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Humans have spread an enormous net over the land. As the largest human artifact on earth, this vast, nearly five million mile (8 million km) road network used by a quarter billion vehicles permeates virtually every corner of North America. The network is both an engineering marvel and an economic success story. Indeed, it provides unprecedented human mobility, greatly facilitates the movement of goods, and stretches the boundary of social interactions. In effect, roads and vehicles are at the core of today’s economy and society. These roads are superimposed on mountains, valleys, plains, and rivers teeming with natural flows. Streams and groundwater flow through the land. Wind carries and deposits seeds, spores, and sediment. Wildlife forage and disperse and may migrate. Fish do too. In effect, nature’s never-ending horizontal flows and movements mold the land mosaic and create its patterns of biodiversity.

These two giants, the land and the net, lie intertwined in an uneasy embrace. The road system ties the land together for us yet slices nature into pieces. Natural processes degrade and disrupt roads and vehicles, requiring continuous maintenance and repair of the rigid network. Conversely, the road system degrades and disrupts natural patterns and processes, requiring management and mitigation for nature. Both effects—nature degrading roads and roads degrading nature—are costly to society. They also increasingly gain public attention.