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Current design guides adopt a deterministic approach to the design of horizontal curves; each factor included in the design is represented by the near-worst-case value. In the context of horizontal curve design, the design procedure is based only on the driver comfort criterion, and data correspond to experiments conducted in the 1930s. Furthermore, current horizontal curve design procedures lack a quantitative evaluation for safety. To overcome those shortcomings, a new design framework is proposed to design horizontal curves; a probabilistic approach is adopted and two criteria are considered: vehicle dynamic stability and driver comfort. Reliability analysis was used to provide a quantitative evaluation for the design in regard to the probability of failure, probability of noncompliance, and reliability index. Outputs of simulation runs in a vehicle dynamics model were used to estimate demand lateral friction and lateral acceleration depending on the geometric characteristics of horizontal curves. In addition, data of an instrumented vehicle experiment were used to develop driver-level models for the distribution of the curve speed and driver comfort threshold. The first-order reliability method was used to estimate the probability of failure, probability of noncompliance, and reliability index. The proposed design framework and developed models were applied in an example to design a horizontal curve for a specific design speed.The high rate of collisions on horizontal curves compared with the rate on other roadway elements makes them one of the most critical elements in the transportation network (1). In that regard, the current practice followed to design horizontal alignment needs to be reviewed. The current design guides adopt a driver comfort criterion to design horizontal curves. That is, horizontal curves are designed to be below a driver's comfort threshold of lateral acceleration, which depends on the drivers' sensation of the forces pushing them to the outside of the curve, or kinaesthesia. Such a threshold is a human factors issue and was determined on the basis of experiments and research conducted between 1920 and 1952 (2). Many developments have since been introduced to the vehicle design to improve safety and ride comfort, and driver behavior has likely evolved as well. In a recent driving experiment using an instrumented vehicle, it was found that drivers tend to accept higher levels of lateral acceleration to maintain their speed and minimize reduction in speed while negotiating horizontal curves (3). These developments would influence drivers' decisions to select appropriate and comfortable speeds on horizontal curves according to their own perception rather than the designer's perception (4).Also, the ability to evaluate the trade-offs between cost and level of safety establishes the need for a quantitative method to evaluate the safety of the design. Unfortunately, the current design guides cannot address this need because horizontal curve safety is included only implicitly in the design. F...

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Many studies have been conducted to develop models to predict speed and driver comfort thresholds on horizontal curves, and to evaluate design consistency. The approaches used to develop these models differ from one another in data collection, data processing, assumptions, and analysis. However, some issues might be associated with the data collection that can affect the reliability of collected data and developed models. In addition, analysis of speed behavior on the assumption that vehicles traverse horizontal curves at a constant speed is far from actual driving behavior. Using the Naturalistic Driving Study (NDS) database can help overcome problems associated with data collection. This paper aimed at using NDS data to investigate driving behavior on horizontal curves in terms of speed, longitudinal acceleration, and comfort threshold. The NDS data were valuable in providing clear insight on drivers’ behavior during daytime and favorable weather conditions. A methodology was developed to evaluate driver behavior and was coded in Matlab. Sensitivity analysis was performed to recommend values for the parameters that can affect the output. Analysis of the drivers’ speed behavior and comfort threshold highlighted several issues that describe how drivers traverse horizontal curves that need to be considered in horizontal curve design and consistency evaluation.