DOTLT1000378The main objective of this study is to evaluate the safety and operation of existing left-turn signal phases at intersections and investigate relevant data to develop proper guidance on when it is appropriate to install each signal type. The study considered protected-only (PO), protected permitted left-turn (PPLT), and flashing yellow arrow (FYA) left-turn phases for the evaluation. The study was conducted in four different folds \u2014 nationwide survey, decision tree modeling, safety analysis, and operation analysis. The nationwide survey revealed that the majority of the respondents indicated FYA as the preferred left-turn signal in terms of operation, followed by PPLT. At the same time, from a safety perspective, PO was desired, followed by FYA. The decision tree revealed several factors such as total left-turn crashes, median types, number of left-turning lanes, speed limit, and annual average daily traffic (AADT) controlling the selection of PO over PPLT. The safety analysis showed almost double left-turn crashes per year at PPLT compared to PO and FYA intersections. Crash modification factor (CMF) of PO over PPLT for total crashes showed that PO was only able to reduce fatal and severe crashes by 25.5% compared to PPLT; however, PO was able to reduce all severe levels of left-turn crashes. It indicates that PO performs better than PPLT from a safety perspective. Before-and-after evaluation at FYA intersections revealed a left-turn crash reduction of 17.73%. In addition, delay analysis showed an average delay of 50.69 seconds per vehicle (sec/veh) at PO, 46.04 at PPLT, and 31.49 at FYA. However, the delay only during the morning peak hour at PO was significantly higher than at PPLT. At other periods, it was not. Delay at FYA was all-time low compared to PO and PPLT, but the outcome from FYA is less robust due to the limited sample size. With left-turn crash reduction by more than 50% and delays not significantly more at all times of the day, the study indicates PO performs better than PPLT. The framework from the decision tree provides key information to help select the suitable left-turn phase between PO and PPLT
