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1 SUMMARY This report presents the results of National Cooperative Highway Research (NCHRP) Project 15- 55, aimed at enhancing the way we understand and assess hydroplaning on roadways. Hydroplaning is a serious problem that is associated with a relatively small but nonetheless significant number of crashes. Traditionally, hydroplaning potential has been approached as a road geometry design problem; however, as with many other safety-related issues, hydroplaning is a complex phenomenon. The original experimental work in the 1960s and 1970s 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. This research represents one of the first attempts to significantly upgrade understanding and methods to predict hydroplaning potential since 1970s. Since the road is only one of the relevant factors, which also include the vehicle, the tires, the environment, and the driver, a full understanding requires a multidisciplinary approach. Project Objective The main objective of this research was to develop guidance to better predict and mitigate hydroplaning potential 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 hydroplaning potential based on the physics behind hydroplaning. Research Approach and Results The research included the following steps: (1) Literature review to identify the gaps in the current practice. (2) Integrated Hydroplaning Model: A key focus of this research was to the development of a research-grade model to address existing gaps, propose a novel definition of hydroplaning, and determine the impact of the various roadway and vehicle factors. (3) Hydroplaning Potential Assessment Tool (Beta Version): The research used the results of the integrated model to developed simplified relationships, which were implemented in a tool developed to apply the Integrated Hydroplaning Model without the need of commercial software. This tool provides highway engineers with practical and simple means for assessing hydroplaning potential (4) Guidance to Predict and Mitigate Hydroplaning Potential which allows agencies to apply the models and tool developed in this research project. Literature Review and Gaps Identified This research started with a comprehensive review of relevant reports, papers, manuals, and research-in-progress briefs to capture the state of practice and state of the art on predicting and mitigating hydroplaning potential on roadways. This review produced a critical assessment of the
2 available models and approaches, recommended the submodels most appropriate for incorporation into an Integrated Hydroplaning Model, and identified the main gaps in current practice. The assessment suggested that the models and approaches used to date had two main limitations and areas for improvements: 1. None of the water accumulation models could accurately predict the water film thicknesses (WFT) on sections of road with irregular surfaces and complex geometries. 2. The existing hydroplaning models could not predict hydroplaning potential for different types of vehicles and tires based on the expected vehicle performance while conducting specific maneuvers on a particular roadway segment. The subsequent task of the project focused on enhancing these two areas. Integrated Hydroplaning Model Understanding Hydroplaning This research developed a research-grade Integrated Hydroplaning Model composed of water accumulation, tire-water-pavement, and vehicle dynamics submodels, developed as separate entities for different applications. The most appropriate modeling approaches and tools specifically for hydroplaning prediction were integrated. Although the original intent was to develop a fully coupled model that could run the water accumulation, tire, and vehicle response simultaneously, this proved impractical and thus, the final model includes a series of submodels that run one after the other. The maximum WFT is first calculated along the pavement and is then taken as a constant during the vehicle performance simulation. Similarly, the interaction between the tire, pavement surface, and water accumulated on the pavement is modeled and the resulting forces are used as an input to the vehicle dynamics model. Water Accumulation Submodel To get away from simplistic ârulesâ and consider the physics of the water flow, as determined by rainfall rate, road geometry, and pavement texture, the Integrated Hydroplaning Model used computational fluid dynamics (CFD) to simulate the formation of the water film. A 3-D CFD model was developed to predict the accumulation of water on multilane highways using ANSYS Fluent software. The model solves the Reynolds-Averaged Navier-Stokes (RANS) equations representing the conservation of mass and momentum using a two-fluid Eulerian model. The water model was developed and verified by comparing the predicted WFTs with experimental results from previous studies on very simple geometries. A sensitivity analysis of the WFT predicted on road segments with complex geometric characteristics was conducted to check the applicability of the model to various type of road geometries and conditions. Vehicle Response Submodel The impact that the water accumulated on the pavement has on the vehicle response was modeled by combining vehicle dynamics, tire, and hydraulic submodels implemented using commercially available modeling and simulation tools. The models estimate the effects of the WFT on a vehicleâs handling capabilities and assess hydroplaning potential for the various operating conditions. The
3 models were used to assess the ability of three types of vehicles (sedan, hatchback and SUV) to brake and maneuver a curve given the WFT and the other conditions (grade, superelevation, curvature, tire condition, etc.). The simulations determined the vehicle limit-handling capabilities and estimated the performance required for each combination of vehicle type, tire condition, WFT, and maneuver. Performance Margin and Hydroplaning Potential The performance margin (PM) is defined as the additional performance capability that can be drawn upon beyond that which is demanded by the current operating condition. This definition requires knowledge of both the current performance requirements and the limits of performance capability with a specific amount of water on the road. The Performance Envelope describes the limit of performance capability that can be drawn upon during any maneuver (at any operating condition) and is developed in terms of the Effective Friction, which varies due to vehicle dynamics and road conditions (including roughness and WFT). The PM for an operating condition is the minimum difference between the Required Acceleration for a given operating condition and the locus of points that define the available acceleration. The PM can then be used to and estimate the potential for a vehicle to hydroplane, or The hydroplaning potential (HP). If the PM is low the HP is high; thus, new designs or exiting roadway segment with low PM should trigger reviews to the geometric design or safety investigations. Hydroplaning Potential Assessment Tool (Beta Version) To simplify the use of the model without the need of commercial software, the research developed simplified relationships from the results of the research-grade hydroplaning model and implemented these relationships in a beta version of a software tool to estimate the PM (Figure 1). Figure 1. Beta version of the HP Assessment Tool.
