National Academies Press: OpenBook

Guidelines for Slope Traversability (2019)

Chapter: Chapter 4. Comparison of Vehicle Characteristics

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Suggested Citation:"Chapter 4. Comparison of Vehicle Characteristics." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Suggested Citation:"Chapter 4. Comparison of Vehicle Characteristics." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Page 60
Suggested Citation:"Chapter 4. Comparison of Vehicle Characteristics." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Page 61
Suggested Citation:"Chapter 4. Comparison of Vehicle Characteristics." National Academies of Sciences, Engineering, and Medicine. 2019. Guidelines for Slope Traversability. Washington, DC: The National Academies Press. doi: 10.17226/25415.
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Page 61

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51 CHAPTER 4. COMPARISON OF VEHICLE CHARACTERISTICS At the directions of the research panel, the researchers compared the characteristics of vehicle types more likely to rollover on slopes, as identified from this crash data analysis, to MASH test vehicles (33). MASH, however, does not specify a particular make and model of the test vehicles. It gives a range of design parameters that the vehicle should fall within, including mass, dimensions, center of mass location, location of engine, location of drive axle, type of transmission, etc. Thus, the comparison between MASH design vehicles and those identified as most likely to rollover on slopes in Chapter 3 was made by extracting dimensional, weight, and other physical characteristics data. Expert AutoStats software, which currently contains over 42,000 cars, pickups, vans, and utility vehicles for 1940’s model years to present, was used to extract the vehicle data of interest (34). The Expert AutoStats software database is commonly used in accident reconstruction problems. Most of the design parameter values in the database are measured values, except the roll, pitch, and yaw moments of inertia. The moments of inertia values are approximate and are based on analytical calculations. The comparison of the MASH vehicles was made with the top five vehicles likely to rollover on slopes for each of the eight vehicle classes. These 40 vehicles were presented previously in Table 3.8. Comparisons were made with the MASH 1100C vehicle (2425-lb small passenger car) and 2270P vehicle (5000-lb pickup truck). Vehicles selected for the 1100C and 2270P were Kia Rio (2006) and Dodge RAM (2005), respectively. These are the most commonly used MASH test vehicles used by the testing labs. The researchers collected data for the following vehicle design parameters.  Curb Weight  Curb Weight, Front Axle  Curb Weight, Rear Axle  Drive Wheels (Front or Rear)  Total Length  Wheelbase  Bumper to Axle Distance (Front)  Bumper to Axle Distance (Rear)  Bumper to Windshield (front)  Bumper to Window (rear)  Maximum Width  Front Track Width  Rear Track Width  Static Stability Factor  Height of Front Bumper Top  Height of Hood Top  Height of Rear Bumper Top  Height of Trunk Top  Turning Cycle (Diameter)  Angle of Tires at Max. Steering  Break Type (ABS or Not)  Transmission Type  C.G. Inches Behind Front Axle  C.G. Inches from Side  C.G. Inches from Ground  Yaw Moment of Inertia  Pitch Moment of Inertia  Roll Moment of Inertia The researchers considered various design parameters and compared them to the MASH pickup and small passenger car vehicles. The researchers also considered how meaningful comparisons could be made between the design parameters and the vehicle’s propensity to

