The Effect of Driver Height on the Death Rate in Single-Vehicle Rollover Accidents

Transcription

1 Paper No.: The Effect of Driver Height on the Death Rate in Single-Vehicle Rollover Accidents Jonathan E. Howson Senior Research Associate Zachry Department of Civil Engineering Texas A&M University College Station, TX Tel: , Fax: Dominique Lord* Associate Professor and Zachry Development Professor I Zachry Department of Civil Engineering Texas A&M University 3136 TAMU College Station, TX d-lord@tamu.edu, Tel: , Fax: D. Lance Bullard Division Head Research Engineer Roadside Safety and Physical Security Division Texas Transportation Institute The Texas A&M University System 3135 TAMU College Station, TX l-bullard@tamu.edu, Tel: , Fax: Transportation Research Board 91 st Annual Meeting January 22-26, 2012 Washington, D.C. November 14, 2011 *Corresponding author Number of words= (4532 Words + 11 Tables + 1 Figure) = 7,582

2 ABSTRACT The average American male age 16 and over is 5-ft, 9-in. (69 inches) tall and the average American female age 16 and over is 5-ft. 4-in. (64 inches). Utilizing data from the National Center for Health Statistics, it was found that only 19.7% of males are 72-in or taller and only 0.15% females reach or exceed 72-in. The statistics on height distribution were compared with the information obtained from the Fatal Accident Reporting System Encyclopedia (FARS) on the number of deaths from single-vehicle rollover accidents as correlated to occupant height. Since individuals 72-in or taller have less headroom (defined as the vertical space between the interior of the roof and the bottom of the seat) in vehicles than the average individual, they therefore have a greater probability of having head injuries when the vehicle rolls over. The average head clearance for the average American male and female is 3.67-in and 4.38-in, respectively; with the head clearance for a 72-in male and female being 2.57-in and 3.60-in, respectively. The results of this study show that male (t-value=4.72, p<0.001) and female (t-value=1.41, p=0.16) drivers 72-in or taller are more likely to be killed in rollover accidents as based on their population percentage; the results for female drivers are less conclusive due to the small sample size. In addition, vehicles were divided into different categories and examined to see if vehicles for certain categories had increased fatalities for male drivers 72-in or taller. It was found that seven of the thirteen vehicle categories showed a higher than average fatality rate for 72-in or taller drivers (all t=values 1.96, p 0.05).

3 INTRODUCTION In 1971, the Federal Motor Vehicle Safety Standard (FMVSS) 216 was implemented to provide a minimum roof strength for passenger cars. The purpose of this standard was to reduce deaths and injuries due to the crushing of the roof into the passenger compartment in rollover accidents. The resistance of the roof to intrusion is determined by a quasi-static test, in which a force of 1.5 times the empty weight of the vehicle or 5,000 pounds, whichever is less, is gradually applied to the roof in the vicinity of the A pillar. The force is applied by a flat test device at a 25 roll angle (sideways) and a 5 pitch angle (forward) to simulate the direction of forces that can be encountered in a rollover. During the test, the roof may show no more than 5 inches of intrusion, as measured by the movement of the test device. In April 1991, FMVSS 216 was updated to include light trucks and vans with an empty weight of 6,000 pounds or less. To accommodate the heavier vehicle weights, the 5,000 poundlimit increased to 6,000 pounds; meaning the vehicle roof has to support 1.5 times the empty vehicle weight or 6,000 lbs (1). The National Highway Traffic Safety Administration (NHTSA) recently updated FMVSS 216 to include vehicles up to 10,000 pounds (FMVSS 216a). It also requires that a roof withstand an applied force equal to 3.0 times the vehicle s weight while maintaining sufficient headroom for an average size adult male. The new standard will be phased in over the coming years, with all vehicles manufactured after September 1, 2015 being required to meet this standard (2). Rollover crashes are actually a low severity type of accident because the impact energy is dissipated over seconds, rather than milliseconds as in a fontal collision (3). Friedman and Nash (4) discovered that the speed at which a vehicle s roof impacts the ground during a rollover event is 5 mph or less. If no roof intrusion occurs, the occupant s head meets the padded roof at this velocity and at low enough forces not to cause serious neck or head injury (4). Studies by Nusholtz et al. (5) and Yoganandan et al. (6) used complete unembalmed cadavers to test the effect of drop height on the formation of cervical spine damage due to impacts to the top of the head. Their main findings relating to this research is that a reasonably healthy person would be able to survive a fall of one meter (3.28 feet) on his or her head if they landed on a reasonably padded surface. A one-meter fall resulted in an impact velocity of 4.5 meters/sec (10.23 mph). Friedman and Nash (4) discovered that when roof intrusion occurred, it took place at the rotational speed of the vehicle, which can be much greater than the roof impact speed. From experiments, the researchers determined that the velocity of an intruding roof had an amplification factor of 3 as compared to the roof impact speed. According to Nusholtz et al. (5) and Yoganandan et al. (6), this increase in impact speed is significant enough to cause vertebra damage or death. The risk of injury from this type of crash is not due to the crash itself, but a lack of occupant protection provided by the vehicle (3). The FMVSS 216 static test used by vehicle manufacturers to design the roof of vehicles applies substantially less force then is seen in an actual dynamic rollover event. Also, this test applies the load in such a way as to allow the windshield to supply 30% of the roof s static strength (4). Friedman and Nash went on to say that in a dynamic rollover event the windshield is fractured or shattered during the first roof contact event and will not provide any support on subsequent roof contact events, making the roof more likely to intrude into the occupant s survival space (4). Therefore, one of the main reasons rollover crashes are so dangerous is because of roof intrusion and roof contact injuries. To protect occupants in a rollover, maintaining headroom is 1