4 The tool uses simplified water accumulation equations and an interpolation method to estimate the PM for one of the three vehicles modeled with new or worn tires under a specific maneuver (defined by the curvature speed and braking) on a specific roadway segment with a specific precipitation level. The tool is supported by a User Manual (provided as an appendix to this report), which summarizes the fundamentals of vehicle hydroplaning, introduces the new definition of HP, and explains the main principles and factors that impact the accumulation of water on the road surface and the response of the vehicle to the accumulated water. The User Manual also discusses the development, data needs, and use of the tool and provides practical examples including assessment of the effectivity of mitigation strategies. Guidance to Predict and Mitigate Hydroplaning Potential The knowledge and know-how developed throughout the research are documented in this Final Report, which provides guidelines for applying the models and tool developed to better predict and mitigate hydroplaning potential on roadways. The guidance provides recommendations on how to identify areas with a high potential of hydroplaning, and to evaluate and select appropriate mitigation solutions. Advantages of the Research Deliverables The main improvements with respect to existing practice for the prediction of HP is that the deliverables from this research provide the following advantages: ï¼ The deliverables propose a new and more comprehensive definition of HP based on the ability of the vehicle to develop tractive forces (longitudinal and lateral) while water is ponded on the road surface. The previously used methods for assessing HP were largely developed in the 1970âs, based on a limited number of field experiments. These methods provided only a hydroplaning speed independent of the characteristics of the road on which the vehicle was driving. This research defines HP based on the ability of the vehicle to conduct a specific maneuver on a specific roadway segment defined by the road geometry, pavement condition (e.g., bumps and curves), and environmental conditions (rainfall). ï¼ The water accumulation model used on this project considers the physics of water flow on complex road alignment and road surface characteristics in three dimensions to estimate the water accumulation on the pavement. Previously existing approaches and tools only considered one-dimensional flow equations that could not be used to accurately predict the WFTs on sections of road with irregular surfaces and complex geometries. This project used three-dimensional models to simulate water flowing on a combination of geometric features and road conditions, and then used the results of the numerical solutions to calibrate a simplified approach for practical implementation. Limitations of the Analysis Approach and the Models Used Because of the complexities of the hydroplaning phenomena and all the factors involved in the interaction between the vehicle, water, and roadway surface, it was necessary to make many simplifications throughout the project.
5 The Integrated Hydroplaning Model required the simplification and/or assumptions with respect to the boundary conditions on the three-dimensional water flow model, the integration of the submodel and the shape of the Performance Envelope, and the number of vehicles and tire types considered in the study. The model development included limited verification based on previous research, field verification, and sensitivity analysis. Furthermore, additional simplifications were needed to be able to run the developed tool without the need to purchase licenses for simulation software. These included the simplification of the water accumulation modeling using an enhanced version of the existing one-dimensional models, the use of only three possible vehicles for the calculation of the PM, and the use of a simple combination of braking and cornering conditions for defining the vehicle performance. Recommendations for Implementation The Beta Version of the Hydroplaning Potential Assessment Tool provides a practical first step for implementing the new hydroplaning definition and makes it possible to assess new and existing roadways; however, further verification and data input enhancements are required before full deployment. In particular, there is a critical need to automate the data input from common roadway design software. Agencies interested in assessing HP can set PM thresholds that are considered low, or hydroplaning investigatory levels, for different road categories. These thresholds can be used for flagging sites with high potential for hydroplaning and triggering safety investigations, as it is done currently for managing pavement friction. The assessment can be conducted at the design stage for new roadways or as part of the periodic monitoring of existing roadways.
