10 Surprising Secrets of America’s Interstate Highway System

America’s Interstate Highway System looks almost inevitable today: a vast web of pavement carrying commuters, vacationers, truckers, and billions of dollars in freight. Follow the blue-and-red shields long enough, and the roads seem capable of leading almost anywhere.

But this network did not emerge naturally from the landscape. It took decades of federal planning, political negotiation, engineering, demolition, and astonishing amounts of concrete. National defense influenced its development, but so did traffic congestion, economic growth, road safety, and the desire to connect cities and industrial centers.

Along the way, the Interstate System acquired its own mythology. Some people insist that portions secretly double as military runways. Others believe every exit number measures the exact distance to a state line. Neither claim is quite true.

The real history is stranger and considerably more complicated. A miserable Army convoy helped shape a president’s thinking. Futuristic rest stops once served as architectural advertisements for entire states. Highway routes destroyed thriving neighborhoods, while newer projects now build bridges for animals displaced by the same roads.

These ten stories reveal the codes, ambitions, myths, and unintended consequences hidden beneath America’s most familiar highways.

Related: 10 Abandoned Transportation Projects That Could Have Changed the Nation

10 Eisenhower’s Miserable Convoy Helped Build the Interstate Age

In 1919, long before becoming president, Dwight D. Eisenhower joined an Army convoy attempting to cross the United States by motor vehicle. The journey was partly an experiment: The War Department wanted to know whether the country’s roads could support the long-distance movement of troops and equipment during an emergency.

The answer was not encouraging.

The convoy left Washington, DC, with 81 vehicles and 282 officers and enlisted personnel. Among them was Lieutenant Colonel Eisenhower. Over the next 62 days, they traveled approximately 3,251 miles (5,232 kilometers) to San Francisco.

Progress frequently deteriorated into mechanical slapstick. Vehicles became trapped in mud or sand. Bridges collapsed under their weight. Engines overheated, tires went flat, and equipment broke down. On the worst roads, the convoy sometimes advanced only a few miles in an entire day.

The experience stayed with Eisenhower. Decades later, after also seeing Germany’s autobahn network, he became an enthusiastic advocate for modern American highways. But the 1919 trip did not single-handedly create the Interstate System. Federal highway planning began before his presidency, and Congress authorized a national system in 1944.

Eisenhower provided the political force needed to fund and accelerate it. The Federal-Aid Highway Act of 1956 established a dependable financing system, set the federal share of construction costs at 90 percent, and launched the enormous building campaign associated with his administration.

The Interstates were intended to support national defense, but they were not simply escape routes for tanks. They were also designed to connect cities, relieve congestion, improve safety, and move people and goods more efficiently.

Still, one of the world’s largest highway networks owes part of its history to a future president spending two miserable months discovering exactly how bad American roads could be.^[1]

9 No, One Mile in Five Is Not a Secret Runway

Few highway facts sound more convincing than the claim that one mile out of every five on an Interstate must be perfectly straight. According to the story, these sections were built as emergency runways so military aircraft could continue operating if enemy attacks destroyed conventional airfields.

It has everything a durable legend needs: war, secrecy, government engineering, and thousands of supposed runways hiding in plain sight.

It also is not true.

No federal law, regulation, policy, or Interstate design standard requires one straight mile in every five. The Federal Highway Administration has searched the legislation and historical record and found no evidence that such a rule ever existed.

Interstate pavement was not universally reinforced to withstand military aircraft. Roads must support heavy traffic, including commercial trucks, but that does not transform every straight section into a concealed airbase. Overpasses, signs, medians, lighting, traffic, and surrounding development would also make many apparently suitable stretches impractical for routine aircraft operations.

Planes have landed on highways during emergencies. Military exercises have occasionally closed selected roads so aircraft can practice operating from them. Some countries have even constructed roads specifically intended to double as runways. Those isolated examples probably helped the American legend spread.

The Interstate System’s full name—the Dwight D. Eisenhower National System of Interstate and Defense Highways—made the story sound even more plausible. Defense was one consideration behind the network, but civilian transportation, commerce, congestion relief, and safety supplied much of its political support.

So the straight pavement ahead may resemble a runway. In a genuine emergency, a pilot might even attempt to use it as one.

