Today’s streets have it harder than their ancestors did. Instead of feet, hooves, and wooden wheels, they shoulder semis and SUVs. As we’ve developed new ways to zip around, we’ve also changed the makeup of the routes on which we travel, transforming gravel paths into asphalt superhighways. Challenges such as extreme weather and carbon emissions mean our expressways must evolve even further, so engineers are turning to futuristic fixes to keep traffic flowing. Here’s the story behind that long and winding journey—and where it’s going next.
Past: In the rearview
1. Rocks and bricks
Humans have been clearing trees and burning brush to transport food and attack our enemies for more than 10,000 years, but Mesopotamians invented some of the first paved roads to make more transit-friendly cities around 3000 B.C. Workers molded thousands of identical clay bricks, dried them, then arranged them like tiles. To keep paths from crumbling every time a cow kicked, they glued the blocks together with bitumen, a naturally occurring semisticky petroleum that we still use as a binder in asphalt today. But this ancient population bothered with the labor-intensive method only on streets with religious significance or high military value.
While all road technology might not lead back to Rome, the Roman Empire did build some of the longest and most durable pathways in the ancient world. Builders recessed rock and gravel layers into the ground for stability. Closer to cities and in other prominent areas, pavers topped these layers with hard stones to create a more polished look.
At its height, around A.D. 100, the Empire presided over a combined 50,000 miles of highways, which allowed soldiers and merchants to move swiftly throughout Europe and Asia Minor. Some of these timeworn passages are still functional today. The Via Appia, which runs from Rome 350 miles southeast to the east coast of Italy, supports automobile traffic on select stretches— though it has required quite a few overhauls over the past two millennia.
2. Sloped mud
As the Industrial Revolution took off in Great Britain during the 1700s, local governments built longer networks of gravel highways, relying on toll collection to finance the construction of new routes. These so-called turnpikes sprang up across the countryside, spreading out from London and connecting cities in England and Scotland. But most roadways were made of small stones piled on top of mud, meaning the slightest sprinkling of rainwater could turn the rocky lanes into mucky heaps of disgusting, dangerous slush.
A civil engineer named John Metcalf had a plan for smoothing things over. His construction crews would slightly slope each new street’s surface and dig deep ditches on either side. This provided proper drainage to keep roadways from caving in due to excess moisture, preventing potholes.
Metcalf famously went the extra mile to advocate for these design changes. Blind from boyhood, he once challenged a colonel to race him to London. Thanks to the rough terrain, Metcalf made his way to the city on foot faster than the military man could get there in his horse-drawn buggy.
3. Packed Gravel and Asphalt
Nineteenth-century Scottish engineer John McAdam, frustrated that even Britain’s best drags were still bumpy, took a new approach: He replaced loose, round stones with rocks crushed into tiny angular bits that he spread out along a path, and then rolled over to press firm. These “Macadam roads” were more durable and weather-resistant—and kinder to carriage wheels—but they were still pretty loosey-goosey.
So, in the 1870s, American engineers began filing patents for bituminous asphalt mixtures known as “binders,” which combine the oily substance with gravel or sand for sleeker street surfaces. The basic cross-section of a road or highway hasn’t changed much since: Builders dig a ditch, lay down a bed of packed soil, spread a layer of crushed stone, and then top it with a smooth, 6-inch layer of either asphalt or concrete. Heavier-duty roads, such as interstates, sometimes have an additional rocky layer at the foundation.
4. Even more asphalt
Around the turn of the 20th century, the U.S. started to roll out roads as we know them, replacing soil paths with hardy concrete. But long-distance routes—such as the Lincoln Highway, which joined New York and San Francisco in 1923—were sometimes no better than dirt ditches etched in the countryside. While riding on the Lincoln with his Army convoy in 1919, Lt. Colonel Dwight D. Eisenhower had an idea: What if this, but better and everywhere? When he became president, Eisenhower masterminded the interstate highway system, its more than 45,000 miles of pavement taking three decades to lay down.
