How does traffic affect erosion of unpaved forest roads?

The relationship between traffic and forest road erosion has been studied for decades, and the answer to the question "what happens when traffic is present on unpaved forest roads?" is simple: erosion increases. However, the answer to the question "why does it increase?" is complex and requires us to consider forest road erosion through an integrated lens. Fully understand-ing how traffic affects forest road erosion will allow us to control forest road erosion effectively. In this synthesis, we look at forest road erosion literature and focus the discussion on the interactions between traffic and erosion. Specifically, we explore four main hypotheses that have been proposed to explain how traffic affects erosion. These hypotheses are discussed in detail, including what data and information are required to evaluate them. In addition to the specific traffic-erosion interactions, we review important factors that interact with traffic to enhance erosion. Finally, we propose a framework that describes forest road erosion as a combination of all limiting factors. This framework can help guide future data collection needs, allow us to form a more holistic understanding of forest road erosion, and ultimately improve predictions of erosion from forest roads.