It is just not the secret military airfield that somebody’s uncle confidently described during a family road trip.^[2]

8 Rest Stops Were Once Tiny Architectural Showpieces

Modern highway rest areas are easy to overlook. Pull in, find the restroom, stretch for three minutes, stare suspiciously at a vending-machine sandwich, and leave.

The earliest roadside stopping places had a more appealing purpose.

In 1929, a Michigan county road commissioner reportedly gave idle snowplow crews a summertime assignment: Build picnic tables from scrap wood. The tables sat beside highways, where motorists could stop, eat, and recover from roads that demanded far more effort than modern driving.

These early waysides were intentionally simple. Many lacked plumbing or elaborate facilities because officials wanted travelers to rest without turning the sites into recreational destinations. Rustic shelters, stone walls, log construction, shade, and scenic surroundings gradually became common features. During the Great Depression, Civilian Conservation Corps crews helped build roadside parks in several states.

The Interstate era transformed the rest area into a genuine safety feature. Controlled-access highways gave motorists fewer opportunities to pull over or visit roadside businesses. Designated stopping places allowed drivers to leave high-speed traffic safely, use a restroom, dispose of trash, and recover before continuing.

By the 1960s, states had discovered that these areas could accomplish something else: advertise regional identity. Rest stops became unofficial state ambassadors, with architecture inspired by tipis, oil derricks, log cabins, adobe buildings, and Space Age modernism. Some displayed public art, historic markers, mosaics, or enormous sculptures visible from the highway.

Many distinctive facilities eventually disappeared as buildings aged and states confronted maintenance costs. Standardized designs were cheaper to construct and easier to operate. Some rest areas closed altogether, while others were replaced by efficient but less memorable buildings.

A rest stop was never meant to rival the destination at the end of the road. For a brief architectural moment, however, even the bathroom break was designed to tell travelers where they were.^[3]

7 The Highways That Bulldozed Entire Communities

The Interstate System promised speed, mobility, and national connection. Inside many American cities, it delivered those benefits by destroying the places that stood in its way.

During the mid-20th century, highway planners routed urban expressways through established neighborhoods. Black, Latino, immigrant, and working-class communities were particularly vulnerable because segregation, redlining, depressed property values, and limited political influence made their land easier to condemn.

Officials frequently presented demolition as urban renewal. On maps, officials could dismiss the selected areas as “blighted” districts requiring redevelopment. On the ground, they contained homes, churches, stores, restaurants, nightclubs, and generations of community relationships.

Highway construction and redevelopment devastated Detroit’s Black Bottom and Paradise Valley. Interstate 81 cut through Syracuse’s predominantly Black 15th Ward. Similar projects damaged communities in Atlanta, Miami, New Orleans, and cities across the country.

Residents could receive payment for property taken through eminent domain, but compensation for a building could not replace an entire commercial district or social network. Renters had even fewer protections, while displaced businesses often lost the customers and community ties that had sustained them.

The damage was not always accidental. Affluent communities possessed lawyers, money, political connections, and organized opposition capable of stopping or redirecting proposed routes. Neighborhoods with less power were more likely to discover that the most “efficient” path ran directly through their homes.

The concrete barriers that remained could reinforce segregation for decades. A highway separated residents from jobs, schools, parks, businesses, and neighboring communities even when the destinations stood only a short distance away.

America’s Interstates connected distant cities with extraordinary efficiency. Within some of those cities, they made neighboring blocks feel worlds apart.^[4]

6 Exit Numbers Can Reveal How Far You Have Traveled

Exit numbers may look like little more than labels, but on many Interstates, they quietly function as distance markers.

Under the milepost system, a state begins measuring an Interstate at its western or southern boundary. An interchange located between mileposts 42 and 43 normally becomes Exit 42. If several ramps appear within that same mile, letters can distinguish them as Exit 42A, 42B, and so forth.

That makes the signs surprisingly useful. If a driver passes Exit 20 and needs Exit 75, the remaining distance is approximately 55 miles (89 kilometers). Emergency dispatchers, maintenance crews, and stranded motorists can also use exit and milepost numbers to describe locations more precisely.

However, this elegant system is not universal.

States traditionally choose between two principal methods. The milepost system assigns numbers by location, while the consecutive system labels interchanges in order: Exit 1, Exit 2, Exit 3, and so on. Under consecutive numbering, Exit 30 is merely the 30th interchange. It could be nowhere near mile 30.

