This paper addresses online modification of reference trajectories for multiple robots with collision avoidance. A smooth mapping dependent on distances between the robots has been proposed to coordinate collision-free motion from given reference trajectories. The trajectories generated by the mapping allow the robots to avoid collisions with each other, and are smooth enough to be tracked by the robots. Moreover, when the reference trajectories are separated from each other, the mapping is identical and the modified trajectories remain unchanged from the original ones. Although the existing method works well for collision avoidance, the modified trajectories often include unnecessary avoidance motion. A new method based on the existing one is proposed to improve the mapping to generate more natural trajectories by eliminating the unnecessary avoidance motion. The new mapping depends not only on the relative distances but also the relative angles between the robots. Finally, the effectiveness of the proposed method is demonstrated by a simulation for three mobile robots.