Present: What’s under our wheels
1. Solid foundations
Apartheid-era South Africa was under sanctions and economically isolated by many other countries, which made buying a highway’s worth of bitumen prohibitively expensive. So engineers in the country came up with an unorthodox solution that was not only cheaper, but also just as effective as conventional methods. Rather than laying down a thin gravel base and slathering it with half a foot of asphalt, South African designers relied on a layer of stone (infused with cement) about a foot thick as the foundation of the path, then placed a 2-inch-thick strip of asphalt on top. Trade opened up after Apartheid ended in the 1990s, but no one raced to repave the region: The unique highways proved to be just as strong and resilient as those in other countries. The clever and effective workaround became a subject of fascination for transportation officials from around the world.
2. Low-friction blacktop
A truly well-designed road won’t just withstand rain, sun, and tire abuse—it’ll also make driving easier and cleaner for the cars passing over it. In Denmark, government researchers will be testing more than 30 miles of highway built with an earth-conscious asphalt that minimizes friction with tires. To lower what’s called “rolling resistance,” the designers embed the upper layer of asphalt with unusually small stones—sometimes smaller than a quarter-inch across—which makes the surface smoother. Cars can coast longer, so drivers need to hit the gas less frequently to maintain a steady speed. Fewer presses of the pedal equals less fuel consumption, which helps minimize emissions. For every million dollars Denmark invests in these streets, motorists could save around $40 million in fuel costs.
3. Limestone bases
Germany’s comprehensive network of famously no-speed-limit autobahn highways are among the best in the world. Their secret sauce: money. Each of the country’s more than 8,000 miles of freeway costs the federal government nearly a million dollars per year in maintenance. As they say: It costs money because it saves money, and Germany’s road-building budget means better materials that cause fewer problems in the long run.
Not only are these bahns twice as thick as the average U.S. highway, but the trenches lining them are sturdy limestone rather than more-common materials such as soil or sand. (The government planned for a few stretches to double as airstrips during World War II, so some sections are actually thick and sturdy enough to survive the force of a landing plane.) Each motorway is also topped with high-quality concrete.
As a result of their resilience, these routes are safer, quieter, easier on cars, and last an average of 20 years longer than American pavement. And even though it’s not as big as the United States, Germany’s web of highways is dense—if you laid all the autobahns end-to-end, they would stretch a third of the way around the planet.
4. Ice/salt road
Especially cold or dry locales can boast boulevards made of unconventional stuff. In arid countries such as Chile, governments sometimes skip asphalt and top roads with a salty compound called bischofite, which occurs naturally in the Atacama desert. It doesn’t make clouds of dust the way sandy roads do, and rare rainfall helps the soluble substance stay in place. In chillier northern climes, frozen rivers become drives linking otherwise-isolated towns from January through April. But they’re melting earlier each year as temperatures rise.
5. Emerging asphalt alternatives
One of the easiest ways to skimp on oily bitumen is to grind up landfill-bound tires and other elastic materials, and mix them into the blacktop. Rubberized highways last nearly two times longer than their traditional counterparts, and can be 50 percent quieter as well. Plus, rubber doesn’t crack as much in extreme heat because, unlike stiff asphalt, the material can expand and contract. California, Spain, and Germany all use rubberized streets, and Japan now features the bouncy stuff in about a fifth of its national roads.
In 2015, the Indian government mandated that cities fill potholes with melted plastic garbage. This technique inspired Scottish engineer Toby McCartney to found MacRebur Ltd., a plastic-road startup that seeks to build byways using the material clogging up our landfills. By grinding thousands of soda bottles down into pellets and mixing the result with standard asphalt, McCartney trims the usual cost of paving materials by as much as 25 percent—while also cutting down on waste.
Some researchers are working on more appetizing alternatives to petroleum. Culinary waste such as soybeans, cooking oil, and even used coffee grounds can boost asphalt’s binding power; the materials’ organic properties allow them to oxidize in much the same way the bitumen does. This practice should also reduce the overall carbon footprint of new construction. But don’t expect to sniff java on the exit ramp anytime soon: Edible leftovers may replace just a small percentage of the sticky bitumen needed for a full-service road.
6. Gravel is back
As small towns across the United States face drastic budget shortfalls, some find themselves without the cash they need for road repair. In at least 27 states, localities have turned to a rugged solution. Instead of repaving damaged paths, they just unpave them—peeling up all the asphalt and leaving swaths of loose rocks in its wake.
