Construction of the Interstate Highway System was proclaimed complete in 1992, though some planned routes were canceled and several routes have stretches that do not fully conform with federal standards. Though much of their construction was funded by the federal government, Interstate Highways are owned by the state in which they were built.
All Interstates must meet specific standards such as having controlled access, avoiding at-grade intersections, and complying with federal traffic sign specifications. The Interstate Highway System is partially financed through the Highway Trust Fund, which itself is funded by a federal fuel tax.
The United States government's efforts to construct a national network of highways began on an ad hoc basis with the passage of the Federal Aid Road Act of 1916, which provided $75 million over a five-year period for matching funds to the states for the construction and improvement of highways. The nation's revenue needs associated with World War I prevented any significant implementation of this policy, which expired in 1921.
In December 1918, E. J. Mehran, a civil engineer and the editor of Engineering News-Record, presented his “A Suggested National Highway Policy and Plan” during a gathering of the State Highway Officials and Highway Industries Association at the Congress Hotel in Chicago. The system would include two percent of all roads and would pass through every state at a cost of $25,000 per mile ($16,000/km), providing commercial as well as military transport benefits.
In 1919 the U.S. Army sent an expedition across the U.S. to determine the difficulties that military vehicles would have on a cross-country trip. Leaving from the Ellipse near the White House on July 7, the Motor Transport Corps convoy needed 62 days to drive 3,200 miles (5,100 km) on the Lincoln Highway to the Presidio army base on San Francisco Bay.
They experienced significant difficulties including rickety bridges, broken crankshafts, and engines clogged with desert sand. Dwight Eisenhower, then a 28-year-old lieutenant, accompanied the trip “through darkest America with truck and tank,” as he later described it.
This new road construction initiative once again provided for federal matching funds for road construction and improvement, $75 million allocated annually. Moreover, this new legislation for the first time sought to target these funds to the construction of a national road grid of interconnected “primary highways “, setting up cooperation among the various state highway planning boards.
A rural stretch of I-5 in California; two lanes in each direction are separated by a large grassy median and cross-traffic is limited to overpasses and underpasses 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. Fairbanks 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 Motor Transport Corps convoy that drove in part on the Lincoln Highway, the first road across America.
He recalled that, “The old convoy had started me thinking about good two-lane highways ... the wisdom of broader ribbons across our land.” Eisenhower also gained an appreciation of the Reichsautobahn system, the first “national” implementation of modern Germany's Autobahn network, as a necessary component of a national defense system while he was serving as Supreme Commander of Allied Forces in Europe during World War II. In 1954, Eisenhower appointed General Lucius D. Clay to head a committee charged with proposing an interstate highway system plan.
Summing up motivations for the construction of such a system, Clay stated, Clay's committee proposed a 10-year, $100 billion program, which would build 40,000 miles (64,000 km) of divided highways linking all American cities with a population of greater than 50,000.
Eisenhower initially preferred a system consisting of toll roads, but Clay convinced Eisenhower that toll roads were not feasible outside the highly populated coastal regions. In February 1955, Eisenhower forwarded Clay's proposal to Congress.
The bill quickly won approval in the Senate, but House Democrats objected to the use of public bonds as the means to finance construction. In June 1956, Eisenhower signed the Federal Aid Highway Act of 1956 into law.
Under the act, the federal government would pay for 90 percent of the cost of construction of Interstate Highways. Each Interstate Highway was required to be a freeway with at least four lanes and no at-grade crossings.
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 Highway System. 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.
Three states have claimed the title of first Interstate Highway. The first contract signed was for upgrading a section of US Route 66 to what is now designated Interstate 44.
On August 13, 1956, work began on US 40 (now I-70) in St. Charles County. Kansas claims that it was the first to start paving after the act was signed.
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. October 12, 1979: The final section of the Canada to Mexico freeway Interstate 5 is dedicated near Stockton, California.
Representatives of the two neighboring nations attended the dedication to commemorate the first contiguous freeway connecting the North American countries. August 22, 1986: The final section of the coast-to-coast I-80 (San Francisco, California, to Tea neck, New Jersey) is dedicated on the western edge of Salt Lake City, Utah, making I-80 the world's first contiguous freeway to span from the Atlantic to Pacific Ocean and, at the time, the longest contiguous freeway in the world.
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 a similar feat was accomplished nearly 120 years prior, the driving of the golden spike of the United States' First Transcontinental Railroad.
