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151 CHAPTER 8. SLOPE TRAVERSABILITY GUIDELINES An errant vehicle that encroaches onto the roadside is faced with a risk of rollover due to the presence of a roadside slope. In general, the steeper the roadside slope, the higher the probability of a vehicle rollover. It is thus important to select the roadside slope by taking into consideration the associated risk of vehicle rollover. It is important to note that rollovers occur on all slopes, including a flat terrain or a nearly flat slope, such as a 1V:10H slope. Similarly, there are many encroachment scenarios in which a rollover may not occur even on a very steep slope, such as a 1V:2H slope. In this research, by simulating a large number of encroachments on different roadside slope configurations, vehicle rollover probabilities were determined for different roadside slope conditions. These rollover probabilities serve as a metric to determine and compare the relative risk of the different roadside slope configurations. There are three key design variables that influence the probability of vehicle rollover on a roadside slope. These are the width of the shoulder, the foreslope, and the width of the foreslope. The combination of these design variables determines the probability of a rollover for an encroaching vehicle. Thus for a given roadside slope, the rollover probability varies based on the other two design variables, i.e. the shoulder width and the width of the foreslope. Figure 8.1 shows the slope traversability guidelines that can be used to determine the vehicle rollover probability for a given combination of roadside design variables. The effect of shoulder width can be aggregated into two types of shoulders â a narrow shoulder that is 4ft wide or less (Figure 8.1a), and a full shoulder that is wider than 4 ft (Figure 8.1b). Once the appropriate chart is selected based on shoulder width, rollover probability can be determined using the combination of the foreslope and the width of the foreslope. In selecting the width of the foreslope, it is recommended that the designer should select the nearest width that is shown on the plots. Due to very high rollover probabilities, slopes steeper than 1V:3H are not included in the guideline and should be avoided. As described previously, vehicle rollover probability on a given slope can vary based on the combination of other design parameters. For this reason, a single rollover probability cannot be ascribed to any particular slope. To use the slope traversability guidelines, a designer should first decide on an acceptable level of risk based on rollover probability. It can be seen from Figure 8.1 that even for the mild 1V:10H slope, the probability of rollover varies roughly from 9% to 13%. Thus a designer or a user agency can select an acceptable rollover probability based on site conditions or as a matter of design policy. This acceptable level of risk based on rollover probability can then be used to determine if a particular slope condition is acceptable or not.
152 (a) (b) Figure 8.1. Slope traversability guidelines.
153 (a) (b) Figure 8.2. Slope traversability guidelines showing probability of vehicle return to roadway.
154 In addition to the rollover probability, a designer or user agency may use Figure 8.2 to determine the probability of a vehicle returning to the roadway after having encroached on a given slope configuration. While rollover probability on a particular slope should be the primary factor in deciding the acceptable level of risk, supplemental information about the percentage of vehicle returns for that slope can be helpful in making this decision. Presented next are some examples of how these guidelines can be used in a practical design scenario. 8.1 EXAMPLE OF USE Problem: A roadside slope of 1V:4H is needed with a 32 ft. wide foreslope. In one case the shoulder will be 6 ft wide whereas in another case it will be 3 ft wide. The user agency considers a 25% rollover probability to be acceptable for this design. It needs to be determined if this design will be acceptable using the slope traversability guidelines. Design Considerations: To determine the acceptability of this design, the first step would be to determine which of two plots in the Figure 8.1 should be used. For the 6 ft wide shoulder, the designer will use Figure 8.1b since it is for shoulders wider than 4 ft. For a foreslope of 1V:4H and a foreslope width of 32 ft, the associated rollover probability can be determined to be 16.5%. Since this is less than the acceptable rollover probability of 25%, the proposed design with a 6 ft shoulder is acceptable. The next step would be to determine the acceptability of this design for the 3 ft shoulder. In this case, Figure 8.1a will be used since it is for shoulders equal to or less than 4 ft wide. Again using the 1V:4H foreslope and a 32 ft foreslope width, the associated rollover probability can be determined to be 21.6%. Since this is less than the acceptable probability of 25%, the proposed design with the 3 ft shoulder is also acceptable. For illustration purposes, now consider the same problem with 1V:3H as the desired foreslope for the design. As before, for the 6ft wide shoulder, Figure 8.1b is to be used. For the 1V:3H slope and a foreslope width of 32 ft, the associated rollover probability is 22.4%. Since this is less than the acceptable probability of 25%, the proposed design with a 6 ft shoulder is acceptable for the 1V:3H slope. With the 3 ft wide shoulder, the designer will refer to Figure 8.1a for narrow shoulders. For a 1V:3H foreslope and a foreslope width of 32 ft, the associated rollover probability is 26.2%, which exceeds the acceptable rollover probability of 25%. Consequently, this design is not acceptable with a 3 ft wide shoulder and 1V:3H slope. In this situation, the designer will have to consider using a shoulder greater than 4 ft, or reducing the slope.