Just how much do U.S. roads matter?

A road is a lot more than just a road, and Richard Forman of Harvard University has started to work out just how much more.

Traffic noise, salt runoff, spread of exotic plants, and other impacts of U.S. roads affect about one-fifth of the nation’s land, Forman calculates. That “preliminary estimate is more likely to rise than to drop,” he says in the February Conservation Biology.

The 6.2 million kilometers of U.S. public roads cover 1 percent of the country, an area the size of South Carolina, and the impacts of those thoroughfares extend farther, Forman argues.

Working from studies in the Netherlands and Massachusetts, he calculated the footprint of each category of road. For example, a primary road feeding 10,000 vehicles a day through a woodland leaves its stamp on a strip 305 meters wide, he estimates. His breakdown shows that roads in rural areas “have by far the greatest total ecological effect,” he reports.

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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