Explosive growth of secondary roads is linked to widespread tropical deforestation.

In the tropics and beyond, roads are key proximate drivers of environmental impacts, including forest fragmentation, fires, mining, and land clearing. Such impacts may be amplified for the initial roads constructed in intact forests-which we term "first-cut roads"-which often promote a rash of associated secondary roads branching off the new infrastructure. These secondary roads in turn can dramatically elevate forest and biodiversity losses. Although widely seen as a conservation concern, the magnitude and effects of secondary road development have not been previously quantified. Without such information, impact assessment procedures for road projects risk misjudging the level of expected forest loss, hampering decision-making. Here, we quantify the environmental impacts of both first-cut and secondary roads in three of the world's major tropical regions where high-quality road maps have recently become available: the Brazilian Amazon, the Congo Basin, and New Guinea. We identified 92 first-cut roads across our study region for which we quantified the length of adjoining secondary roads and the area of related forest loss and degradation. On average, we found 4.8, 9.8, and 49.1 km of secondary road for every kilometer of first-cut road in the Congo Basin, New Guinea, and Brazilian Amazon regions, respectively. Forest loss and degradation associated with these secondary roads was remarkably heavy, being 31.5, 22.2, and 305.2 times greater, respectively, than that directly linked with first-cut roads. Our findings provide key insights into the potential scale and extent of forest loss and degradation that will emerge with proposed roads and development corridors in tropical forests.