One challenging problem about connected automated vehicles is to optimize vehicle trajectories considering realistic constraints (e.g. vehicle kinematic limits and collision avoidance) and objectives (e.g., travel time, fuel consumption). With respect to communication cost and implementation difficulty, parsimonious trajectory planning has attracted continuous interests. In this paper, we first analyze the feasibility conditions for a general continuous-time trajectory planning problem and then propose an analytical solution method for two important boundary trajectory problems. We further propose a discrete-time model with a more general objective function and a certain sparsity requirement that helps parsimonious planned trajectories. This sparsity requirement is implemented with a l1 norm regulatory term appended to the objective function. Numerical examples are conducted on several representative applications and show that the proposed design strategy is effective.