4 2 very important. Headroom is defined as the vertical distance between the interior of the roof and the seat bottom. A study by Partyka (7) showed that vehicles in eighty percent of rollover crashes, with two or more vehicle quarter turn rolls, sustained vertical roof intrusion (which included the roof top, roof side rails and front/rear headers). The roof is part of the structural support of a vehicle and is therefore a critical component in keeping the occupant safe. If a roof crushes substantially during an accident, from a failure of the side rails, headers, or support pillars, catastrophic injuries can occur. Often, this decreased headroom results in the occupant s head impacting some portion of the vehicle causing death, paralysis, or brain damage. A study by NHTSA found that head injury increased when headroom was reduced after a rollover event; with the risk of head injury from roof contact substantially increased when headroom (preversus post-crash) was decreased by more than 70% (8). To date, research has not been performed to examine the influence of driver s height on the risk of death when a rollover occurs using observed data collected in the field. Most of the previous work has been conducted in a controlled environment and was usually associated with the development of standards. Therefore, the objective of this paper is to examine the effect of driver height on the death rate of single-vehicle rollover crashes. The study objective was accomplished using data collected from the National Center for Health Statistics, NHTSA s Fatal Accident Reporting System (FARS) for the time period , and a database containing headroom vertical distance for several vehicle body types and vehicle makes. Risk values were estimated for men, women and different categories of vehicles. DATA DESCRIPTION Height Distribution FMVSS 216 is designed to protect a population of height up to approximately the average size adult male. As mentioned earlier, the average male adult is 69-in tall (9). However, all adult males are not 69-in tall. The height distribution of a random sampling of adult males and/or females should follow a normal distribution. McDowell et al. (9) from the National Center for Health Statistics published their height distribution data as percentiles, as seen in Table 1. This data were used in calculate the percentage of males and females that are 72-in or taller. TABLE 1 Height Distribution of Males and Females in Percentiles (9) Men Percentile Height Women Percentile Height Sitting Height Bardeen (10) conducted a study to determine the relation between stature (standing height) and sitting height. His findings are summarized in Tables 2 and 3 for males and females, respectively (10). Bardeen (10) found that a 69-in tall adult male has a sitting height of in and a 64-in adult female has a sitting height of in. It is also noted that the difference

5 3 between the sitting heights of an average adult male and a male of 72-in. is 1.1 inches. This difference increases to over 2-in when the individual is 75-in or taller. Bardeen (10) found that females have a lower sitting height as compared to a male of the same height. A female of 72-in has roughly the same sitting height as a 69-in male. TABLE 2 Stature and Sitting Heights of Adult Male Americans (10) Stature Number of Mean Sitting Height Inches cm. Individuals Inches cm

6 4 TABLE 3 Stature and Sitting Heights of Adult Female Americans (10) Stature Number of Mean Sitting Height Inches cm. Individuals Inches cm Vehicle Headroom Vehicle headroom is defined as the vertical distance from the seat to the bottom of the roof liner. The amount of vehicle headroom afforded varies from vehicle to vehicle, but in general, pickup trucks and SUVs offer more headroom than passenger and sports cars. On the other hand, pickups and SUVs are more likely to be involved in a rollover accident due to their higher center of gravity. The FARS database places every vehicle into one of 15 different vehicle body type categories. By looking at categories of vehicles instead of all vehicles together, vehicles that are more dangerous to people of above average height can be identified. An important note is that soft-top convertibles are not regulated by the FMVSS 216 standard. The next step is to determine the average headroom for each category. The first step taken to accomplish this was to determine the top selling vehicles in the US for The statistics for 2009 are used because FARS only had data published to 2009 (at the time the study was completed). This list will represent the vehicles with the highest volume on the roadways and will be used to determine the headroom for the different vehicle categories. The top 20 list is shown in Table 4 below (11).

