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155 CHAPTER 9. SUMMARY AND CONCLUSIONS This report includes details of the research performed under NCHRP Project 17-55 for developing of guidelines for roadside slope traversability. A detailed literature review was conducted at the start of the project. This was followed by crash data analysis to determine the types of vehicles that are most likely to rollover on slopes. The slope traversability guidelines were developed through extensive use of vehicle dynamics simulations. A total of 43,000 simulations were performed in this research. The simulation analyses were performed for 60 unique roadside slope configurations that were a combination of a range of shoulder widths, foreslopes, and foreslope widths. For each of the 60 terrains, 720 unique vehicle encroachments were simulated. The encroachments were a combination of a range of encroachment speeds, angles, driver inputs, and vehicle types. To develop guidelines that are representative of the real world crash scenarios, the researchers weighted each of the 43,000 simulations according to the probability of their occurrence in the real world. This was done by developing probability distributions for all of the encroachment variables using the NCHRP 17-22 crash database and vehicle sales data. These probability distributions were then used to determine a combined probability of vehicle rollover for each of the 60 roadside slope configurations. Once a combined rollover probability was assigned to each of the 60 terrains, the influence of the various roadside slope design variables was evaluated to arrive at the slope traversability guidelines. The researchers determined that the probability of the vehicle rollover on a slope is influenced by three key design variables. These are the width of the shoulder, the roadside foreslope, and the width of the foreslope. Simulation analyses revealed that the interaction of the vehicle with the bottom of the slope has a significant destabilizing effect on vehicle stability for steeper slopes. Thus the width of the slope is an important factor that should be taken into consideration. In the past, traversability of different slopes has been defined as ârecoverable,â âtraversable, non-recoverable,â and âcritical,â depending on whether the vehicle is able to come back to the road, reach the bottom of the slope, or has a high chance of a rollover, respectively. In this research, it was noted that for each foreslope type, there is a certain probability of rollover, regardless of the steepness. Thus rollovers can occur on flat terrain or mild slopes such as 1V:10H. Furthermore, there are additional design features, such as the width of the shoulder and the slope, that have significant influence on the stability of a vehicle traversing a slope. For these reasons, assigning one particular outcome type to a specific foreslope leaves a ditch designer unaware of the true hazard of the slope. The guidelines emanating from this research are thus presented in the form of a probability of rollover that is defined as function of the foreslope, foreslope width, and the shoulder width. This format provides the designer or the user agency greater flexibility in defining an acceptable risk of encroaching a roadside slope in the form of an acceptable rollover probability.
156 In Chapter 8, the guidelines have been presented in a format suitable for incorporation in the AASHTO Roadside Design Guide. It is envisioned that the implementation of the research results will occur through the adoption of the guidelines by AASHTO and their publication in the AASHTO Roadside Design Guide. Their implementation and use will improve the level of safety for motorists and reduce the number of serious injury crashes associated with rollover on roadside slopes.