TRIPLE LEFT-TURN LANES AT SIGNALIZED INTERSECTIONS

The use of triple left turns is seen as a means to meet constantly increasing traffic demand at signalized intersections. Although triple left turns have been in use for a number of years and their operational benefits are documented, they are still considered a relatively new design alternative that many agencies are reluctant to approve. While some excellent guidelines appear in the literature, there are no universal standards that apply. The objectives of this project were to investigate the current usage of triple left turns from an operational, safety and modeling perspective and to develop guidelines for triple left-turn installations based on these findings. The project tasks included: review of existing studies of operations and safety, as well as guidelines used by other states for double and triple left turns; conduct of operational analyses to determine the capacity parameters of multiple left-turn lanes; modeling of triple left-turn lane operations with available intersection and arterial performance analysis software products; assessment of the safety performance of triple left turns, identifying crash patterns with associated intersection features and comparing the safety performance of triple and double left-turn lane configurations; and development of recommended criteria for adding turn lanes and/or new construction that includes triple left-turn lanes. The results of these analyses suggested that triple left turns can produce substantial operational benefits. However, an increase in crash rates was noted at some locations. It was also observed that existing traffic models do not treat triple left turns explicitly and can therefore only offer an approximation of their operation. The principal product of the study was a set of guidelines recommended for incorporation into the Florida Department of Transportation's Traffic Engineering Manual.