To create traffic simulations of high visual-fidelity, each component part needs to be designed and modelled in great detail. Roads can be created manually, but this can become a time-consuming and laborious process when modelling large-scale networks. Therefore, automated techniques for generating road networks efficiently and effectively, is highly desirable in both urban-planning and entertainment industries. In this paper we present a novel sketch-based tool to semi-automate the design, creation, and visualisation of road networks across both flat and undulating terrains. Our tool is guided by input sketches and a combination of prioritised constraints, including the curvature of roads, their inclination, and the volume of ground that would be displaced during construction. We introduce ‘Influence Regions’ which are user-specified areas of the terrain that influence the path of the roads generated, and are used to attract or repel roads to/from certain obstacles or designated areas, such as forestation, listed buildings, marshland, etc. A user study is conducted to evaluate the usability of the system and the quality of roads generated in a diverse range of scenarios. The results indicate that our system is both user-friendly and able to produce roads that are true to the user's intention.