August 10, 1990: The final section of coast-to-coast I-10 (Santa Monica, California, to Jacksonville, Florida) is dedicated, the Pap ago 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.
I-95 was made a continuous freeway in 2018, and thus I-70 remains the only original Interstate with a discontinuity. Traveling in either direction, I-70 traffic must exit the freeway and use a short stretch of US-30 (which includes a number of roadside services) to rejoin I-70.
The interchange was not originally built because of a legacy federal funding rule, since relaxed, which restricted the use of federal funds to improve roads financed with tolls. Solutions have been proposed to eliminate the discontinuity, but they have been blocked by local opposition, fearing a loss of business.
Expansion The Interstate Highway System has been expanded numerous times. For example, I-49, added to the system in the 1980s as a freeway in Louisiana, was designated as an expansion corridor, and FHA approved the expanded route north from Lafayette, Louisiana, to Kansas City, Missouri.
In 1966, the FHA designated the entire Interstate Highway System as part of the larger Pan-American Highway System, and at least two proposed Interstate expansions were initiated to help trade with Canada and Mexico spurred by the North American Free Trade Agreement (NAFTA). Long-term plans for I-69, which currently exists in several completed segments (the largest of which are in Indiana and Texas), is to have the highway route extend from Tamaulipas, Mexico to Ontario, Canada.
I-40 in Memphis, Tennessee was rerouted and part of the original I-40 is still in use as the eastern half of Sam Cooper Boulevard. Though local opposition led to the cancellation of this project in 1981, bridges and ramps for the connection into the “north split” remain visible.
I-70 in Baltimore was supposed to run from the Baltimore Beltway (Interstate 695), which surrounds the city to terminate at I-95, the East Coast thoroughfare that runs through Maryland and Baltimore on a diagonal course, northeast to southwest; the connection was cancelled on the mid-1970s due to its routing through Wynn Falls-Leakin Park, a wilderness urban park reserve following the Wynn Falls stream through West Baltimore. This included the cancellation of I-170, partially built and in use as U.S. Route 40, and nicknamed the Highway to Nowhere.
I-80 in San Francisco was originally planned to travel past the city's Civic Center along the Panhandle Freeway into Golden Gate Park and terminate at the original alignment of I-280 / SR 1. Similarly, more than 20 years later, Sacramento canceled plans to upgrade I-80 to Interstate Standards and rerouted the freeway on what was then I-880 that traveled north of Downtown Sacramento.
I-83, southern extension of the Jones Falls Expressway (southern I-83) in Baltimore was supposed run along the waterfront of the Tabasco River / Baltimore Harbor to connect to I-95, bisecting historic neighborhoods of Fells Point and Canton, but the connection was never built. I-95 was originally planned to run up the Southwest Expressway and meet I-93, where the two highways would travel along the Central Artery through downtown Boston, but was rerouted onto the Route 128 beltway due to widespread opposition.
This revolt also included the cancellation of the Inner Belt, connecting I-93 to I-90 and a cancelled section of the Northwest Expressway which would have carried US 3 inside the Route 128 beltway, meeting with Route 2 in Cambridge. Being freeways, Interstate Highways usually have the highest speed limits in a given area.
Speed limits are determined by individual states. From 1975 to 1986, the maximum speed limit on any highway in the United States was 55 miles per hour (90 km/h), in accordance with federal law.
Typically, lower limits are established in Northeastern and coastal states, while higher speed limits are established in inland states west of the Mississippi River. Currently, rural speed limits elsewhere generally range from 65 to 80 miles per hour (105 to 130 km/h).
Several portions of various highways such as I-10 and I-20 in rural western Texas, I-80 in Nevada between Farley and Winnemucca (except around Love lock) and portions of I-15, I-70, I-80, and I-84 in Utah have a speed limit of 80 mph (130 km/h). Other Interstates in Idaho, Montana, Oklahoma, South Dakota and Wyoming also have the same high speed limits.
The maximum speed limit on I-90 is 50 mph (80 km/h) in downtown Cleveland because of two sharp curves with a suggested limit of 35 mph (55 km/h) in a heavily congested area; I-70 through Wheeling, West Virginia, has a maximum speed limit of 45 mph (70 km/h) through the Wheeling Tunnel and most of downtown Wheeling; and I-68 has a maximum speed limit of 40 mph (65 km/h) through Cumberland, Maryland, because of multiple hazards including sharp curves and narrow lanes through the city. In some locations, low speed limits are the result of lawsuits and resident demands; after holding up the completion of I-35E in St. Paul, Minnesota, for nearly 30 years in the courts, residents along the stretch of the freeway from the southern city limit to downtown successfully lobbied for a 45 mph (70 km/h) speed limit in addition to a prohibition on any vehicle weighing more than 9,000 pounds (4,100 kg) gross vehicle weight.
