What Happens When Animals Cross the Road | Science

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As highways encroach ever further into animal habitats, drivers and wildlife are in greater danger than ever. And off the beaten path, decaying old forest roads are inflicting damage as well. “Roads are this incredibly disruptive force all over the planet that are truly changing wild animals’ lives and our own lives in almost unfathomable, unaccountable ways,” says science journalist Ben Goldfarb, author of the 2023 book Crossings: How Road Ecology Is Shaping the Future of Our Planet.

Goldfarb wrote about this problem for the March 2024 issue of Smithsonian. For Earth Day, we’ll talk to him about what’s being done to make the relationship between roads and lands more harmonious, and we’ll meet Fraser Shilling—a scientist at the University of California, Davis, who’ll tell us what he’s learned from his rigorous scholarly examination of … roadkill.

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A transcript is below. To subscribe to “There’s More to That,” and to listen to past episodes on the devastating effects of wildfires, a NASA mission to capture asteroid dust and the 2024 North American total solar eclipse, find us on Apple Podcasts, Spotify or wherever you get your podcasts.


Chris Klimek: Fraser Shilling was out driving in California one day when he saw something unusual in the road.

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Fraser Shilling: There was this brown, fluffy thing, and I thought, “What is that? It’s such a strange-looking animal.”

Klimek: Most people don’t have a habit of stopping to check out roadkill when they see it on the highway, but this is Fraser’s job. He actually studies roadkill. More specifically, he’s the director of the Road Ecology Center at the University of California, Davis.

Shilling: I’ve done some sketchy pullovers on interstates, because if it’s a porcupine, if it’s a bear, I really want to make sure that’s what it is.

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Klimek: Road ecology is the study of how roads and highways impact local ecosystems. So, to Fraser, a dead animal in the road is important scientific evidence.

Shilling: I think it’s a really important activity, obviously, and I have to do my part. I can’t just expect other people to collect the data.

Klimek: But on this day in particular, it was a false alarm.

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Shilling: And I pulled over, and it was a teddy bear.

Klimek: From Smithsonian magazine and PRX Productions, this is “There’s More to That,” the show that may definitively solve, right here in this episode, why a chicken would want to cross a road. This week, just in time for Earth Day and spring migration season, we’ll learn all about road ecology, what our roads are doing to our ecosystems and how we can fix it. I’m Chris Klimek.


Klimek: One dead squirrel or dead deer in a road might not be that much cause for concern, but if you keep finding dead deer in the same stretch of road, then there’s obviously a problem, both for the deer and for the people that use that road.

Shilling: This has happened to me. I’ve driven around a curve, you don’t have time to stop if you see something around that curve, and I had, in one stretch of Highway 12 in California, three male deer within a mile of each other. They’re just standing in or about to enter the road. Very alarming. I don’t think I would’ve died. I was probably only going 50, but it definitely would’ve been a noticeable impact on my life. But most of the animals are not a safety concern. Most of the animals that are being hit are smaller, like newts. There are places where newts are migrating across roads between where they spend their adult phase and where they’re going to reproduce. They’re just annihilated by traffic.

And some areas, you think, “Well, they’ve always been doing that, so what’s the big deal?” But where it becomes a big deal is that you get fewer and fewer and fewer newts over time. Part of that is just loss from the regular traffic that’s occurring, but also, as you increase traffic, you’re increasing the number of newts that are getting killed, and, eventually, you’re going to wipe out the population. These are real-time ecological disasters, some of them.

Klimek: Do people generally get it, or does it take a bit of explaining for you to say like, “No, this is actually valuable data that we can collect and learn from?”

Shilling: Well, at the beginning, as you might imagine, there were people trying to be funny, ways of asking questions. I had a SiriusXM station interview, probably the weirdest media discussion about roadkill that I’ve had. But it was interesting. You’ve got these shock jocks, initially they were making fun of it, but then they started to get into it.


Ben Goldfarb: There are just so many different ways in which our transportation infrastructure disrupts animal lives.

Klimek: Ben Goldfarb is the author of an acclaimed book called Crossings: How Road Ecology Is Shaping the Future of Our Planet.

Goldfarb: The dead deer or raccoon or squirrel we’ve all seen by the side of the road, that’s just the tip of the iceberg. Roads are this incredibly disruptive force all over the planet that are truly changing wild animals’ lives and our own lives in almost unfathomable, unaccountable ways.