Though pebbled pathways can generate throat-clogging dust and tend to get a little messy in the rain, they work just fine for routes that don’t see a lot of heavy traffic. And while you’d be right to worry about sharp little stones wearing down your Goodyears, some experts argue that supersize potholes in busted-up asphalt can be way more dangerous for drivers than good ol’ gravel is. Any rough spots that do form are much easier and cheaper to fix: All you need to do is get some more rocks and fill in the craters.
Future: The road ahead
Shifts in temperature and moisture mean cracks are always forming, and we often don’t know where they are until they’re really dangerous. But what if a freeway could repair itself? Scientists are experimenting with forms of fissure-fixing pavement. In the Netherlands, researcher Erik Schlangen of the Delft University of Technology laced a stretch of asphalt with a matrix of steel-wool fibers, turning the surface into one big conductor. When cracks start to form, the government passes by with a massive magnet on a truck, which makes the metal contract, closing the gap.
The Dutch are already employing Schlangen’s threads on a dozen roads, but even-more-radical solutions are also in the works. Su Jun-Feng of Tianjin Polytechnic in China, who has worked with Schlangen in the past, has tested dispersing small capsules of an expanding chemical polymer known as “rejuvenator” throughout a few Tianjin streets. Whenever fissures start to form, the capsules expand to fill up the chasms. This patch-up job halts the decay of the road while making the aging pavement less brittle—which means it’s less likely to crack again later.
2. Climate-proof creations
Extreme heat due to global warming poses one of the biggest threats to drivers across the U.S. High temps cause asphalt to split more quickly, which means pavement in toasty places will decay faster than governments can repair it. Although the Southwest will see the most scorching days, roads in the Midwest might also suffer—they weren’t built to withstand heat at all.
Some cities, such as Los Angeles, have started to paint blacktop with light hues so it absorbs less sun. But preventing future fissures might require new materials.
And as monster storms and rising tides trigger ever-bigger and more-frequent floods, traditional drainage systems in coastal areas from Georgia to Cambodia become overwhelmed. Miami, one of the world’s most vulnerable cities to sea-level rise, has already started taking preventative measures such as raising roads off the ground and building dozens of anti-flooding pump stations. But low-lying countries such as Belize might have to renovate much of their existing infrastructure. U.K. startup Topmix is experimenting with permeable pavements that slurp up thousands of gallons of rain; their mixture omits the usual layer of fine-crushed stone to allow moisture to slip through to the dirt beneath.
3. Pro-robot designs
While early models still might be a tad crash-prone, self-driving cars are coming. And they’ll eventually lead us to change our streets. Lanes, for example, will get skinnier once they don’t have to accommodate shaky human hands. Roads will also have to become “smart,” communicating instructions via embedded sensors rather than with more-traditional visual cues. Radio transmitters could take over right-of-way regulation from stoplights, and satellite pings could mark detours in place of signs. Engineers will also have to find new ways to manage construction zones along highways—car cameras have a hard time knowing which messages, from cones to hand signals to barricades, trump others. Some startups, such as UC Berkeley-born Hyperlane, have crafted novel proposals for highway upgrades like self-driving-only lanes. There’s not much time: By some estimates, fleets of robot taxis and buses could roll in during the next decade.
4. Solar panels
With every passing year, humanity needs more power: Global energy consumption could rise by around 25 percent by 2050, to the equivalent of nearly 150 billion barrels of oil a year. All that energy has to come from somewhere—so, some engineers wonder, why not get it from our streets? Open roads tend to absorb plenty of sunlight, which you surely know if you’ve ever stepped on blacktop while barefoot. Those toasty rays could power your dishwasher. In 2017, China debuted a mile-long stretch of “solar highway,” a raft of panels stuck beneath a plasticlike polymer thick enough to tolerate the weight of a vehicle. Energy from the array could funnel to streetlights above or houses nearby. Idaho startup Solar Roadways has tested similar technology, but both prototypes face serious financial hurdles: One square meter costs more than 90 times as much as the same swath of regular asphalt.