On May 8, 2021, at the crack of dawn, shreds of mist crept from the chilly fields onto Ziendeweg, a country road south of Amsterdam. The rush hour traffic caused by commuters using the road to bypass jams on the highway had not yet picked up. But another activity was taking place. All along the four-kilometer-long road, small groups of people were carrying bundles of white crosses and quietly beginning to erect them on the roadside. When the sun came up, the first motorists were greeted by an eerie spectacle: 642 crosses marked the precise spots where dozens of animals had been killed by vehicles during the past few years. Each cross displayed the common name of each animal, a drawing of the animal and a QR code that linked to the roadkill incident logged on the citizen science platform Observation International.

This guerilla campaign was the brainchild of biologist Bram Koese, who was frustrated by the large numbers of otter and waterfowl deaths from speeding traffic and the lack of response from local authorities. Koese decided to take matters into his own hands, and by midmorning, his parade of crosses was featured on local and national news, suitably embarrassing the municipality.

While they do not all share this intensity of activism, community roadkill monitoring programs such as Koese’s are ongoing worldwide. In fact, because road authorities themselves do not routinely keep track of animals killed by traffic—and if they do, it is only because such collisions pose a risk to human road-users—most of the data come from citizen scientists. These amateur investigators have turned up evidence revealing that some species are being driven toward extinction because of traffic.

An early effort along these lines was started in 1992 by Brewster Bartlett, aka “Dr. Splatt,” then a science teacher at Pinkerton Academy, a high school in New Hampshire. He used the school’s very first e-mail server to exchange students’ sightings and post them to a bulletin board. Since then technology has improved, and roadkill monitoring is now conducted through the use of dedicated apps or online citizen science platforms.

In Belgium, which has Europe’s densest road network, drivers can use speech recognition on the app ObsMapp to report and log roadkill. In Israel, a roadkill mapping project relies on a feature in the navigation app Waze. Motorists can tap an icon depicting the face of a porcupine that has crosses for eyes and its tongue sticking out whenever they spot a dead animal.

In 2020 Clara Grilo of the University of Aveiro in Portugal and her colleagues pulled together data from 90 European roadkill surveys and concluded that, on Europe’s roads, 194 million birds and 29 million mammals die annually. Similar calculations suggest that, each year, more than 350 million vertebrate animals are killed by traffic in the U.S.

Astronomical as those numbers for larger animals may be, they pale in comparison with the amounts of insects and other smaller creatures that perish on the road. To get a handle on that, Arnold van Vliet of Wageningen University & Research in the Netherlands and his colleagues devised a citizen science project specifically focused on insect mortality. Drivers were asked to take a daily photograph of all the insects squished on their license plates, record their car’s mileage and then scrub the license plate to start with a clean slate the next day. By extrapolating from the nearly 18,000 dead insects thus tallied, the group came up with estimates that, if extended globally, would mean that 228 trillion insects are killed each year on the world’s 36 million kilometers of roads.

Community scientists are not just mapping roadkill; they are also mapping the roads themselves. They do so because that figure of 36 million kilometers is little more than a crude estimate—and it is rapidly becoming outdated. The world’s road networks are, in fact, expected to increase by 25 million kilometers by midcentury. The open license project OpenStreetMap aims to create a world map made by the general public for the general public. In 2016, a team of researchers used it to calculate that roads carve up the world’s land into no fewer than 600,000 roadless parcels. Half of them are less than one square kilometer, and only 7 percent or so are more than 100 square kilometers. In other words, we live in a world that is completely shattered into tiny road-encircled fragments.

And that, Grilo says, is bad news for the world’s species. She and her team combined information from the International Union for the Conservation of Nature’s Red List of Threatened Species and the existing data on roadkill and worked out the risk that roadkill poses for specific species. While some, such as the Eurasian blackbird (Turdus merula), suffer huge losses—a whopping 35 million become roadkill per year—populations are able to absorb the losses without noticeable traffic-induced declines in numbers. Other species are not so lucky. The hazel grouse (Tetrastes bonasia) in Eurasia, the maned wolf (Chrysocyon brachyurus) in South America and the brown hyena (Hyaena brunnea) in southern Africa are likely to be literally driven to extinction by road traffic in the next few decades.

So roadkill is not just the unavoidable but inconsequential collateral damage that inspires the crude humor of books such as the faux field guide Flattened Fauna, The Roadkill U.S.A. Coloring and Activity Book or the lyrics of Loudon Wainwright III’s song “Dead Skunk,” “You got your dead skunk in the middle of the road stinkin’ to high heaven.” Vehicles continue to be overlooked environmental forces that are likely to decimate more and more animal populations. While mitigation measures such as “ecoducts,” underpasses and fencing are helpful, they usually protect just one or a few species.

Perhaps more powerful are community awareness projects such as the one started by Koese. The scientific data the researchers gathered are just statistics, but hundreds of shrines erected for the killed stoats, weasels, swallows, owls, frogs and geese produce a visual impact that drives home the message to road users and builders that roadkill is not a laughing matter. Unfortunately, some members of a local community that Ziendeweg runs through were not impressed by the white crosses last year, Koese says with regret. “Two days after we erected them, they had run down each and every one of the crosses,” he says.