Left turn movements are considered to be the highest risk movements through an intersection, and can result in head on, angle, or side impact crashes. The flashing yellow arrow (FYA) for use as a permissive left turn phasing indication is relatively recent, first appearing in the 2009 edition Manual on Uniform Traffic Control Devices (MUTCD). Permissive phases have historically been communicated to drivers with various traffic signal indications. This study investigated how real-world vehicle trajectories can be collected and used to evaluate the conflict potential associated with permissive left turn movements that use the FYA and to consider how this data source could improve signal timing. A field study was completed which used a novel mobile radar sensor and data collection and processing software to map vehicle movements while making a turn through three intersections with a FYA. The sensor data from four experimental locations was compared to video data to determine an initial validation procedure and a method to process the data into a usable form.