Free-Gap Evaluation for Two-Lane Rural Highways

Transportation Research Record: Journal of the Transportation Research Board, No. 2223, Transportation Research Board of the National Academies, Washington, D.C., 2011, pp. 9–17. DOI: 10.3141/2223-02 Operating speed is most frequently represented by the 85th percentile speed (V85) of vehicles passing at a given road location, in a nonplatoon condition, and it is usually determined by spot speed measurements. The Highway Capacity Manual (HCM) 2000 recommends that the number of observations for V85 calculation be equal to or greater than 100 (3). Therefore, one critical issue for studies of operating speeds is to define when a platoon condition is present. Another relevant question is related to the identification of the road volume for which the number of vehicles in a nonconstrained operation is enough for V85 estimation. In most studies on the development of operating speed prediction models, a nonplatoon condition is usually defined by means of minimum headway between successive vehicles in a traffic stream. However, these studies do not describe the procedure adopted to establish the reference value taken. Also, this traffic measure is affected by the type of the two successive vehicles considered. Equipment available for automatic speed data collection is, in some cases, able to collect headway and gap values simultaneously. The gap between two successive vehicles, being the interval between the rear bumper of the first vehicle and the front bumper of the second as the vehicles pass a point on the roadway, is not affected by the vehicles’ type. Therefore, the definition of platoon condition based on gap values can be useful for general applications. In this context, this paper aims to present and test a procedure to define a gap value between two successive vehicles from which the vehicles can be considered as traveling in a nonplatoon condition. This reference value is referred to as “free gap.” In addition, the procedure allows for the identification of the traffic volume suitable for ensuring the sample size required for V85 measurements. Therefore, the procedure is proposed as the initial activity to be performed for V85 data collection for operating speed evaluation and modeling. This paper is organized into five sections. After this introductory section, a brief literature review is presented on headway reference values considered for platoon definition in different operating speed studies. The third and fourth sections describe, respectively, the procedure proposed and its application to Portuguese conditions. This application is the initial step for a broad operating speed data collection activity planned to be performed at Portuguese roads for the development of a respective operating speed prediction model. The last section presents this study’s main conclusions.