I-93 in Franconia Notch State Park in northern New Hampshire has a speed limit of 45 mph (70 km/h) because it is a parkway that consists of only one lane per side of the highway. On the other hand, Interstates 15, 80 and 84 in Utah have speed limits as high as 70 mph (115 km/h) within the Salt Lake City, Cedar City, and St. George areas, and I-25 in New Mexico within the Santa Fe and Las Vegas areas along with I-20 in Texas along Odessa and Midland and I-29 in North Dakota along the Grand Forks area have higher speed limits of 75 mph (120 km/h).
The system has also been used to facilitate evacuations in the face of hurricanes and other natural disasters. An option for maximizing traffic throughput on a highway is to reverse the flow of traffic on one side of a divider so that all lanes become outbound lanes.
This procedure, known as contraflow lane reversal, has been employed several times for hurricane evacuations. After public outcry regarding the inefficiency of evacuating from southern Louisiana prior to Hurricane Georges landfall in September 1998, government officials looked towards contraflow to improve evacuation times.
In Savannah, Georgia, and Charleston, South Carolina, in 1999, lanes of I-16 and I-26 were used in a contraflow configuration in anticipation of Hurricane Floyd with mixed results. In 2004 contraflow was employed ahead of Hurricane Charley in the Tampa, Florida area and on the Gulf Coast before the landfall of Hurricane Ivan ; however, evacuation times there were no better than previous evacuation operations.
Engineers began to apply lessons learned from the analysis of prior contraflow operations, including limiting exits, removing troopers (to keep traffic flowing instead of having drivers stop for directions), and improving the dissemination of public information. As a result, the 2005 evacuation of New Orleans, Louisiana, prior to Hurricane Katrina ran much more smoothly.
According to urban legend, early regulations required that one out of every five miles of the Interstate Highway System must be built straight and flat, to be usable by aircraft during times of war. The association's present numbering policy dates back to August 10, 1973.
While numerous exceptions do exist, there is a general scheme for numbering Interstates. Primary Interstates are assigned one- or two-digit numbers, while shorter routes (such as spurs, loops, and short connecting roads) are assigned three-digit numbers where the last two digits match the parent route (thus, I-294 is a loop that connects at both ends to I-94, while I-787 is a short spur route attached to I-87).
Odd route numbers increase from west to east, and even-numbered routes increase from south to north (to avoid confusion with the U.S. Highways, which increase from east to west and north to south).
Numbers divisible by five are intended to be major arteries among the primary routes, carrying traffic long distances. Primary north-south Interstates increase in number from I-5 between Canada and Mexico along the West Coast to I95 between Canada and Miami, Florida along the East Coast.
Major west–east arterial Interstates increase in number from I-10 between Santa Monica, California, and Jacksonville, Florida, to I-90 between Seattle, Washington, and Boston, Massachusetts, with two exceptions. There are no I-50 and I-60, as routes with those numbers would likely pass through states that currently have U.S.
Several two-digit numbers are shared between road segments at opposite ends of the country for various reasons. Some such highways are incomplete Interstates (such as I-69 and I-74) and some just happen to share route designations (such as I-76, I-84, I86, I-87, and I-88).
Some of these were due to a change in the numbering system as a result of a new policy adopted in 1973. Previously, letter-suffixed numbers were used for long spurs off primary routes; for example, western I84 was I80 N, as it went north from I80.
Additionally, due to Congressional requirements, three sections of I-69 in southern Texas will be divided into I-69W, I-69E, and I-69C (for Central). AAS HTO policy allows dual numbering to provide continuity between major control points.
For example, I75 and I85 share the same roadway in Atlanta ; this 7.4-mile (11.9 km) section, called the Downtown Connector, is labeled both I75 and I85. In rare instances, two highway designations sharing the same roadway are signed as traveling in opposite directions; one such wrong-way concurrency is found between Asheville and Fort Chi swell, Virginia, where I81 north and I77 south are equivalent (with that section of road traveling almost due east), as are I81 south and I77 north.
