Trajectory Based Operations (TBO) and 4-D trajectory (4DT), i.e. latitude, longitude, altitude and time, are core concepts in Next Generation Air Transportation System (NextGen). Assuring safety of TBO involves ensuring there are no conflicts between multiple 4DT trajectories. If the complete position profile (i.e. latitude, altitude and longitude for all time) is prescribed and perfectly followed, it is straightfoward to tell whether there would be conflicts by simply comparing all 4DTs, for all time. However, because a complete position profile implies a fully specified speed profile, making pilots conform to a fully specified speed profile for the entire flight will require even more workload for Air Traffic Controller (ATC) and leave less autonomy for airlines than current spacing-based operation, which is unlikely to be welcomed by the industry. Trajectory based concepts thus encounter a tradeoff: how much autonomy over speed should be assigned to airlines to ensure conflict-free trajectories while simulaneously reducing ATC workload? This paper aims to answer this question by conducting a safety analysis on a simple but fundamental leader-follower example. 33 scenarios are created based on different combinations of initial conditions, scheduled timings, and positions. It is found out that one scenario is guaranteed unsafe, 15 scenarios are guaranteed safe, 16 scenarios are safe in certain conditions and one scenario requires a full speed profile to guarantee safety. The analysis results show that, with certain structure prescribed to the 4DT trajectory, it is realistic enough to ensure conflict-free trajectories and assign full autonomy over speed to pilots. Potential application of the results to a speed advisory system for both strategic and tactical trajectory planning is also discussed in the end.