When alien archaeologists exhume the rubble of human civilization, they may conclude that our raison d’être was building roads. Some 40 million miles of roadways encircle the Earth, from the continent-​spanning Pan-​American Highway to the hundred thousand miles of illegal logging routes that filigree the Amazon. Our planet is burdened by perhaps 3,000 tons of infrastructure for every human, nearly a third of an Eiffel Tower per person. Roads predate the wheel: Mesopotamian builders began laying mud-​brick paths in 4000 B.C.E., centuries before anyone thought to drop a chariot onto a couple of potter’s disks. Today it’s impossible to imagine life without the asphalt arteries that connect goods with markets, employees with jobs, families with each other. “Everything in life is somewhere else,” wrote E.B. White, “and you get there in a car.”

Roads are both logistical essentials and cultural artifacts. They epitomize freedom—​the “architecture of our restlessness,” per Rebecca Solnit, the “two lanes [that] take us anywhere,” per Bruce Springsteen. To us, roads signify connection and escape; to other life-​forms, they spell death and division. Sometime during the 20th century, scientists have written, roadkill surpassed hunting as “the leading direct human cause of vertebrate mortality on land.” Name your environmental ill—​dams, poaching, megafires—​and consider that roads kill more creatures with less fanfare than any of them. (More birds die on American roads every week than were slain by the Deepwater Horizon oil spill, with the road deaths accompanied by a fraction of the hand-​wringing.) And it’s only getting worse as traffic swells. A half-​century ago, just 3 percent of land-​dwelling mammals met their end on a road; by 2017 the toll had quadrupled. It has never been more dangerous to set paw, hoof or scaly belly on the highway.

Roads distort the planet in other, more insidious ways. No sooner was Rome’s Via Cassia completed around 100 B.C.E. than its surface began to shed sediment into Lago di Monterosi, spawning algal blooms that permanently distorted the lake’s ecosystem. Phytophthora lateralis, an invasive fungus that attacks cedar trees, hitchhikes in the patterns of truck tires. The little red fire ant, a merciless insect notorious for stinging the eyes of elephants, has exploited logging tracks to spread through Gabon 60 times faster than it would have otherwise. A 2000 study found that pavement itself blanketed less than 1 percent of the United States, yet its influence—​the “road-​effect zone,” to use ecological jargon—​covered up to 20 percent. Park your car on the shoulder and bushwhack half a mile into the woods, and you’ll still see fewer birds than you would in an unroaded wilderness. Hike two miles more, and you’ll still see fewer mammals. If you’re a Kerouac reader, you grew up steeped in the dogma that highways represent freedom. If you’re a grizzly bear, they might as well be prison walls.

The repercussions of roads are so complex that it’s hard to pinpoint where they end. British Columbia’s caribou herds have dwindled to furtive bands, in part because logging and mining roads have permitted the ingress of wolves—​a human-​caused disaster disguised as natural predation. Nearly a fifth of America’s greenhouse gas emissions are coughed out by cars and trucks, and the transportation sector is the fastest-​growing contributor to climate change; meanwhile, the rise of electric vehicles, whose batteries depend on lithium and other metals, has catalyzed a mining boom that threatens to disfigure landscapes in places as disparate as Chile, Zimbabwe and Nevada. Even habitat loss, the most thorough eraser of wildlife, is a road problem. Before you can log Alaska’s rainforests or convert Bornean jungles into oil-​palm monocultures, you need roads to transport the machinery in and the product out. Roads are, you might say, the routes of all evil.

Yet roads select winners as well as losers. Arizona’s highways funnel rainfall into ditches and thus soften desert soils for pocket gophers, whose tunnels parallel the shoulder like subway lines. Vultures, ravens and other cunning scavengers are ascendant, their diets subsidized by roadkill. Butterflies whose prairies have been devoured by cornfields find succor in unkempt strips of roadside milkweed. In Britain such habitat is called the “soft estate”—​a suggestion that roads are capable of creating new ecosystems, even as they shatter existing ones. A biologist once led me beneath a highway bridge to show me hundreds of little brown bats roosting in its crevices, seemingly unbothered by the traffic thumping overhead.

Crossings: How Road Ecology Is Shaping the Future of Our Planet

An eye-opening account of the global ecological transformations wrought by roads.

Considering the outsize effects of roads, it’s perhaps surprising that they didn’t truly receive their scientific due until the late 20th century. One afternoon in 1993, a landscape ecologist named Richard Forman was standing in his Harvard University office with a few students, admiring a satellite photograph of a forest. Forman was expounding on the forest’s features—​where the water flowed, why people had put houses where they had, how the animals moved through it—​when he paused. “I noticed the long slice going diagonally across the image,” he recalled to me. “It was a two-​lane road through the forest. I said, gee, we know a lot about the ecology of everything else in this image, but we don’t know much about the ecology of that.” Inspired by inattention, Forman soon coined an English term: “road ecology,” defined loosely as the study of how “life change[s] for plants and animals with a road and traffic nearby.”