Generally, roads are enormous sources of pollution, right? Our cars are constantly bleeding cadmium and copper and zinc and microplastics. One of the big issues that scientists have only recently discovered is that tire particles are a huge problem. I think there’s something like 6 million tons of tire particles that enter the environment every year, and they contain this chemical called 6PPD, which kills salmon in huge numbers.

Another big issue is invasive species. In Oregon, there’s a fungus that actually rides in truck tire treads and gets dispersed up the road network that way and kills trees. There’s all kinds of novel agents, both chemical and biological, that are using these roads to spread through our forests.

Klimek: These particularly toxic roads, are they concentrated in a few geographic areas, or are they dispersed all over?

Goldfarb: I think it’s a pretty widespread problem, but road salt, which is in some ways probably the most transformative, consequential pollutant along our road networks, and obviously that’s something that we use as a de-icing chemical. So that’s really a Northern issue. I think Minnesota is the most profligate user of de-icing salt, and that’s turning all of these freshwater rivers and lakes and streams into functionally brackish estuaries. There are some cases where ocean crabs have entered these freshwater ecosystems, because that’s just how salty they’ve gotten.

And then, another big issue, too, is that: Look, animals like salt. If you’ve got these salty roadsides and you’re luring all of these deer and moose and other critters to the roadside, well that’s also a huge roadkill issue.

Klimek: Are there other de-icing agents available that don’t have such severe consequences for the environment?

Goldfarb: Beet juice has been used in some places. It doesn’t smell great, so it hasn’t really caught on, and it’s also a little bit eerie to see bright red bloody-looking roads that are covered in beet juice. So the quest for a universally beloved, non-salt de-icer continues.

Klimek: Yeah. On the beet juice note, I do use a citrus-based chain degreaser on my bicycle. It’s ground up orange peels or something that they claim is eco-friendly and as effective as any artificial chemical. So I hope that’s right.

Goldfarb: Well, the fact that you’re getting around via bicycle, that’s a big win right there. So, Chris, you’re doing pretty good, man.

Klimek: Is there any way in which our roads are a good thing for animals?

Goldfarb: It depends who you are, right? The scavengers, for example, the turkey vultures or the coyotes that use roadkill as this resource, essentially. Or think about the Midwest, we’ve turned all of the landscape into corn and soy monoculture, and some of the only strips of native prairie vegetation remaining are those roadsides and road medians that end up being pretty good habitat for animals like monarch butterflies. Roads are ultimately ecosystems in their own right, and every ecosystem has winners and losers.

Klimek: Yeah. You opened the door to this a little bit when you mentioned de-icing salt, but how do roads alter biodiversity more broadly than just animals being struck by cars?

Goldfarb: I think a lot about that barrier effect. These walls of traffic that animals don’t even attempt to cross in many places. Lots of big interstate highways actually have very little roadkill, because animals never even try to cross the highway. And yet, they’re having enormous impacts on wildlife distribution. You end up, in some cases, with very inbred populations. Famously, in Southern California, there’s this cluster of mountain lions living near Los Angeles surrounded by freeways. And those animals have ended up having to mate with their own daughters and granddaughters and even great-granddaughters because they just can’t cross the highway to escape this little island of habitat, and no new animals can cross to enter the population.

So even without killing animals directly, these roads are dramatically changing their lives and influencing where they can live and who they can mate with.

Klimek: So, conversely, how are humans impacted by animals in the roadway?

Goldfarb: Roadkill is a really dangerous event for drivers as well as for animals. There are up to 2 million large animal crashes in this country every year, most of them with white-tailed deer, and several hundred drivers die in those incidents. And road collisions with animals are costing society more than $8 billion every year, in vehicle repairs and hospital bills and tow trucks and so on. This epidemic of wildlife-vehicle collisions is a human public health and safety crisis, in a lot of ways.

Klimek: Are there other ways in which animals have adapted to this influx of road construction?

Goldfarb: Certainly animals have ingenious strategies for living alongside all of this infrastructure. In Chicago, there’s this very famous population of urban coyotes that looks both ways and crosses at the crosswalks. They’re very intelligent animals.

