Guidance to Predict and Mitigate Dynamic Hydroplaning on Roadways (2021)

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87 6. CONCLUSIONS AND RECOMMENDATIONS 6.1. CONCLUSIONS The project has produced a novel, transformational approach to estimate HP based on the physics behind hydroplaning. Using advanced fluid dynamics, tire, and vehicle response models, the project has developed a new way to assess the safety risks associated with vehicle hydroplaning. This novel approach represents a quantum leap with respect to traditional approaches that were based on a handful of experimental studies. 6.1.1. Research Products The main products include the following:  An advanced, research-grade Integrated Hydroplaning Model, which addressed some of the existing gaps in hydroplaning analysis and can be used to simulate and study various types of road, environment, and vehicle combinations. The model is composed of water accumulation and vehicle response submodels, developed as separate entities for different applications. These models were enhanced and integrated specifically for hydroplaning prediction and validated using limited experimental data. 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 are run one after the other. The maximum WFT is first calculated along the pavement and 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.  A new definition of HP in terms of the PM developed using the results of the advanced model. The potential for a vehicle to hydroplane and lose control on an specific road segment with different WFTs was defined in terms of the PM, which 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 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.  A beta version of a Hydroplaning Potential Assessment Tool to provide highway engineers with practical and simple means for assessing the impact of roadway features and other factors on HP. The tool was also validated using available, yet limited, data sets.

88 The tool can be run without the need of commercial software. It uses a simplified water accumulation model 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 user can then compare the computed PM with a defined threshold for the type of roadway being analyzed and determines if it is appropriate. 6.1.2. 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:  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 1970’s, 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).  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. 6.1.3. Limitation of the Analysis Approach and the Implemented Model 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.  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 beta version of the 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.

89  Finally, the proposed risk analysis approach was not implemented as part of the project because it was not possible to get a hydroplaning crash data set representative of the entire country. Thus, a simplified approach based on defining threshold PM values was proposed. 6.2. RECOMMENDATIONS FOR FUTURE RESEARCH Although the developed approach, models, and tool represent a significant advance in the understanding and prediction of HP further improvements are possible. The main short-term possible improvements or enhancements include  Improving the water accumulation model through the collection of additional experimental data using non-contact devices for measuring the WFT;  Refining the models to predict HP by collecting additional experimental data using instrumented vehicles; and  Developing an enhanced Hydroplaning Potential Assessment Tool that incorporates the enhancements. Complete research need statements for these potential follow-up projects are include in Appendix B. The goal of the prepared research need statements is to refine the models developed under NCHRP 15-55 (water accumulation, vehicle, tire, and tire-water-pavement interaction), verify the hydroplaning risk assessment process with new experimental results, and produce an updated practical tool to facilitate the implementation of the outcomes of the project. 6.2.1. Improve the Water Accumulation Model This first potential continuation research effort would focus on obtaining new experimental data using a vehicle with a non-contact device for measuring water thickness, conducting additional computer simulations under a wider range of conditions, and refining and enhancing the water accumulation model using the new experimental and computer-generated data. 6.2.2. Refine the Models to Predict Hydroplaning Potential for Light Vehicles The second potential enhancement aims at obtaining experimental results using an instrumented vehicle on a test track to perform standard vehicle dynamic maneuvers. These additional data can be used to refine existing models and adjust the developed standards for HP and risk as needed. 6.2.3. Develop an Enhanced Hydroplaning Potential Assessment Tool Finally, once the two previously described research efforts are completed, it is recommended to enhance the Integrated Hydroplaning Model with their findings and produce an updated, more practical version of the Hydroplaning Potential Assessment Tool using additional simulations produced with the enhanced model and the experimental data collected. This effort could also implement the recommended risk assessment approach and verify the impact of the WFT on wet accident rates using information collected in various states if available. 6.3. RECOMMENDATIONS FOR IMPLEMENTATION NCHRP Project 15-55 proposed a new approach to estimate HP that considers the physics of water flow on complex road alignments to estimate the water accumulation on the pavement and defines HP based on the ability of the vehicle to conduct a specific maneuver on a specific roadway segment.

90 The HP is defined as a function of the PM, which is a measure of any additional performance capability that is available beyond the performance required by the current operating condition. Once the PM is estimated for a given vehicle in a given operating condition, the potential for hydroplaning can be established. 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. It is recommended that agencies interested in assessing HP can set PM thresholds that are considered low, or hydroplaning investigatory levels, for different types of road. These thresholds can be used for flagging sites with high potential for hydroplaning and triggering safety investigations, as is done currently for managing pavement friction (AASHTO, 2008). The assessment can be conducted at the design stage for new roadways or as part of the periodic monitoring of existing roadways. Thus, the implementation of the research deliverables will require that agencies define policies to: (1) verify new road projects using virtual audits to identify potentially dangerous locations with high hydroplaning potential before finalizing the geometric design; and/or (2) monitor existing roads to pinpoint road segments with high HP identify possible mitigation treatments, and evaluate the cost-effectiveness of these treatments. Examples of possible approaches for monitoring include the following:  A traditional “hot-spot” approach that analyzes crashes and pinpoints segments with a high number of wet-related crashes. These locations can be analyzed following the NCHRP 15- 55 approach and subjected to a more detailed road safety audit if the PM is lower than the agency-defined investigatory level for the road category.  A systemic approach that uses a network-level HP assessment as part of the agency’s asset management business processes. This approach can use the data collected by multipurpose automatic road-monitoring equipment to feed a network-level version of the tool to highlight areas with deficient PMs, which can be triggered for investigation and consideration for the agency’s highway safety improvement programs. In addition, it is recommended that work continue on enhancements to the Hydroplaning Potential Assessment Tool to automate the data input from common roadway design software. Collaboration with the roadway design software developers can also support the implementation of the proposed approach as part of their offered analysis tools. For example, Bentley’s OpenRoads Designer currently offers an Aquaplaning tool (Bentley, 2020). Furthermore, if the agency has access to reliable historical records of hydroplaning-related crashes and their locations, these data can be used to define safety performance functions that include PM as one of the influential parameters. The safety performance functions can in turn be used to estimate the potential crash reductions for the feasible hydroplaning mitigation treatments and to select the most effective solution based on a cost-benefit approach. Other activities that can help dissemination and implementation of the research products include: preparation of technical papers, lectures, and presentations at professional meetings and conferences, developing a short technical brief that can be widely distributed among all the stakeholders, organizing one or more technical webinars and a workshop at the Annual Meeting of the Transportation Research Board, and preparing a short informational video.