7 5 TABLE 4 Top 20 Best Selling Vehicles: 2009 (11) Top 20 Best Selling Vehicles: Total Yearly Sales Ford F-Series P/U 413,625 2 Toyota Camry 356,824 3 Chevrolet Silverado P/U 316,544 4 Toyota Corolla 296,874 5 Honda Accord 290,056 6 Honda Civic 259,722 7 Nissan Altima 203,568 8 Honda CR-V 191,214 9 Ford Fusion 180, Dodge Ram P/U 177, Ford Escape 173, Chevrolet Impala 165, Chevrolet Malibu 161, Ford Focus 160, Toyota RAV4 149, Toyota Prius 139, Hyundai Sonata 120, GMC Sierra P/U 111, Chevrolet Cobalt 104, Hyundai Elantra 103,269 Using the vehicle categories listed in FARS with the list of vehicles shown in Table 4, a composite list can be formed with example vehicles in each category and their corresponding headroom (using the Specs & Features -> Interior tabs at (12). Table 5 presents an example of a vehicle used in each category. The full list of vehicles in Table 5 is still not complete, as it leaves out many makes and models of vehicles; however, it does contain many of the most popular, and therefore abundant vehicles on the roadways. There were two main considerations which went into choosing the vehicles for the categories described in Table 5. The first was the popularity of the vehicle and the second was choosing vehicle models that existed both in 2009 and 1999 so as to capture the change in headroom of vehicles over time. The last four columns of Table 5 contain the clearance between the driver s head and the bottom of the roof liner for a 69-in adult male and a 72-in adult male, respectively. It can be seen in Table 5 that the clearance between the driver s head and the bottom of the roof liner is less than 5-in for every vehicle category except truck tractor for individuals 72-in or taller. Vehicle designs allow the roof to intrude 5-in into the passenger cabin during a rollover event, as per FMVSS 216. FARS Encyclopedia The data needed to complete this work was obtained from FARS. Relevant information from years was queried to obtain the relevant information needed for this work. Information prior to 1999 did not include information regarding driver height and was therefore not applicable. Table 6 shows the data fields selected to extract the relevant information from FARS.

8 6 Type TABLE 5 Head Clearance by Vehicle Category (12) 2009 Statistics 1999 Statistics Headroom Averge for Type Headroom Averge for Type 69" Adult Male 72'' Adult Male Convertible Ford Mustang Door Sedan Honda Civic Door Sedan Ford Focus Door Sedan Toyota Camry Door Sedan Volkswagen Golf Station Wagon Subaru Outback Compact Utility Honda CR-V Large Utility Chevy Tahoe Utility Station wagon Chevy Suburban Minivan Example Chrysler Town and Country Total Average Head Clearance Large Van Chevy Express Compact Pickup Toyota Tacoma Standard Pickup Ford F Truck Tractor Freightliner Coronado ~ ~

9 7 TABLE 6 Data Field Selected in FARS Category Relevant Information Specific Information Vehicles Persons Drivers Body Type Most Harmful Event Rollover Injury Severity Restraint System Used Seating Position Sex Driver Height (Feet) Driver Height (Inches) All Rollover 1st Event Fatal Lap & Shoulder Front Left - Driver All All The selection criteria are as follows: Body Type Determine if any vehicles are more dangerous to individuals over 72-in. Most Harmful Event The rollover event should be the most harmful event. Rollover The rollover was chosen as the first event to ensure no other factors lead to the driver s death; such as an impact with another vehicle prior to the vehicle rolling over. Injury Severity Examine whether driver height has any influence on death rate in rollover accidents. Therefore, only fatalities are considered. Restraint System Used Only consider drivers properly belted into the vehicle. Unbelted drivers are much more likely to be killed regardless of height. Seating Position FARS only records height of drivers; therefore, only the driver is considered. This eliminates children and causes the search to look at people 15+ years old. However, it reduces the number of results in the search and causes the results to omit the affect of other occupant seating positions. Sex Separate male and female fatalities since each gender has a separate height distribution curve. Driver Height (Feet) Used to separate total fatalities into one-foot height increments. This is used to determine percent fatalities over 6-ft. Driver Height (Inches) Further breaks down total fatalities into increments of one inch. All RESULTS Height Distribution To determine the percentage of males and females taller than 72-in it is necessary to convert the percentile data (Table 1) to a standard normal distribution. The standard normal distribution has a mean value of zero and a standard deviation of 1. The equation needed to convert a normal distribution to a standard normal distribution is: (1) where, Z is the standard normal random variable, X is normal random variable, μ is the average of the normal distribution, and σ is the standard deviation of the normal distribution. When converting from percentiles, the standard deviation is not known, but the Z value is equal to 95%

