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6 1. INTRODUCTION 1.1. BACKGROUND The American Association of State Highway and Transportation Officials (AASHTO) Glossary defines hydroplaning as âa condition where one or more tires of a moving vehicle are separated from the pavement by a film of water; usually due to a combination of depth of water, pavement surface texture, vehicle speed, tread pattern, tire condition, and other factors.â Hydroplaning is a serious problem that is associated with a relatively small but significant number of crashes. Statistics from various parts of the world indicate that approximately 15 to 20% of all road traffic crashes occur in wet weather conditions. Although there are no detailed statistics on the exact causes of these crashes, it is believed that low skid resistance and hydroplaning are major factors leading to many of these crashes. The original experimental work in the 1960âs and 1970âs resulted in a definition of hydroplaning based on a hydroplaning speed, and hydroplaning research since then has focused on improving the models to predict this hydroplaning speed. The National Cooperative Highway Research Program (NCHRP) Project 1-29, âImproved Surface Drainage of Pavementsâ proposed design guidelines for reducing hydroplaning on new and rehabilitated pavements and developed the PAVDRN computer program to support the implementation of these guidelines (Anderson et al., 1998). The study focused on the accumulation of water on the pavement and identified the areas that need to be considered in order to analyze and eventually minimize the potential for hydroplaning. More recently, a study carried out in Florida by Gunaratne et al. (2012) summarized the most widely known models for determining WFT and HP, and then compared them against experimental data and wet weather crash statistics obtained from the Florida Department of Transportation (DOT). The study formulated an alternative set of hydroplaning models and investigated the impact of operational changes, such as speed reduction, as they relate to rainfall and traffic volume along with the pavement properties that impact WFT. In addition to the road geometry and surface characteristics, other aspects critical to assessing the potential for hydroplaning include the weather, vehicle, tire type and condition, maneuver, and speed. This research represents one of the first attempts to significantly upgrade understanding and methods to predict HP since 1970s. The project integrated all factors contributing to hydroplaning, using the best available submodels to create a comprehensive, integrated model to predict HP. This model was used to produce a practical tool and guidelines for planning and design to mitigate hydroplaning on new roadways, and to select the most effective, site-specific strategies for reducing hydroplaning on existing roadways. 1.2. PROJECT OBJECTIVE NCHRP 15-55 built on past empirical studies (focused on road geometrics and hydraulics). Traditionally, hydroplaning has been approached as a road geometry design problem; however, as with many other safety-related issues, hydroplaning is a complex phenomenon. The road is only
7 one of the relevant factors, which also include the vehicle, the tires, the environment, and the driver, and thus a full understanding requires a multidisciplinary approach. The main objective of the research was to develop guidance to better predict and mitigate HP on roadways. The guidance will help transportation agencies to identify areas with a high potential of hydroplaning, and to evaluate and select appropriate mitigation solutions. The guidance can be applied to all types of roadways, considers site-specific factors such as geometric design and pavement surface characteristics, and can be used for new construction, reconstruction, and maintenance/retrofit projects. The guidance includes an enhanced definition of HP based on the physics behind hydroplaning. In addition to this report, the final products include a beta version of a tool that can be used to predict hydroplaning on roadways. The tool provides a practical first step for implementing the new hydroplaning definition and allows to assess HP. 1.3. RESEARCH APPROACH OVERVIEW To fully understand hydroplaning, it is necessary to consider all the factors that affect the interaction between the vehicle and the road when there is accumulation of water on the road surface. This project developed a comprehensive model that integrates the best available knowledge for each of its components. Based on a resulting fundamental understanding of the problem, this model was then simplified to model a wide range of situations to assess areas with high potential for hydroplaning and evaluate mitigation solutions. A key aspect of the project is to understand how the ability of the vehicle to develop tractive forces (longitudinal and lateral) is affected by the presence of water on the road surface under a variety of environmental conditions. Road geometry and pavement condition (e.g., curves and bumps) affects vehicle dynamics (roll, pitch, bounce) and the resulting loading and unloading of vertical forces on the tires. The vertical force at any instant affects the ability of the tire to generate tractive forces, some of which (the lateral forces) are required to hold the vehicle on a desired path. Braking and throttle commands (longitudinal forces) reduce the tractive effort that is available to generate the lateral forces and vice versa. Vertical tire forces also affect the dynamics of the fluid between the tires and the road surface. Again, the ability to generate tractive forces depends on the fluid dynamics and the tire dynamics. Finally, the amount of fluid on the road surface at any location depends on the road geometry, surface texture, environment, and hydraulics. The overall approach to the project is summarized in Figure 2. The work began by identifying the best available techniques for modeling each part of the hydroplaning phenomenon. The research then evaluated the various available models, identified potential gaps in the knowledge, and developed a plan for integrating the best practices for modeling water accumulation and the response to the vehicle to the water ponding while pursuing different maneuvers. The research then assembled a fundamental theory to explain the hydroplaning phenomena, developed mathematical models that capture this underlying theory, verified these models using empirical data, integrated them in a research-grade Integrated Hydroplaning Model. The results of the integrated model were used to propose a novel definition of hydroplaning and develop simplified equations, which were implemented in a Hydroplaning Potential Assessment Tool beta version. This beta version of the tool provides highway engineers with practical and simple means for assessing HP.
8 Figure 2. Modeling approach summary.
9 1.4. OVERVIEW OF THE REPORT This Final Report summarizes and documents the work conducted on the project. This first chapter introduced the objectives of the project and the overall approach adopted for accomplishing these objectives. Chapter 2 covers a critical review of previous efforts and models. Chapter 3 presents the development of the various submodels and the Integrated Hydroplaning Model. Chapter 4 illustrates the developments and functionality of the simplified Hydroplaning Potential Assessment Tool. Chapter 5 presents mitigation solutions for sites with high HP. Chapter 6 summarizes the effort and presents the findings, conclusions, and recommendations. Finally, the stand-alone manual for the beta version of the Hydroplaning Potential Assessment Tool is presented in Appendix A.
