Analysis of Naturalistic Driving Study Data: Offset Left-Turn Lanes

The purpose of this research project was to demonstrate one application of the Strategic Highway Research Program 2 (SHRP 2) Naturalistic Driving Study (NDS) data, showing how these data can be used to answer a highway safety question and provide guidance for the implementation of safety countermeasures. The specific safety countermeasure evaluated in this project is offset left-turn lanes. The research team evaluated the effect of left-turn lane offset on gap-acceptance behavior, which serves as a surrogate safety measure for this study. Specifically, the objective was to evaluate left-turning gap rejection and acceptance by a large number of drivers at a large number of intersections with a broad distribution of left-turn lane offsets (ranging from negative to positive). The primary source of data for this study consisted of the forward- and rear-facing cameras placed in the vehicles of NDS drivers. Clips of these videos were obtained for instances in which NDS drivers made a left-turning maneuver at one of the study intersections. From the videos, data were collected for a large number of variables, most importantly, the start and end time of each gap rejected or accepted by the turning driver. The primary analysis for the study was a logistic regression to predict the critical gap length for each offset category, both when sight obstructions were present and when they were not. Differences in critical gap between specific offset categories were not found to be statistically significant; however, the data clearly showed that negative-offset left-turn lanes resulted in longer accepted gaps than positive-offset left-turn lanes because opposing left-turn vehicles were much more likely to cause a sight restriction at negative-offset left-turn lanes than at positive-offset left-turn lanes. Therefore, even intersections with opposing left-turn lanes with a minimally negative offset have the potential to benefit from shifting the lanes to a positive offset. In addition, the data did not show that offsetting the left-turn lanes to make them less negative would have a substantial effect on safety or operational concerns.