People often choose one route when traveling from point A to point B and a different route when traveling from point B to point A. To explain these route asymmetries, we propose that people rely on a heuristic (the initial segment strategy, or ISS) during route planning. This heuristic involves basing decisions disproportionately on the straightness of the initial segments of the routes. Asymmetries arise because the characteristics that favor selection of a particular route in one direction will usually differ from those that favor selection when traveling in the opposite direction. Results from five experiments supported these claims. In the first three experiments, we found that subjects' decisions were asymmetric and involved a preference for initially straight routes. In Experiment 4, we confirmed that the ISS is a heuristic by demonstrating that people rely on it more when under time pressure. However, people can choose the optimal route when instructed to do so. In Experiment 5, we generalized the findings by having subjects select routes on maps of college campuses. Taken together, the results indicate that the ISS can account for asymmetries in route choices on both real and artificial maps.