In this article we address the problem of the traversability of the trajectory of an agricultural robot defining the conditions insuring a safe displacement according the notion of "obstacle". Unlike other approaches that try to detect and to avoid obstacle, we propose the concept of Allowable Speed Trajectories which depends on the vehicle capabilities, its dynamics constraints, its speed and the 3D rendering of the environment. We make a dynamic study to estimate the acceleration of inertial center of vehicle taking into account geometry of the environment and the trajectory to follow. Then we propose some solutions to adapt the speed and/or the trajectory of the agricultural vehicle according criteria linked to the mission objective. Results in simulation and real conditions show the performance of our algorithm in different scenarios.