There are even cases of evolution that have occurred due to road construction. There’s a very famous example in Nebraska where cliff swallows, which are those birds who build their little mud nests on highway overpasses and bridges, they’ve actually evolved over time to have shorter wings. Because if you have a long wing as a bird, that’s good for flying long, straight directions, whereas having a short wing is good for maneuverability and making lots of tight rolls and turns to avoid an 18-wheeler. The long-wing swallows have gotten weeded from the population by roadkill, and the shorter-wing swallows remain, and now the whole population is becoming less susceptible to roadkill.

That’s just incredible to think about, right? That evolution is usually this process that unfolds over the course of thousands or millions of years, but roads and cars are such a powerful selective pressure that they’re literally driving evolution in a matter of decades.

Klimek: Have road construction techniques evolved over the decades? Are we building them in a more eco-conscious way now or not so much?

Goldfarb: It is true that roads are one of the technologies that are least amenable to disruption. One thing we’ve become much more cognizant of, and better about, is the need to build wildlife crossings: overpasses and underpasses and tunnels that allow animals to safely cross highways. And, typically, whenever there’s a big highway modification or expansion, they’ll include some wildlife crossings. We’ve got the equipment out there already—let’s just put it in a tunnel or something like that to facilitate animal movements.

Klimek: And from what we’ve seen, do animals use these crossings when we build them? Do they figure out that’s a safer way to get across the eight lanes or however many there are?

Goldfarb: Absolutely. Yeah, crossings are extremely effective. Typically, they reduce vehicle collisions by 90 percent or so, in part because, typically, you’ve got a crossing and then you’ve got roadside fences that funnel the animals to the crossings and allow them to safely cross the highway. So there’s lots of research showing that animals definitely use these things.

And in many cases, they actually pay for themselves. Sometimes the transportation department will propose a new $5 million wildlife overpass, and everybody shakes their head about the idea of spending $5 million on helping elk cross a highway. But actually, by preventing all of these really dangerous, expensive crashes with animals and vehicles, these crossings are actually recouping their own construction costs. And that’s a big part of the reason that so many transportation departments around the country are really embracing them.

Klimek: What do these crossings look like? Are they similar to what a pedestrian bridge or tunnel would be?

Goldfarb: In some ways, yeah. The basic technology isn’t all that different, but you want to make them look like habitat. You want an animal to feel comfortable crossing this novel, weird structure. So typically, the overpasses especially will have shrubs and even whole trees and dirt.

And one of the cool things that’s happening now in road ecology is that we’re thinking about different species. It used to be that engineers and biologists were very focused on the big animals, the deer and the elk. And now we’re also thinking, “Well, wait a second, what does a meadow vole or a snake or a lizard need to feel comfortable on these crossings?” You tend to see lots of rock piles and log jams and other little micro-habitat features that might induce an animal to run across.

Klimek: Yeah. I know you mentioned deer specifically as one of the major sources of roadkill and accidents. Are there other significant categories of animals that changed their patterns as a result of these crossings being made available?

Goldfarb: There are incredibly successful crossings for grizzly bears and pronghorn antelope and salamanders. There have been crossings built for this incredible diversity of species, and they’re really effective. But it’s important to really think about what different species need.

For example, the difference between black bears and grizzly bears. Grizzly bears were plains animals who lived out into the prairies. That was where Lewis and Clark saw them in eastern Montana. So they like to be out in the open. They like having a big, open bridge to walk across so they can confront their enemies with their power and speed. Whereas black bears are more forest dwellers and more comfortable in tighter spaces, potentially, and they’re typically happier using smaller underpasses that a grizzly bear would probably avoid.

So different species just have different requirements for these crossing structures, and that’s one of the things that road ecologists do, is to think, “OK, in this given place where we want to build one of these crossings, what are the species we have to account for, and how do we account for them in the design of this structure?”

Klimek: Salamanders is not one of the species I was picturing as I was reading the excerpt from your book Crossings. So tell us more about that. How do you get a salamander to cross where you want it to cross?

Goldfarb: Amphibians, even though they’re small, they’re also migratory. They travel proportionately very large distances, and they’re typically moving between their upland forest habitat, going down to their breeding ponds, and they’re often moving in large numbers on these warm, wet spring nights. The problem is that we tend to build our roads in the same low-lying areas where water collects and amphibians breed. So in many cases, you get these big squishing events of salamanders and frogs and toads and other amphibians. Again, those warm, wet spring nights in the Northeast are just the most dangerous times. Yeah, the phrase “massive squishing event” is actually in a road ecology textbook.

Klimek: Oh, wow.