Examples of the auxiliary Interstate Highway numbering system. An odd hundreds' digit means the route connects at only one end to the rest of the interstate system, known as a “spur route” (see I-310 and I-510 in image).
An even hundreds' digit means the route connects at both ends, which could be a bypass route (which has two termini) (see I-210 and I-810 in image) or a radial route (known also as a beltway, belt line, or circumferential route) (see I-610 in image). Auxiliary Interstate Highways are circumferential, radial, or spur highways that principally serve urban areas. These types of Interstate Highways are given three-digit route numbers, which consist of a single digit prefixed to the two-digit number of its parent Interstate Highway.
Spur routes deviate from their parent and do not return; these are given an odd first digit. Circumferential and radial loop routes return to the parent, and are given an even first digit.
Unlike primary Interstates, three-digit Interstates are signed as either east–west or north-south, depending on the general orientation of the route, without regard to the route number. Map of routes in Puerto Rico that receive funding from the Interstate program, but are not signed as Interstate Highways Map of routes in Alaska that receive funding from the Interstate program, but are not signed as Interstate Highways The Interstate Highway System also extends to Alaska, Hawaii, and Puerto Rico, even though they have no direct land connections to any other states or territories.
However, their residents still pay federal fuel and tire taxes. The Interstates in Hawaii, all located on the most populous island of Oahu, carry the prefix H.
These Interstates connect several military and naval bases together, as well as the important cities and towns spread across Oahu, and especially the metropolis of Honolulu. Furthermore, these routes were neither planned according to nor constructed to the official Interstate Highway standards.
On one- or two-digit Interstates, the mile marker numbering almost always begins at the southern or western state line. As with all guidelines for Interstate routes, however, numerous exceptions exist.
Three-digit Interstates with an even first number that form a complete circumferential (circle) bypass around a city feature mile markers that are numbered in a clockwise direction, beginning just west of an Interstate that bisects the circumferential route near a south polar location. In other words, mile marker 1 on I-465, a 53-mile (85 km) route around Indianapolis, is just west of its junction with I-65 on the south side of Indianapolis (on the south leg of I-465), and mile marker 53 is just east of this same junction.
Under the latter system, a single mile with multiple exits may be assigned letter suffixes, for example on I890 in New York. AAS HTO defines a category of special routes separate from primary and auxiliary Interstate designations.
Interstate Highways and their rights-of-way are owned by the state in which they were built. The last federally owned portion of the Interstate System was the Woodrow Wilson Bridge on the Washington Capital Beltway.
The new bridge was completed in 2009 and is collectively owned by Virginia and Maryland. Maintenance is generally the responsibility of the state department of transportation.
However, there are some segments of Interstate owned and maintained by local authorities. About 70 percent of the construction and maintenance costs of Interstate Highways in the United States have been paid through user fees, primarily the fuel taxes collected by the federal, state, and local governments.
The federal gasoline tax was first imposed in 1932 at one cent per gallon; during the Eisenhower administration, the Highway Trust Fund, established by the Highway Revenue Act in 1956, prescribed a three-cent-per-gallon fuel tax, soon increased to 4.5 cents per gallon. Initially, that fund was sufficient for the federal portion of building the Interstate system, built in the early years with “10 cent dollars”, from the perspective of the states, as the federal government paid 90% of the costs while the state paid 10%.
The rest of the costs of these highways are borne by general fund receipts, bond issues, designated property taxes, and other taxes. However, any local government contributions are overwhelmingly from sources besides user fees.
As decades passed in the 20th century and into the 21st century, the portion of the user fees spent on highways themselves covers about 57 percent of their costs, with about one-sixth of the user fees being sent to other programs, including the mass transit systems in large cities. Some large sections of Interstate Highways that were planned or constructed before 1956 are still operated as toll roads.
Others have had their construction bonds paid off, and they have become toll-free, such as in Connecticut (I95), Maryland (I95), Virginia (I95), and Kentucky (I65). A view of I-75 in Atlanta, Georgia, featuring HOV lanes running alongside the medians American suburbs have expanded, the costs incurred in maintaining freeway infrastructure have also grown, leaving little in the way of funds for new Interstate construction.
This has led to the proliferation of toll roads (turnpikes) as the new method of building limited-access highways in suburban areas. Some Interstates are privately maintained (for example, the VMS company maintains I35 in Texas) to meet rising costs of maintenance and allow state departments of transportation to focus on serving the fastest-growing regions in their states.