This explains why an exit number can suddenly seem less sensible after a driver crosses a state line. Interstate mileposts normally restart at zero, and the neighboring state may follow a different numbering method altogether.

Even mileage-based numbers are approximate. An interchange at mile 8.7 ordinarily receives the number 8 rather than 9. The number therefore identifies the mile interval containing the exit, not its precise distance from the border.

So exit numbers do contain a navigation system—just not the perfectly uniform national code people sometimes imagine. They can tell motorists where they are, how far they have traveled, and roughly how much road lies ahead.

First, however, the driver must know which numbering system the state has decided to use.^[5]

5 The Interstate System Was Never Truly Finished

America’s Interstate Highway System was supposed to take approximately 12 years to build. Instead, more than three decades passed before officials could credibly declare the original network complete.

One of the final and most difficult gaps lay in Colorado’s Glenwood Canyon. The narrow gorge carried the Colorado River between steep rock walls, leaving engineers little room for a modern freeway. Earlier proposals inspired fierce opposition from residents and environmentalists who feared that a conventional design would destroy the canyon.

Construction on the final 12.5-mile (20-kilometer) section of I-70 began in 1980. Instead of simply blasting a broad path through the landscape, engineers squeezed the highway between the river, canyon walls, and an existing railroad.

They built approximately 40 bridges and viaducts, three tunnels, retaining walls, and lengthy elevated sections. At several points, they built the eastbound lanes directly above the westbound lanes to reduce the highway’s footprint. Some equipment had to be lowered into the canyon by cable because ordinary construction access would have caused too much damage.

The section opened on October 14, 1992—Eisenhower’s birthday—and was widely celebrated as the completion of the original Interstate System. It cost about $490 million, making it one of the most expensive rural highway projects constructed in the United States at the time.

But “complete” did not mean finished forever. New routes have since been added, existing highways have received Interstate designations, and missing connections have continued to open. A gap in I-95 was finally eliminated in 2018, 26 years after the Glenwood Canyon celebration.

The Interstate System is less like a monument completed on a particular day than a living network that continually expands, changes, and repairs itself.

The original map may have been declared finished in 1992. The roadwork, naturally, never ended.^[6]

4 Lady Bird Johnson Tried to Tame the Billboard Explosion

As Interstate highways spread across the country, billboards followed them. Companies recognized that a captive audience moving at highway speed offered an enormous advertising opportunity. Signs multiplied beside rural roads, city approaches, and landscapes that had previously offered little more than trees, farms, or mountains.

Not everyone appreciated the transformation.

Claudia “Lady Bird” Johnson, the first lady of the United States, became one of the most visible advocates for protecting roadside scenery. Her campaign helped produce the Highway Beautification Act of 1965, sometimes nicknamed “Lady Bird’s Bill.”

The law did not ban billboards on federal land, as some claim. Instead, it required states to exercise “effective control” over outdoor advertising along Interstate highways and other designated federal-aid roads. States that refused could lose 10 percent of certain federal highway funds.

That sounds strict, but the final law contained substantial compromises. Official signs and advertisements located on the property of the advertised business remained permissible. States could also allow regulated billboards in commercial and industrial areas. Sign removal frequently required compensation for property owners and advertising companies, making widespread elimination expensive.

The result was control, not eradication. States developed rules governing billboard size, lighting, spacing, and location, while advertisers continued operating wherever the law allowed.

Four states went much further. Hawaii, Alaska, Vermont, and Maine adopted state laws prohibiting most conventional billboards. Their restrictions did not all arise directly from the federal act—and some predated it—but together they created stretches of roadway remarkably free from commercial advertising.

The Highway Beautification Act did not restore every scenic horizon or defeat the billboard industry. It did establish that the view from a public highway had public value.

Apparently, even Congress recognized that no mountain could compete with a 48-foot advertisement for discount fireworks.^[7]

3 Interstate Numbers Contain a Built-In Navigation Code

The numbers on Interstate signs are not random. Once drivers understand the system, those familiar red, white, and blue shields become a crude map of the country.

Primary Interstates generally carry one- or two-digit numbers. Odd-numbered routes run north and south, while even-numbered routes run east and west. The smallest north–south numbers begin in the West, and the smallest east–west numbers begin in the South.

That is why I-5 follows the West Coast while I-95 runs along the East Coast. I-10 crosses the southern United States, while I-90 travels through the northern part of the country.

