Evaluation of the Passing Process on Two-Lane Rural Highways

The main purpose of this research was to develop models to quantify the major components of the passing process. A second goal was to compare the results with existing highway-design models and to arrive at conclusions about the applicability of the existing models. Additional objectives were to evaluate several time elements of the passing process, such as the response time of drivers from the arrival of a proper gap until the start of the passing maneuver. The evaluation is based on an analysis of data that were collected by videotaping six tangent two-lane highway sections from high vantage points and from a helicopter hovering above one site. Normal driver behavior was not disturbed during the data collection process. About 1,500 passings were recorded; of these, 54 percent were characterized as “single passing,” in which one driver passed a single, slower vehicle. About half of all passing maneuvers were found to involve two cars; in the other half, at least one truck was overtaken. A model showing the relationship between the speed of the impeding vehicle and the passing distance was calibrated, and the implications for highway-design procedures were discussed. Several of the findings had unique safety implications, such as the very short headway before the start of the passing maneuver and very short driver-reaction times. The primary results of the analyses enabled determination of the required passing distances and, therefore, the sight distances needed for different design speeds and various traffic combinations. Additional safety-related aspects are evaluated and discussed.