52 rollover. It was concluded that using the Static Stability Factor and the Roll Moment of Inertia to compare stability and rollover propensity was most meaningful. Shown in Figure 4.1 are the ranges of Static Stability Factors (SSF) for vehicles most likely to rollover on slopes, for each vehicle class. The higher SSF value implies greater stability. The SSF values for the MASH pickup and small car are also shown for comparison purposes. Similarly, shown in Figure 4.2 are the ranges of roll moments of inertia for vehicles most likely to rollover, along with the roll moments of inertia values for MASH vehicles. As presented in Chapter 3, the researchers had determined the relative risks of slope- related rollovers of one particular vehicle body type relative to the rest of the vehicle types using the FARS data. The results of this crash data analysis are presented again in Figure 4.3. Various similarities can be observed between the crash data analysis results (Figure 4.3) and the results of the vehicle SSF and roll inertia ranges (Figures 4.1 and 4.2). For instance, the 2-door and 4-door sedan have the lowest relative risk of slope related rollovers (see Figure 4.3), and are the most stable (i.e. have higher SSF (see Figure 4.1)) and have lower roll moment of inertia (see Figure 4.2). The compact and large utility vehicles have a high relative risk of rollover (see Figure 4.3), and are less stable due to low SSF (see Figure 4.1) and have a higher roll moment of inertia. Similar trends exist for other vehicle types. The exception is the large van, which has a relatively small relative risk of rollover from the actual crash data analysis (see Figure 4.3), but is much more unstable (i.e. lower SSF, see Figure 4.1) and has a relatively high roll moment of inertia (see Figure 4.2). It must be noted that trends generated by comparing vehicle design features such as SSF and roll moment of inertia do not take into account many other factors that are somewhat accounted for in the crash data. So differences are expected. Rollover studies have been conducted for decades by NHTSA, NAS, and other research institutes. Over the years, many “vehicle factors” and “driver maneuvering behavioral factors” have been researched. NHTSA has consistently defended the use of SSF as a key “vehicle factor” in rating vehicle rollover propensity through various studies. See, for example, the following report for a historical review.  National Academies of Science. NHTSA's Rating Systems for Rollover Resistance: An Assessment, Transportation Research Special Report 265, 2002. NHTSA’s current rollover estimates are based on rollover risk models developed in its New Car Assessment Program (NCAP) (35). It should be noted that there are many other factors that can influence the vehicle’s stability. There is no straightforward way to establish a relationship between various design parameters and the propensity for rollover on slopes. Furthermore, if the traversability guidelines are developed using just MASH vehicles, it is not possible to make meaningful adjustments to the guidelines based on differences in vehicle design parameter of other vehicle classes in comparison to MASH vehicles. Because of these concerns, the researchers eventually performed detailed simulation analyses using the MASH vehicles (small passenger car and pickup truck) and some of the vehicle types identified as likely to rollover from the crash database. This was

53 considered that most efficient and sound methodology for incorporating different types of vehicle in the development of the traversability guidelines. The exercise of comparing difference vehicle properties to MASH vehicles did, however, guide the researchers in selecting vehicle makes and models of the non-MASH vehicles for use in the simulation analyses. Results of these simulations will be presented in following chapters. Figure 4.1. Static stability factor comparison of vehicles most likely to rollover on slopes.

54 Figure 4.2. Roll moment of intertia comparison of vehicles most likely to rollover on slopes. Figure 4.3. Relative risks of slope-related rollovers for one particular vehicle body type to that of the rest of the body types and associated confidence intervals. Relative Risks of Slope-Related Rollovers for One Particular Body Type to That of the Rest of the Vehicle Types and Associated Confidence Limits (Data Source: FARS, 2004-2010; Posted Speed Limit: 45-75 mph) 0.0 1.0 2.0 3.0 4.0 5.0 2-Dr Sedan 4-Dr Sedan Compact Utility Large Utility Minivan Large Van Compact Pickup Standard Pickup Vehicle Body Type R el at iv e R is k (v s. A ll O th er T yp es C om bi ne d)

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TRB’s National Cooperative Highway Research Program (NCHRP) has released a pre-publication version of Research Report 911: Guidelines for Slope Traversability, which includes guidelines for determining the traversability of roadside slopes considering the characteristics of the current passenger vehicle fleet.

As part of development of this report, researchers performed full-scale traversability tests and compared the performance of the vehicles with the simulations performed for the same test conditions.

Rollovers are the leading cause of fatalities in single vehicle ran-off-road (SVROR) crashes. Analysis of six years of data from the National Automotive Sampling System Crashworthiness Data System indicates that 31% of SVROR crashes result in a rollover. Approximately 75% of these rollover crashes are initiated by vehicles digging into the ground on embankments or in ditches after encroaching onto the roadside.

Development of NCHRP Research Report 911 was prompted by concern that some roadside slope conditions that have for many years been considered traversable for passenger cars may not be traversable for light trucks. With the steadily increasing percentage of light trucks in the vehicle fleet, further research was needed to determine what should be considered as safe sideslope conditions for today’s vehicle fleet. Proper assessment of slope traversability may help reduce the number of rollover crashes and associated fatalities.

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