Goldfarb: There are a number of great salamander and frog tunnels, these little narrow passages that go under roadways. You could drive over them a thousand times and never know they were there, but they do tend to work really well.

Klimek: The roads we drive on every day are only one of Ben’s concerns. Ben recently wrote an article for Smithsonian magazine about roads that have fallen out of use. He says that you can’t just leave an old, decaying road to sit and expect nature to reclaim it.

Goldfarb: There’s just this huge road density out there. In some places, there are more roads per square mile in national forests than there are in New York City, which is pretty hard to fathom. And those roads, even though they’re out in the middle of nowhere, they still have a big environmental impact.

What my story’s about, in a lot of ways is, OK, what do we do about those impacts? If roads cause problems in these otherwise wild areas, can we eliminate those roads? And that’s what the Forest Service and its many partner organizations are doing in many cases, is getting in there with the same heavy machinery that built the roads—in some cases, the big, yellow Tonka toys—and just tearing that roadbed up and allowing nature to reclaim it. Which is really exciting.

Klimek: So generally, if one wants to decommission a road safely with minimal environmental impact, how could that be done?

Goldfarb: One of the challenges is that often the soil is really compacted. You’ve got 30 years of big, heavy logging trucks rolling down these dirt roads, and so all of that pressure and weight over time has really compacted the soil. So it’s super-hard for any vegetation to really effectively take root there. What firms that do road decommissioning and the Forest Service does is rip up that roadbed to loosen up the soil, and then you can replant it, and that vegetation will have a much greater chance of success.

It’s funny, I visited a lot of these sites where road decommissioning was in progress, and it looks like a war zone. The earth is just ripped up everywhere, and there are saplings lying over the road that they tear up and use to cover the roads so that seedlings and wildflowers and stuff can shelter in the vegetative cover. So the whole thing looks like a tornado went through or something like that.

But you come back in 20 years, and it truly looks like a forest. I visited a bunch of sites in Idaho and Montana where roads were decommissioned 20 or 30 years ago, and you truly would have no idea that a road had ever been there, if there wasn’t a scientist telling you so. So it can be pretty inspiring.

Klimek: What are the barriers to this always being done in the most conscientious way? Expense? Politics? A combination of factors? Why doesn’t this always happen the way we might wish?

Goldfarb: You put your finger on the two big ones. Expense and politics. The expense, the U.S. Forest Service, this giant federal agency that manages something like 190 million acres of American public land, is also the largest road manager in the world, I think. Unbeknownst to most people, the Forest Service has something like 370,000 miles of road. You get to the moon and most of the way back on Forest Service roads.

In general, you’re looking at $5,000 to $15,000 per mile of decommissioned road—that tends to add up quickly. The Forest Service is also chronically a funding-challenged agency. So much of its budget goes toward fighting wildfires, and there’s often very little left over for anything else, including road decommissioning. So expense is definitely a big one.

And then there’s also, oftentimes the Forest Service proposes closing some roads, and there’s a lot of uproar from locals who don’t want to see those roads taken out of commission. So it can definitely be politically contentious at times.

Klimek: To back up a few decades, how did the Forest Service become the keeper of these tens of thousands of miles of road?

Goldfarb: Initially, a lot of those roads were built with really good intentions. The Forest Service was created in the early 1900s, and its first generation of rangers basically said, “We have been tasked with stewarding these forests, and we need roads to do that. We need to be able to fight fires and to remove trees that have been killed by beetles and keep an eye on the elk population. We need these roads to manage this land.” That was where a lot of those early roads came from, I would say.

And then in the 1950s, after World War II, there was this huge economic boom, a lot of home construction going on. And a lot of the private timber lands in America had been clear-cut already, and those national forests were the site of all of this industrial logging. And suddenly those early roads, those Forest Service roads, became the basis for this vast new network of logging roads. And in many cases, it was these private timber companies that the Forest Service was effectively paying to build logging roads on public land.

And so that’s where, when we talk about forests that have higher road densities than New York City, what we’re talking about are these incredibly dense networks of logging roads. One biologist told me that you go to some forests and it looks like the loggers must have driven to every single tree, because the roads are just so thick. And it’s actually very poignant to read the journals and memoirs of some of these early Forest Service rangers, as I did, because they talk about the pain of seeing these forests that they love just totally overrun with roads that they helped facilitate.

Klimek: Here’s the good news: Ben says there’s a lot of cause for optimism right now.

