We study the motion-planning problem for a car-like robot whose turning radius is bounded from below by one and which is allowed to move in the forward direction only (Dubins car). For two robot configurations σ, σ , let (σ, σ ) be the shortest bounded-curvature path from σ to σ . For d 0, let (d) be the supremum of (σ, σ ), over all pairs (σ, σ ) that are at Euclidean distance d. We study the function dub(d) = (d) − d, which expresses the difference between the bounded-curvature path length and the Euclidean distance of its endpoints. We show that dub(d) decreases monotonically from dub(0) = 7π/3 to dub(d * ) = 2π, and is constant for d d * . Here d * ≈ 1.5874. We describe pairs of configurations that exhibit the worst-case of dub(d) for every distance d.this is a natural and fundamental question related to motion planning with bounded curvature, it is also a relevant question that has repeatedly appeared in the literature, with only partial answers so far.Dubins [11] showed that the shortest curvature-constrained path between two configurations consists of at most three segments, each of which is either a straight segment or a circular arc of radius one. Using ideas from control theory, Boissonnat et al. [7], in parallel with Sussmann and Tang [26], gave an alternative proof. Sussmann [25] extended the characterization to the 3-dimensional case. Bui et al.[10] discussed how the types of optimal paths partition the configuration space, and also proved that optimal paths for free final orientation have at most two segments [6]. Significant work has been done on the problems of deciding whether a bounded-curvature path exists between given configurations among different kinds of obstacles and finding the shortest such path [12,5,21,15,3,4,2,1,9,8].At least two interesting problems have been studied where not configurations but only locations for the robot are given. The first problem considers a sequence of points in the plane, and asks for the shortest curvature-constrained path that visits the points in this sequence. In the second problem, the Dubins traveling salesman problem, the input is a set of points in the plane, and asks to find a shortest curvature-constrained path visiting all points. Both problems have been studied by researchers in the robotics community, giving heuristics and experimental results [22,19,20]. From a theoretical perspective, Lee et al.[18] gave a linear-time, constant-factor approximation algorithm for the first problem. No general approximation algorithms are known for the Dubins traveling salesman problem (the approximation factor of the known algorithms depends on the smallest distance between points).All this work depends on some knowledge of the function dub. Lee et al. [18], for instance, prove that the approximation ratio of their algorithms is max(A, π/2 + B/π), where A = 1 + sup{dub(d)/d | d 2} and B = sup{dub(d) + d | d 2}. They claim without proof that dub(d) 2π for d 2 and derive from this that A = 1 + π and B 5π/2 + 3, leading to an approximation ratio of about 5.03. We ...