In order to restrict non-yielding maneuvers of left-turning vehicles, an optimal design of left-lane line extensions is proposed to solve the problem. A field observation was conducted to collect a data set of left-turning vehicles at the beginning of a green phase at two similar intersections (one with a permitted phase and the other with a protected phase). The comparative analysis shows no significant difference in the speed distribution using either a permitted phase or a protected phase, but it reveals that a permitted phase can lead to a larger acceleration when the left-turn vehicles pass through the conflict points. Those indicate the existence of non-yielding maneuvers of left-turn vehicles at signalized intersections with a permitted phase. Optimal designed left-lane line extensions contain two types of segments, circular curves and transition curves, and they are only related to four geometry parameters of an intersection. The proposed method is easy to use and it can offer reference for intersection channelization and traffic organization.