10 8 or and X is equal to the value corresponding to the 5 th or 95 th percentile. Solving for the standard deviation yields: (2) Once the standard deviation is known, it is possible to use Equation (1) to solve for the Z value corresponding to a height of 72-in. Table 7 displays the results from these calculations. Table 7 Percentage of Males and Females 72-in or Taller Group Sigma Z Percentage Male % (1.93%) Female % (0.20%) Standard error Crash Data The results from FARS, with the inputs given in Table 6, are summarized in Tables 8 and 9. These tables show the total number of deceased males and females, the number over 72-in, the percent total, and average and standard deviation, respectively. Table 8 Percent of Males Over 72-in Killed in Rollover Accidents Males Year Total Over 72-in Standard % Total Average Deceased Deceased Deviation

11 9 Table 9 Percent of Females Over 72-in Killed in Rollover Accidents Females Year Total Over 72-in Standard % Total Average Deceased Deceased Deviation It is clear that the average death rate for males and females in Table 8 and 9, respectively, is greater than their respective populate percentage, as shown in Table 7. The next step is to determine whether or not the values in Tables 8 and 9 are statistically significant at a 95% confidence interval. Since the actual standard deviation is not known, the confidence interval used is:, / 3 where, X is the sample mean (average percentage killed in rollover accidents),, / is the value from the t-distribution with probability α and n-1 degrees of freedom, s is the standard deviation of the random sample, and n is the sample size. In this research, the sample size is 11 and the corresponding t-value is Applying Equation 3 to the values calculated in Tables 8 and 9, the following confidence intervals can be computed (Table 10). Table 10 displays the proportion of death rates along with the 95% confidence intervals for male and female drivers, respectively. By taking account the uncertainty associated with the height distribution, males (tvalue=4.72, p<0.001) and females (t-value=1.41, p<0.16) 72-in and taller are more likely to be killed in a single-vehicle rollover collision as compared to their population percentage (shown in Table 6). However, the results are less conclusive (i.e., marginally significant) for females due to the small sample size. TABLE 10 Confidence Intervals for Male/Female Death Rates (Percent) Sex Lower Bound Average Upper Bound Male Female Vehicle Category Are some vehicles more dangerous to drivers over 72-in then others? To analyze this hypothesis, the percent of people 72-in and taller killed in each vehicle category contained in FARS was

12 10 computed for each of the study years. Only data for males was used due to the limited number of female fatalities and corresponding high degree of variability. The average was taken over the years of interest as well as the standard deviation of the values. In addition, uncommon vehicle categories with little or no data for each year were removed due to the variability of the results. The final results are shown in Table 11 below. TABLE 11 Percent Fatalities of Male Drivers 72-in and Taller by Vehicle Category Body Type Years < 6,000 lbs Average Standard Deviation Convertible Door Sedan Door Sedan Door Sedan Station Wagon Compact Utility ,000 lbs Large Utility Utility Station Wagon Minivan Large Van Compact Pickup Standard Pickup Truck Tractor Total From Table 11, it is seen that the vehicle categories with the highest fatality percentage are: large utility, utility station wagon, standard pickup, and truck-tractor. Conversely, it can also be noted that these vehicle categories have some of the largest amount of headroom for the drivers. However, recall that FMVSS 216 only applies to vehicles under 6,000 pounds, and vehicles under 6,000 pounds have to support 1.5 times the empty vehicle weight or 6,000 pounds; whichever is less. Many of the vehicles under those vehicle categories exceed the 6,000 pound weight limit, and therefore do not have any regulation for roof strength. It should be pointed out, however, that some manufacturers still elect to meet the standards to promote the roof strength of their vehicles. Thus, it is possible that this attribute could influence the results for this category of vehicles, but there is no way to know unless the company is contacted directly. Most of the remaining vehicles have empty vehicle weights very close to 6,000 pounds, meaning the roof only has to support 6,000 pounds and not the full 1.5 times the vehicle weight (in theory). The combination of these factors may increase the likelihood of roof intrusion into the occupant s survival space. Individuals 72-in or taller will have less survival space, and this could increase the likelihood to be fatally injured by the roof intrusion. Other factors, such as the age of the occupant, seating position or the number of rotations, could also play a role in the