He did not immediately attract disciples. When a major government committee invited Forman to present his new field to transportation higher-​ups the next year, he was met with polite laughter. “You’re not here to make us stop running over animals, are you?” one engineer asked, cocking an eyebrow. As the 1990s wore on, though, road ecology gained steam. Forman and other pioneers published papers, wrote textbooks, held conferences that lured curious officials. “All of a sudden,” Forman said, “it became mainstream.”

Road ecology inverts our oldest joke about animals and transportation: Why did the chicken cross the road? Embedded in that chestnut is an assumption—​that the road is inviolable and eternal, as fixed in its course as a river. The road is a given; it’s the fowl whose actions demand explanation. But the riddle’s logic is backward. It’s the animals who have always moved, the road that’s the upstart. A better question might be: Why did the road cross the land?

This framing isn’t always comfortable. When we don’t ignore roads, we nevertheless dismiss their toll as the inevitable cost of modernity. Other forms of human-​caused animal death are deliberate: We pull the trigger, set the trap, order the cheeseburger. But few among us ever flatten an animal on purpose. Like most people, I at once cherish animals and think nothing of piloting a 3,000-​pound death machine. The allure of the car is so strong that it has persuaded Americans to treat 40,000 human lives as expendable each year; what chance does wildlife have? One summer, in Alaska, I hit a feisty songbird called a yellow-​rumped warbler—​a death I didn’t discover until I found the delicate splash of feathers wedged in the grille the next day. I’d killed not with malice but with mobility. “We treat the attrition of lives on the road like the attrition of lives in war,” the writer Barry Lopez lamented. “Horrifying, unavoidable, justified.”

This is particularly true in the United States, home to the world’s longest road network, at four million miles. Our midcentury automotive revolution spawned not only highways but also parking lots, driveways, suburbs, pipelines, gas stations, car washes, drive-​throughs, tire shops and strip malls—​a totalizing ecosystem engineered for its dominant organism, the car. For all its grandeur, though, America’s highway network is relatively static. Although we spend almost $200 billion on our roads annually, most goes toward repair rather than new construction. Granted, American wildlands are hardly safe from ill-​conceived development: Florida, for one, has been scheming up new toll roads in panther habitat, and even routine highway maintenance projects have an uncanny knack for adding lanes and worsening traffic. Even so, our country’s asphalt limbs have mostly ceased to elongate, petrified into something like their eternal shape.

Instead, we’re exporting our autocentric lifestyle. More than 25 million kilometers of new paved road lanes will be built worldwide by 2050, many through the world’s remaining intact habitats, a concrete wave that the ecologist William Laurance has described as an “infrastructure tsunami.” Astoundingly, as of 2016, three-​quarters of the infrastructure that will exist by the middle of this century had yet to be built. Although it’s easy to denounce the tsunami, I benefit from roads as much as anyone: I eat avocados trucked from California; I get pizza delivered to my doorstep; I rely on America’s marvel of a highway system to reach friends and hospitals and airports. (And I confess to feeling what one Volkswagen ad campaign called Fahrvergnügen, the pleasure of driving.) Roads pose the same queasy conundrum as climate change: Having profited wildly from growth, can wealthy nations deny less-​developed countries the benefits of connectivity?

Road ecology offers one path through this thicket. North America and Europe constructed their road networks with little regard for how they would affect nature and even less comprehension of how to blunt those effects. Today, in theory, we know better. Road ecology has revealed the perils of reckless development and pointed us toward solutions. Over the last several decades, its practitioners have constructed bridges for bears, tunnels for turtles, rope webs that allow howler monkeys to swing over highways without descending to the forest floor. On Christmas Island, red crabs clamber over a steel span during their beachward migrations; in Kenya, elephants lumber beneath highways and railroads via passages as tall as two-​story houses. And road ecology has yielded more than crossings: We’ve also learned to map and protect the migrations of cryptic animals, to design roadsides that nourish bees and butterflies, and to deconstruct the derelict logging tracks that lace our forests—​proof that old mistakes need not be permanent.

Here in the U.S., we’re entering a period that might fairly be considered the golden age of road ecology. Once, we ignored our infrastructure’s cataclysmic ecological toll; today, we’re newly focused on remediating it. The 2021 Infrastructure Investment and Jobs Act alone allocated billions of dollars to the cause: Its programs include a $350 million grant program for new wildlife crossings; $250 million to fix or obliterate derelict roads and trails in national forests; and $1 billion to repair road culverts that block salmon, herring and other migratory fish from reaching their spawning grounds. The coming years will undoubtedly be transformative ones for our road network. Whether we can ever truly undo the harms of our concrete-encrusted world is far less certain.

Excerpted from Crossings: How Road Ecology Is Shaping the Future of Our Planet by Ben Goldfarb (W. W. Norton & Company). Copyright © 2023 by Ben Goldfarb. All rights reserved.

Get the latest Science stories in your inbox.