  To generate safe and real-time trajectories for an autonomous vehicle in
dynamic environments, path and speed decoupled planning methods are often
considered. This paper studies speed planning, which mainly deals with dynamic
obstacle avoidance given the planning path. The main challenges lie in the
decisions in non-convex space and the trade-off between safety, comfort and
efficiency performances. This work uses dynamic programming to search heuristic
waypoints on the S-T graph and to construct convex feasible spaces. Further, a
piecewise Bezier polynomials optimization approach with trapezoidal corridors
is presented, which theoretically guarantees the safety and optimality of the
trajectory. The simulations verify the effectiveness of the proposed approach.