13 11 probability of being killed in a rollover. Further research is thereforee needed to examine the combination of thesee factors influence the probability of dying as a function of driver height. Hypothetically, if the height of the driver did not affect the fatality rate of single vehicle rollover accidents, then the fatality rate for men and women would be the same as their respective height distributions; shown in Table 6. Assuming this to be true, it is possible to see which vehicle categories increasee the likelihood for individuals 72-in or taller dying in a rollover accident. This is accomplished by using the data given in Table 11 and Equation (3) to construct a 95% confidence interval. If the lower bound of the confidence interval exceeds the population percentage, 19.71% for males, then males 72-in or taller have an increased probability of dying in that category of vehicle. The confidence intervals of the different vehicle categories are plotted along with the population percentagee as shown in Figure 1. 95% Confidence Intervals by Vehicle Category Percent Fatality of Individual 6ft or Over Vehicle Category Population Percentage Vehicle Category FIGURE 1 Confidencee Intervals by Vehicle Category Seven of the thirteen vehicle categories have an increased fatality rate for individuals 72- in or taller; all resultss at statistically significant at the 5% %-level (all t= =values 1.96, p 0.05). The high number of vehicles that have an increased fatality rate for 72-in or taller males is not surprising when compared to the free headroom above the driver s head, shown in Table 5, for the different categories of vehicles.

14 12 SUMMARY AND CONCLUSIONS The overall conclusion of this paper suggests that an individual 72-in or taller, male or female, involved in a single-vehicle rollover collision has a greater likelihood of being killed based on their population percentage than individuals of a height less than this value. More specific observations are as follows: Taller individuals have a greater sitting height and thus less distance between the top of their head and the bottom of the roof liner. Roof intrusion is probably more likely to cause severe head and neck injuries due to this decreased distance. However, other factors could also influence the risk of being killed, as discussed above, and should be examined further. Both men and women, 72-in or taller, are more likely to be killed in a single-vehicle rollover collision then their respective population percentage. This conclusion was reached using a 95% confidence interval. Vehicles close to or exceeding the 6,000-pound weight limit specified by FMVSS 216, regardless of the headroom provided, appear to be more risky to individuals 72-in or taller. The authors believe this occurs due to roofs not able to support the dynamic rollover weight of the vehicle, leading to increased roof intrusion. This statistic might begin to decrease as the new regulations imposed by FMVSS 216a are implemented. It should be pointed out that these results are influenced by an important limitation. Because FARS only records the height of the driver even if multiple occupants were killed in the collision, several vehicle categories have very limited data. This creates a higher standard deviation, which makes the final results not statistically significant for these categories of vehicles. Future work in this area show focus on obtaining a larger data set with more detailed information, such as the number of rotations, the driver s sitting position, and the medical condition of the driver. This additional information could also help rule out confounding factors, such as tall people buying vehicles with larger headroom space. Obtaining data from other sources, National Automotive Sampling System (NASS) Crashworthiness Data System (CDS), General Estimates System (GES), National Center for Statistics and Analysis (NCSA) will increase the number of cases and possibly provide a greater depth of information. Another avenue of future research would be to calculate the population percentage and number of individuals killed in single-vehicle rollover accidents for each height increment and for different age groups. This type of analysis, however, would require a much larger dataset. REFERENCES 1. NHTSA. Federal Motor Vehicle Safety Standards, Roof Crush Resistance. U.S. Department of Transportation, Washington, D.C., Standard No. 216, NHTSA. Federal Motor Vehicle Safety Standards, Roof Crush Resistance; Upgraded Standard. U.S. Department of Transportation, Washington, DC, Standard No. 216a, Young, D., Grzebieta, R., Rechnitzer, G., Bambach, M. and Richardson, S. Rollover Crash Safety: Characteristics and Issues. Proceedings from the 5th International Crashworthiness Conference (ICRASH2006). Athens, Greece, Friedman, D. and Nash, C.E. Advanced Roof Design for Rollover Protection. Paper No. 01-S12-W-94, 2001.

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