Parts of the Interstate System might have to be tolled in the future to meet maintenance and expansion demands, as has been done with adding toll HOV / HOT lanes in cities such as Atlanta, Dallas, and Los Angeles. Although part of the tolling is an effect of the SAFETEALU act, which has put an emphasis on toll roads as a means to reduce congestion, present federal law does not allow for a state to change a freeway section to a tolled section for all traffic.
While federal legislation initially banned the collection of tolls on Interstates, many of the toll roads on the system were either completed or under construction when the Interstate Highway System was established. Congress also decided that it was too costly to either build toll-free Interstates parallel to these toll roads, or directly repay all the bondholders who financed these facilities and remove the tolls.
Thus, these toll roads were grandfathered into the Interstate Highway System. Toll roads designated as Interstates (such as the Massachusetts Turnpike) were typically allowed to continue collecting tolls, but are generally ineligible to receive federal funds for maintenance and improvements.
Some toll roads that did receive federal funds to finance emergency repairs (notably the Connecticut Turnpike (I-95) following the Minus River Bridge collapse) were required to remove tolls as soon as the highway's construction bonds were paid off. In addition, these toll facilities were grandfathered from Interstate Highway standards.
A notable example is the western approach to the Benjamin Franklin Bridge in Philadelphia, where I-676 has a surface street section through a historic area. Policies on toll facilities and Interstate Highways have since changed.
The Federal Highway Administration has allowed some states to collect tolls on existing Interstate Highways, while a recent extension of I-376 included a section of Pennsylvania Route 60 that was tolled by the Pennsylvania Turnpike Commission before receiving Interstate designation. A new addition of the Manual on Uniform Traffic Control Devices in 2009 requires a black-on-yellow “Toll” sign to be placed above the Interstate trailblazer on Interstate Highways that collect tolls.
Legislation passed in 2005 known as SAFETEA-LU, encouraged states to construct new Interstate Highways through “innovative financing” methods. SAFETEA-LU facilitated states to pursue innovative financing by easing the restrictions on building interstates as toll roads, either through state agencies or through public–private partnerships.
However, SAFETEA-LU left in place a prohibition of installing tolls on existing toll-free Interstates, and states wishing to toll such routes to finance upgrades and repairs must first seek approval from Congress. Interstate Highways financed with federal funds are known as “chargeable” Interstate routes, and are considered part of the 42,000-mile (68,000 km) network of highways.
Federal laws also allow “non-chargeable” Interstate routes, highways funded similarly to state and U.S. Several Interstate shield design proposals submitted by the Texas Highway DepartmentInterstate Highways are signed by a number placed on a red, white, and blue sign.
The shield design itself is a registered trademark of the American Association of State Highway and Transportation Officials. In the original design, the name of the state was displayed above the highway number, but in many states, this area is now left blank, allowing for the printing of larger and more-legible digits.
Signs with the shield alone are placed periodically throughout each Interstate as reassurance markers. Interstate business loops and spurs use a special shield in which the red and blue are replaced with green, the word “BUSINESS” appears instead of “INTERSTATE”, and the word “SPUR” or “LOOP” usually appears above the number.
The green shield is employed to mark the main route through a city's central business district, which intersects the associated Interstate at one (spur) or both (loop) ends of the business route. The route usually traverses the main thoroughfare(s) of the city's downtown area or other major business district.
A city may have more than one Interstate-derived business route, depending on the number of Interstates passing through a city and the number of significant business districts therein. Over time, the design of the Interstate shield has changed.
In 1957 the Interstate shield designed by Texas Highway Department employee Richard Oliver was introduced, the winner of a contest that included 100 entries; at the time, the shield color was a dark navy blue and only 17 inches (43 cm) wide. The Manual on Uniform Traffic Control Devices (MUTED) standards revised the shield in the 1961, 1971, and 1978 editions.
For many years, California was the only state that did not use an exit numbering system. It was granted an exemption in the 1950s due to having an already largely completed and signed highway system; placing exit number signage across the state was deemed too expensive.
Newer signs along the freeways follow this practice as well. California, however, still does not use mileposts, although a few exist for experiments or for special purposes.
In 2010–2011, the Illinois State Toll Highway Authority posted all new mile markers to be uniform with the rest of the state on I90 (Jane Addams Memorial/Northwest Tollway) and the I94 section of the Prostate Tollway, which previously had matched the I294 section starting in the south at I80/I94/IL Route 394. Exit numbers correspond to Interstate mileage markers in most states.