Goldfarb: Earlier we were talking about funding being one of the primary limitations for road decommissioning. And now, there’s just a lot more funding available, really thanks to these two giant pieces of legislation passed under the Biden administration, the Bipartisan Infrastructure Act and the Inflation Reduction Act. And both of those giant laws have different pots of money embedded within them that can be used for road decommissioning.

In the Infrastructure Act, there’s this thing called the Legacy Roads and Trails Program, which is, basically, $250 million for road restoration and rehabilitation. And then, in the IRA, the Inflation Reduction Act, there’s also all of this money that can be used by the Bureau of Land Management, which is the Forest Service’s sister agency, for road restoration. So there are just these big new pots of money coming online now and being distributed. And everybody I talked to for this story was just really excited about the prospects for road removal in the years ahead.

Klimek: That Smithsonian story you wrote was really focused on the removal of forest roads, rural roads, but what about the freeways and roads we were discussing earlier that remain heavily used? Are there ways of reducing the environmental harm that they cause?

Goldfarb: Yeah, it’s a great question. I think that one of the exciting things in that bipartisan Infrastructure Act that also has money for road removal, is that it also has $350 million for those new wildlife crossings that we were talking about. Which is easily the largest pot of money for animal passages ever put together. Historically, it’s been the Western states that have built a lot of these animal passages, but now states like Pennsylvania and South Dakota and Nebraska are getting interested.

I think that in the next five to ten years, thanks to this big federal grant program, we’re going to have lots more wildlife crossings popping up all over the country. And granted, that’s not going to solve the problem of roads in nature, obviously, but hopefully it’ll at least help to alleviate some of the really negative impacts.

Klimek: Smithsonian magazine contributor Ben Goldfarb is the author of Crossings: How Road Ecology Is Shaping the Future of Our Planet. This has been a really illuminating conversation, Ben. Thank you.

Goldfarb: Thank you so much, Chris. Yeah, I appreciate your time and interest.

Klimek: To read Goldfarb’s latest article in Smithsonian about safely decommissioning roads, and to learn more about how to report roadkill sightings to Shilling’s database at UC Davis, check out the links in our show notes.


Klimek: And speaking of Shilling, we couldn’t leave you without sharing one more story from him. We like to end all of our episodes with a “dinner party fact.” This is an anecdote or piece of information to stoke the conversation at your next social gathering. And for me, well, I can’t stop thinking about what Fraser told me about the culinary aspect of his roadkill research. Hold onto your dinners, folks.

Shilling: It falls a little bit into that shock jock kind of category of, “Oh, roadkill is so weird. What is that? What are you talking about?” But there’s a huge population of people that do collect and eat animals fresh off the road. I’ve done that. I’ve stopped on the side of I-5, 101, 395, and I have sliced out parts of deer from a fresh carcass and taken them home.

Klimek: Don’t knock it until you’ve tried it, I guess.

Shilling: Steak in a grocery store or chicken, how many days ago was that thing alive? But I would bet anything that the meat I’m cutting out from inside a deer that was killed a day ago has way less bacteria on it than that steak in a supermarket.

Klimek: After the New York Times published an article about his research in 2010, Fraser got an unexpected call.

Shilling: A chef in San Francisco called me up and said, “Hey, I do these unique meals for wealthy people, and we want to do a really just incredible dinner made from roadkill. Can I use your system to find out where to get something?” And I thought about it and I said, “Yeah, actually,” because our reporting’s real-time. So I said, “Well, how about this?” I knew he was in San Francisco, “I’m going to look at our system, as soon as something comes in that looks like it was probably fresh, especially if there’s a photograph, I’m going to forward the location to you, and you can just zip out there and go get it.”

And he did. He did exactly that, and did a meal of raccoon, which I was kind of surprised about. And rabbit, which makes more sense, based on that data collection. It was not at all legal, but definitely interesting.

Klimek: “There’s More to That” is not legal advice, but it is a production of Smithsonian magazine and PRX Productions. From the magazine, our team is me, Debra Rosenberg and Brian Wolly. From PRX, our team is Jessica Miller, Genevieve Sponsler, Adriana Rozas Rivera, Ry Dorsey and Edwin Ochoa. The executive producer of PRX Productions is Jocelyn Gonzales. Our episode artwork is by Emily Lankiewicz. Fact-checking by Stephanie Abramson. Our music is from APM Music.

I’m Chris Klimek. Thanks for listening.

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