Highway Act of 1956.

 

According to information liaison specialist Richard Weingroff, the Pennsylvania Turnpike could also be considered one of the first Interstate Highways. On October 1, 1940, 162 miles (261 km) of the highway now designated I 70 and I 76 opened between Irwin and Carlisle. The Commonwealth of Pennsylvania refers to the turnpike as the Granddaddy of the Pikes.

 

Milestones in the construction of the Interstate Highway System include:

• October 17, 1974—Nebraska becomes the first state to complete all of its mainline interstate highways with the dedication of its final piece of I-80.

 

• August 22, 1986—The final section of the coast-to-coast I-80 (San Francisco, California, to New York City) is dedicated on the western edge of Salt Lake City, Utah. The section spanned from Redwood Road to just west of the Salt Lake City International Airport. At the dedication it was noted that coincidentally this was only 50 miles (80 km) from Promontory Summit, where the golden spike of the United States' First Transcontinental Railroad was laid 120 years prior.

 

• August 10, 1990—The final section of coast-to-coast I-10 (Santa Monica, California, to Jacksonville, Florida) is dedicated, the Papago Freeway Tunnel under downtown Phoenix, Arizona. Completion of this section was delayed due to a freeway revolt that forced the cancellation of an originally planned elevated routing.

 

• September 12, 1991—I-90 becomes the final coast-to-coast Interstate Highway (Seattle, Washington to Boston, Massachusetts) to be completed with the dedication of an elevated viaduct bypassing Wallace, Idaho. This section was delayed after residents forced the cancellation of the originally planned at-grade alignment that would have demolished much of downtown Wallace. The residents accomplished this feat by arranging for most of the downtown area to be declared a historic district and listed on the National Register of Historic Places; this succeeded in blocking the path of the original alignment. After the dedication residents held a mock funeral celebrating the removal of the last stoplight on a transcontinental Interstate Highway.

 

• October 14, 1992—The original Interstate Highway system is proclaimed to be complete with the opening of I-70 through Glenwood Canyon in Colorado. This section is considered an engineering marvel with a 12-mile (19 km) span featuring 40 bridges and numerous tunnels and is one of the most expensive rural highways per mile built in the United States. Although this was claimed the final section of Interstate Highway to open, at the time this section was dedicated there were still missing interchanges elsewhere in the system, making some Interstate Highways not contiguous.

 

The initial cost estimate for the system was $25 billion over 12 years; it ended up costing $114 billion (adjusted for inflation, $425 billion in 2006 dollars) and took 35 years.

 

1992–present

Although the system was proclaimed complete in 1992, two of the original interstates, I-95 and I-70, are not contiguous because they are missing interchanges. Both of these discontinuities are due to local opposition which has blocked efforts to build the necessary connections to fully complete the system.

 

Because of the cancellation of the Somerset Freeway, I-95 is discontinuous in New Jersey. Authorized by the federal government in 2004, the Pennsylvania Turnpike/Interstate 95 Interchange Project is scheduled to connect the separate sections of I 95 to form a continuous route, completing the final section of the original plan. Construction began in 2010.

 

There is a missing interchange between the Pennsylvania Turnpike and I-70 near Breezewood, Pennsylvania, where traffic must use a few blocks of US 30, which are cluttered with services, to rejoin I-70. Although solutions have been proposed through the years to complete the discontinuity on I-70, they have been blocked by local opposition.
 

Additional spurs and loops/bypasses remain under construction, such as I-485 in North Carolina, which has been under construction since the 1980s. A few main routes not part of the original plan remain under construction, such as I-22 in Tennessee, Mississippi, and Alabama, and the extension of I-69 from Indiana to Texas. Officials have also identified some non-Interstate corridors for future inclusion into the system, either by construction of new Interstate routes or upgrade of existing roads to Interstate standards.