On I19 in Arizona, however, length is measured in kilometers instead of miles because, at the time of construction, a push for the United States to change to a metric system of measurement had gained enough traction that it was mistakenly assumed that all highway measurements would eventually be changed to metric; proximity to metric-using Mexico may also have been a factor, as I19 indirectly connects I10 to the Mexican Federal Highway system via surface streets in Nogales. Mileage count increases from west to east on most even-numbered Interstates ; on odd-numbered Interstates mileage count increases from south to north.
Some highways, including the New York State Thruway, use sequential exit-numbering schemes. Exits on the New York State Thruway count up from Yonkers traveling north, and then west from Albany.
The first section makes up the Major Keegan Expressway in the Bronx, with interchanges numbered sequentially from 1 to 14. From Albany north to the Canadian border, the exits on I87 are numbered sequentially from 1 to 44 along the Adirondack North way.
These two exits share a number but are located 150 miles (240 km) apart. States in which Interstate exits are still numbered sequentially are Connecticut, Delaware, Massachusetts (although efforts to use mile-based exit numbers began in 2020), New Hampshire, New York, Rhode Island, and Vermont; as such, five of the main Interstate Highways that remain completely within these states (87, 88, 89, 91, and 93) have interchanges numbered sequentially along their entire routes.
Locating a sign on the ground to the side of the highway, mostly the right, and is used to denote exits, as well as rest areas, motorist services such as gas and lodging, recreational sites, and freeway names Attaching the sign to an overpass Mounting on full gantries that bridge the entire width of the highway and often show two or more signs Mounting on half-gantries that are located on one side of the highway, like a ground-mounted sign The Heaviest traveled : 374,000 vehicles per day: I-405 in Los Angeles, California (2008 estimate ).
Longest (east–west) : 3,020.54 miles (4,861.09 km): I-90 from Boston, Massachusetts, to Seattle, Washington. Longest (north-south) : 1,908 mi (3,071 km): I-95 from the Canadian border near Houston, Maine, to Miami, Florida.
Shortest (two-digit) : 1.69 mi (2.72 km): I-69W in Laredo, Texas. The Shortest segment between state lines : 453 ft (138 m): Interstate 95 / I-495 (Capital Beltway) on the Woodrow Wilson Bridge across the Potomac River where they briefly cross the southernmost tip of the District of Columbia between its borders with Maryland and Virginia.
The Longest concurrency : 278.4 mi (448.0 km): I-80 and I-90 ; Gary, Indiana, to Elyria, Ohio. Most states served by an Interstate : 15 states plus the District of Columbia: I-95 through Florida, Georgia, South Carolina, North Carolina, Virginia, DC, Maryland, Delaware, Pennsylvania, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, and Maine.
Most Interstates in a state: 32 routes: New York, totaling 1,750.66 mi (2,817.41 km) Most primary Interstates in a state: 13 routes: Illinois Most Interstate mileage in a state : 3,233.45 mi (5,203.73 km): Texas, in 17 different routes. The Fewest Interstates in a state: 3 routes: Delaware, New Mexico, North Dakota, Puerto Rico, and Rhode Island Fewest primary Interstates in a state: 1 route: Delaware, Maine, and Rhode Island (I-95 in each case).
The Least Interstate mileage in a state : 40.61 mi (65.36 km): Delaware, in 3 different routes. Following the passage of the Federal Aid Highway Act of 1956, the railroad system for passengers and freight declined sharply, but the trucking industry expanded dramatically and the cost of shipping and travel fell sharply.
Suburbanization became possible, with the rapid growth of easily accessible, larger, cheaper housing than was available in central cities. Tourism dramatically expanded as well, creating a demand for more service stations, motels, restaurants and visitor attractions.
There was much more long-distance movement to the Sun Belt for winter vacations, or for permanent relocation, with convenient access to visits to relatives back home. In rural areas, towns and small cities off the grid lost out as shoppers followed the interstate and new factories were located near them.
The system had a particularly strong effect in the Southern United States, as most Southern states had not previously been able to afford the construction of major highways. The construction of the Interstate Highway System facilitated the relocation of heavy manufacturing to the South and spurred the development of Southern-based corporations like Walmart and FedEx.