Three-digit Interstates contain another layer of information. Their final two digits identify the main Interstate they are associated with. I-495, for example, is connected to I-95.

The first digit usually indicates the type of route. An even first digit commonly identifies a bypass, connector, or partial beltway that meets its parent Interstate at both ends. An odd first digit usually marks a spur that connects with the parent at only one end before heading toward a city, airport, or other destination.

The code is clever, but it is not foolproof. Three-digit numbers can be reused in different states, so several unrelated highways may share the same designation. An even-numbered auxiliary route does not necessarily form a complete circle, and geographic complications have produced assorted exceptions.

Some numbers also disappear entirely. No I-50 or I-60 exists because either designation could be confused with existing US Routes 50 and 60 in states where the highways might overlap.

The system cannot replace a map, but it can tell an observant traveler a remarkable amount. An even two-digit shield suggests east or west. An odd one points north or south. Three digits announce a highway branching from a larger parent.

A secret code printed several feet high on signs all over the country.^[8]

2 Some Interstate Bridges Were Built for Animals

An Interstate highway may connect human communities while splitting an animal population in two. The pavement creates a dangerous barrier between feeding grounds, seasonal habitats, and breeding areas that wildlife may have used for generations.

Animals continue trying to cross. The results can be deadly for them and for the humans traveling at highway speed.

One solution looks like a piece of landscape suspended above the traffic. Wildlife overpasses are broad bridges covered with soil and vegetation, often paired with fencing that directs animals toward the safe crossing. Underpasses, enlarged culverts, and passages beneath elevated roadways accomplish the same purpose for other species.

A striking example stands near Trappers Point outside Pinedale, Wyoming. Each year, thousands of pronghorn migrate through the region between seasonal ranges. US Highway 191 cut across their route, forcing the animals onto pavement filled with fast-moving vehicles.

Wyoming constructed two overpasses and several underpasses along the highway, with fencing guiding wildlife toward them. Motion-activated cameras soon recorded pronghorn, mule deer, elk, and other animals using the structures.

The crossings did more than produce charming photographs. Since the project’s completion in 2012, wildlife-vehicle collisions in the area have fallen by more than 80 percent. The structures also helped preserve a migration corridor instead of forcing animals to abandon it.

Success depends on location and design. A bridge placed somewhere animals do not naturally travel will accomplish little. Fencing must be long and secure enough to prevent animals from simply walking around it, and different species may prefer different kinds of passages.

Done properly, however, wildlife crossings can protect motorists, reduce costly crashes, and reconnect fragmented habitat.

The Interstate System spent decades teaching animals that concrete meant danger. Now engineers are building pieces of wilderness that teach them where it is safe to cross.^[9]

1 Some Interstates Never Leave Their Own State

The word “Interstate” appears to contain a simple promise: This road must travel from one state to another.

Hawaii makes that interpretation rather difficult.

The island of Oahu contains three primary Interstate highways—H-1, H-2, and H-3—plus the auxiliary H-201. None can cross a state boundary without encountering several thousand miles of ocean.

They are not honorary highways or mislabeled state roads. They are genuine members of the Interstate Highway System, built with federal funding and required to meet Interstate standards.

That is possible because “Interstate” describes the national system rather than the travels of every individual route. A highway can belong to a network that supports national transportation and defense without physically crossing a state line. Even in the contiguous United States, some Interstate routes remain entirely within one state.

Hawaii’s highways received a special H prefix to prevent confusion with mainland routes. They connect Honolulu with military installations, residential communities, and other important destinations across Oahu. H-3, which passes through tunnels and across elevated viaducts amid the Koʻolau Mountains, ranks among the country’s most visually dramatic Interstate highways.

Alaska and Puerto Rico also possess federally designated Interstate routes despite lacking road connections to the contiguous system. Those highways ordinarily use local route numbers on their signs rather than the familiar Interstate designations.

The apparent contradiction has helped create another myth: that Hawaii’s highways received the label solely because they connect military bases. Defense connections mattered, particularly in federal funding decisions, but a military destination is not what makes a road an Interstate.

The name belongs to the system, not necessarily the geography of one road.

So America has Interstate highways that never cross a state line, never reach another island, and cannot be driven to any other Interstate in the country.

Even the highway network occasionally uses “interstate” as more of a team name.^[10]