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61.1 Background The design of the ramp-freeway junction is a critical com- ponent in the overall safety performance of a controlled access facility. Interchanges present the greatest safety and operational problems for drivers, as most freeway crashes occur in the vicinity of interchanges (Lunenfeld, 1993). Inter- changes are inherent points of conflict involving entering and exiting traffic. Both entry and exit maneuvers place increased demands and workload on drivers associated with naviga- tional decision making, speed changing, and tracking. The combination of these demands results in an increased likeli- hood of driver error. Freeway ramps consist of the following three elements: ⢠The freeway mainline ramp terminal, ⢠The ramp proper, and ⢠The crossroad terminal. This report primarily focuses on the freeway mainline ramp terminal element of freeway ramps (i.e., acceleration and deceleration lanes), but also evaluates operations of vehicles along the ramp proper. Chapter 10 of the American Associa- tion of State Highway and Transportation Officials (AASHTO) A Policy on Geometric Design of Highways and Streets (AASHTO, 2004) (hereinafter referred to as the Green Book) describes recommended dimensional guidance for freeway mainline ramp terminals, including both entrance and exit terminals. Such guidance focuses on the speed relationship of the ramp proper to each ramp terminal and the linear requirements to affect appropriate speed-change behavior. Design values for freeway mainline ramp terminals in the Green Book rely on outdated research, much of which was conducted prior to the development of the Interstate sys- tem. The underlying design guidance is, therefore, based on assumptions and vehicle performance characteristics that may no longer be applicable. It is also clear that driver perfor- mance has changed over the years, with particular focus on the increasing proportion of older drivers and the increasing number of trucks in the traffic stream. 1.2 Objective and Scope The objective of this research is to develop improved design guidance for freeway mainline ramp terminals based on modern driver behavior and vehicle performance capa- bilities. This research addresses the following questions: ⢠Is the fundamental AASHTO model or set of operational assumptions behind freeway mainline ramp terminal design sufficient to describe the full range of design parameters? ⢠Are acceleration and deceleration rates inherent in the AASHTO speed-change lane (SCL) models appropriate for todayâs driver population and vehicle fleet? ⢠Should trucks be used as the design vehicle for freeway mainline ramp terminal design for all ramps, or for ramps under certain design conditions? ⢠Are there important differences in the substantive opera- tional performance of parallel and tapered SCLs? ⢠How does driver behavior differ for low-speed ramps com- pared with higher speed ramps? ⢠What are the implications of any design changes on cur- rent roadway practice and existing designs? The research methodology was formulated to address these questions central to the design of mainline freeway ramp terminals. The scope of this research includes only independent free- way mainline ramp terminals (i.e., outside the influence of upstream or downstream operations of other freeway main- line ramp terminals). This research does not investigate the influence of ramp or interchange spacing. In addition, this research focuses on single-lane freeway mainline ramp termi- nals and does not specifically address freeway mainline ramp S e c t i o n 1 Introduction
7 terminals with two or more lanes. Finally, this research does not address weaving areas where a continuous auxiliary lane is provided between entrance and exit terminals, nor does it address issues specifically related to ramp metering. Based upon the findings and conclusions from this research, proposed changes to the next edition of the Green Book are provided. 1.3 Overview of Research Methodology The research conducted during this study was divided into two phases. In Phase I, the literature was summarized to gain knowledge concerning prior research on the safety performance of freeway mainline ramp terminals, vehicle performance char- acteristics of passenger cars and trucks, human factor consid- erations in the design and operation of freeway mainline ramp terminals, and the operational performance of freeway mainline ramp terminals. Next, the research team evaluated conceptual models to explain both the merging and diverging processes. The research team also reviewed truck-related crashes near free- way ramps to assess an appropriate design vehicle for freeway mainline ramp terminals. Following these three tasks, a research plan was formulated for execution in Phase II. The primary Phase II efforts consisted of two types of field studies to examine both vehicle performance and driver behavior near freeway mainline ramp terminals. An observa- tional field study was performed in which laser guns, cameras, and traffic classifiers were used to collect speed, acceleration/ deceleration, and distance information on a large number of vehicles at several freeway entrance and exit ramps. A driver behavior study was also performed in which subjects drove an instrumented vehicle along a selected course, entering and exiting the freeway using designated entrance and exit ramps. Speed, acceleration/deceleration, and location infor- mation were gathered in this study, as well. Data from the driver behavior study were used to supplement the data col- lected during the observational study. Data from both studies were reduced and analyzed to develop proposed changes to the next edition of the Green Book, to improve design guid- ance for freeway mainline ramp terminals. 1.4 Outline of Report This final report documents the entire research effort. The remainder of this report is organized as follows. Section 2 summarizes the literature related to freeway mainline ramp terminals. Section 3 summarizes the conceptual models uti- lized by AASHTO in determining minimum acceleration and deceleration lane lengths for entrance and exit terminals, respectively. Section 4 presents an analysis of truck-related crashes near freeway mainline ramp terminals. Section 5 describes an observational study conducted to gather speed and distance information for a large number of merging and diverging vehicles covering a range of ramp types and merge/ diverge types. Section 6 presents a behavioral study designed to collect detailed information on a limited number of driv- ers to investigate driver behaviors while performing merge and diverge maneuvers onto and off of freeways. Section 7 presents the conclusions from this research, including future research needs. Section 8 presents the references cited in this report. The appendices to this report, which are available on the TRB website at http://www.trb.org/Main/Blurbs/167516. aspx include the following. Appendix A presents an aerial view of the study locations included in the observational study. Appendix B provides histograms of acceleration rates from the observational study. Appendix C presents the verbal instructions given to participants in the behavioral study. Appendix D presents proposed changes to the next edition of the Green Book, based upon the findings and conclusions of this research.