^ This counts the suffixed routes in Texas (I-35E, I-35W, I-69E, I-69C, and I-69W) as auxiliary routes or parts of the same primary Interstate and not separate primary Interstates. Federal -Aid Highway Act of 1956, Creating the Interstate System”.
^ a b Office of Highway Policy Information (August 30, 2019). Table HM-20: Public Road Length, 2018, Miles By Functional System (Report).
^ Office of Highway Policy Information (December 2017). Table VM-1: Annual Vehicle Distance Traveled in Miles and Related Data, 2016, by Highway Category and Vehicle Type (Report).
“A Suggested National Highway Policy and Plan”. ' Clearly Vicious as a Matter of Policy': The Fight Against Federal -Aid”.
The Roads That Built America: The Incredible Story of the U.S. Interstate System. “The Federal -State Partnership at Work: The Concept Man”.
“Fighting Traffic: U.S. Transportation Policy and Urban Congestion, 1955–1970”. “Timeline of Notable Events of the Interstate Highway System in California”.
“America Celebrates 30th Anniversary of the Interstate System”. ^ “Around the Nation: Transcontinental Road Completed in Utah”.
United States Gross Domestic Product deflation figures follow the Measuring Worth series. “Decades in the Making, I-95, Turnpike Connector Opens to Motorists”.
Interstate I-49 Expansion Corridor in Southwest District of Missouri. ^ New Mexico State Highway and Transportation Department (2007).
State of New Mexico Memorial Designations and Dedications of Highways, Structures and Buildings (PDF). Santa Fe: New Mexico State Highway and Transportation Department.
“The National Highway System: A Commitment to America's Future”. “ One-Way-Out”: Contraflow Freeway Operation for Hurricane Evacuation” (PDF).
“Contraflow Implementation Experiences in the Southern Coastal States” (PDF). ^ a b c d e f American Association of State Highway and Transportation Officials (January 2000).
American Association of State Highway and Transportation Officials. “Highway Numerology Muddled by Potholes in Logic”.
“Was I-76 Numbered to Honor Philadelphia for Independence Day, 1776?” Proposed I-41 in Wisconsin and partly completed I-74 in North Carolina respectively are possible and current exceptions not adhering to the guideline.
Highways with the same numbers will be retained in the states upon completion of the Interstate routes. “FHA Route Log and Finder List”.
“FHA Route Log and Finder List: Additional Designations”. ^ a b American Association of State Highway and Transportation Officials (January 2000).
“Establishment and Development of the United States Numbered Highways (PDF). American Association of State Highway and Transportation Officials.
“When did the Federal Government begin collecting the gas tax?” “Funding For Highways and Disposition of Highway-User Revenues, All Units of Government, 2007”.
“On 40th birthday, Interstates Face Expensive Midlife Crisis”. “1st Toll Project Proposed for I-20 East: Plan Would Add Lanes Outside I-285” (PDF).
99–599: Surface Transportation Assistance Act of 1978 ^ American Association of State Highway Officials (September 19, 1967). Standard Highway Signs (2004 English ed.).
^ American Association of State Highway and Transportation Officials (2006). American Association of State Highway and Transportation Officials.
^ American Association of State Highway Officials (1958). Manual for Signing and Pavement Marking of the National System of Interstate and Defense Highways.
Washington, DC: American Association of State Highway Officials. ^ National Joint Committee on Uniform Traffic Control Devices; American Association of State Highway Officials (1961).
^ National Joint Committee on Uniform Traffic Control Devices; American Association of State Highway Officials (1971). Guide Signs: Conventional Roads” (PDF).
Manual on Uniform Traffic Control Devices for Streets and Highways (1971 ed.). Washington, DC: Federal Highway Administration.
Manual on Uniform Traffic Control Devices for Streets and Highways (1978 ed.). Washington, DC: Federal Highway Administration.
^ Office of Highway Policy Information (July 27, 2010). Most Travelled Urban Highways Average Annual Daily Traffic (AAT) > 250,000 (Report).
2008 Highway Performance Monitoring System (PMS). ^ American Association of State Highway and Transportation Officials (n.d.).
“Table 1: Main Routes of the Dwight D. Eisenhower National System Of Interstate and Defense Highways as of October 31, 2002”. ^ “Table 3: Interstate Routes in Each of the 50 States, District of Columbia, and Puerto Rico”.
“The Dwight D. Eisenhower National System of Interstate and Defense Highways : The Road to Success?” Long Beach, California: Society for History Education.