Abstract: We present Drive, a system for the conceptual layout of 3D path networks. Our sketch-based interface allows users to efficiently author path layouts with minimal instruction. Our system incorporates some new and noteworthy components. We present the break-out lens, a novel widget for interactive graphics, inspired by break-out views used in engineering visualization. We also make three contributions specific to path curve design: First, we extend our previous work to fit aesthetic paths to sketch strokes with constraints, using piecewise clothoid curves. Second, we determine the height of paths above the terrain using a constraint optimization formulation of the occlusion relationships between sketched strokes. Finally, we illustrate examples of terrain sensitive path construction in the context of road design: automatically removing foliage, building bridges and tunnels across topographic features and constructing road signs appropriate to the sketched paths.