Efficient path planning has an utmost importance in the domain of autonomous navigation. Even though shortest path planning has been well discussed in the past, the same techniques might not always be used to find the fastest path in outdoor environments due to the inability of mobile agents to travel at their peak speed everywhere in irregular terrains. Mobility maps are an effective way for dealing with such irregularities. In this paper, we first introduce a grid-based mobility maps for

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... senting speed limitations in outdoor terrains. Then, we propose a heuristic for finding the fastest path on such maps. The proposed heuristic is proven to be both admissible and consistent. Therefore, it can be used with A*-like heuristic search algorithms for obtaining fastest paths efficiently. Simulation results provided in this paper verify the optimality of paths that are found with the help of the proposed heuristic.