Vehicle and Driver Attributes Affecting Distance from the Steering Wheel in Motor Vehicles.
Evidence exists that women are at greater risk of injury than men as a result of road crashes. In Australia, serious injury rates (resulting in hospitalization) have fallen more sharply for male than for female car drivers, and in 1995 the serious injury rate per 100 million km driven was 8.35 for female drivers and 7.25 for male drivers (Attewell, 1998).
Similar crash patterns have been observed in the United States. From 1983 to 1990, there was a decrease in fatal accidents per distance traveled (Massie, Campbell, & Williams, 1995). Although women had a lower fatality rate than men during this period, they had a higher rate of injury.
Reasons for the greater risk of injury in women may relate to the car (e.g., age and size, type of accident) or to the driver (e.g., physical structure and driving experience; Over, 1998). Another factor in the type and severity of injury outcomes is the steering wheel assembly. In 1948, Woodward noted the role of the steering wheel in chest injuries, and since that time his conclusions have been confirmed and extended.
Dalmotas (1980) studied injuries among 314 fully restrained occupants, of whom 223 were drivers. He found that 60% of drivers suffered an injury of moderate or greater than moderate severity (Abbreviated Injury Scale Score of 2 or more; Committee on Injury Scaling, 1980) to the head or face, 33% to the shoulder/chest, and 15% to the upper extremities. The steering assembly was involved in 20% of head injuries, 82% of facial injuries, 40% of shoulder/chest injuries, and 63% of upper extremity injuries.
Blower and Campbell (1994) analyzed crash records from the National Accident Sampling System Crashworthiness Data System for the years 1988 to 1992. For drivers of vehicles without air bags (N = 12 302), the steering assembly was the most common cause of injury, causing 3.4 of the 12.4 total injuries per 100 drivers.
Differences in sitting distance between men and women may help to explain the higher rate of injury sustained by female drivers. Parkin, Mackay, and Cooper (1995) used distances between the head and the steering wheel to show that women sit closer to the steering wheel than do men and are therefore more prone to head injury from it. Other studies, primarily concerned with injury sustained from air bag deployment, have also found that women sit closer to the steering wheel than do men (De Leonardis, Ferguson, & Pantula, 1998; Segui-Gomez et al., 1999).
The current study was designed to confirm that female drivers sit closer to the steering wheel than do male drivers and to investigate whether this expected difference in sitting position is attributable to differences in the physical dimensions of men and women. Vehicle size was also included in the design. Other research has indicated that driver sitting position varies with car size (De Leonardis et al., 1998).
The study was conducted in car parks in Newcastle, New South Wales, Australia, by two final-year university students who were trained in measuring body dimensions and distances from the driver to the steering wheel. There were 50 male and 50 female drivers in each of three car sizes: small, medium, and large. Sports cars, utilities, and vans were excluded.
To recruit the required 300 participants, we approached 393 drivers. Participants ranged in age from 18 to 75 years; 42% of drivers were younger than 25 years, and 8% were over the age of 60.
The make, model, and year of manufacture of the car were used to determine the size of the car based on vehicle specification data held by the Australian Transport Safety Bureau. Small vehicles were defined as those with a mass equal to or less than 1150 kg, medium vehicles as those 1151 - 1300 kg, and large vehicles as greater than 1300 kg.
The driver's height and weight (in footwear), lower limb length (from the lateral condyle of the tibia to the ground; in shoes), upper arm length (from the acromion process to the lateral epicondyle of the humerus), and lower arm length (lateral epicondyle of the humerus to the styloid process of the ulna) were measured with the driver standing outside the car. The driver's age was also recorded.
Drivers were asked to sit in their usual driving positions in their cars, and distances were taken between the nasion (junction of the forehead and the nose) and the top of the steering wheel (NT), the nasion and the center of the steering wheel (NC), the sternum and the center of the steering wheel (SC), and the xiphisternum and the base of the steering wheel (XB; see Figure 1). Each distance was measured twice. A third measurement was taken if a discrepancy of more than 1 mm occurred. The average measurement was recorded in the database.
After checking for normality, we used a two-factor analysis of variance (ANOVA) to compare drivers' distances from the steering wheel by car size (small, medium, and large) and driver gender. Where the ANOVA showed a significant difference in the means, the distances from the wheel in the three car sizes were compared using Bonferroni significance levels of .017 for each of the three comparisons. The 1st, 5th, 25th, 50th, 75th, 95th, and 99th percentiles were calculated for driver height, NT, NC, SC, and XB and plotted for men and women on the same graph.
Linear regression was then used to determine whether the differences in sitting position were attributable to the drivers' physical dimensions rather than their gender. For each of the regression models, Minitab (Ryan, Joiner, & Ryan, 1985) was used to obtain the best subset of the five anthropometric measurements, the age of the driver, and the gender and car size factors. Model assumptions of normality of the residuals and constancy of the variance of the error terms were verified (Stata Corp., 1995). The final model was checked for outlying and influential observations (Neter, Kutner, Nachtsheim, & Wasserman, 1996).
Measurements were taken for 300 male and female drivers of small, medium, and large cars. The driver of a Volkswagon Kombi was excluded because the steering wheel position in this vehicle is totally different from that in standard vehicles. After this exclusion, there were 49 men and 50 women in the large-car group and 50 men and 50 women in each of the other car size groups.
The participants' mean height was 165 cm (SD = 7.3) for women and 178 cm (SD = 7.3) for men. For drivers of small, medium, and large cars, the mean heights (range) for men were 178 cm (160-188 cm), 179 cm (165-209 cm), and 178 cm (157-191 cm), respectively. The corresponding mean heights for women were 165 cm (150-182 cm), 165 cm (149-183 cm), and 164 cm (149-180 cm), respectively. The participants' mean weight was 65 kg (SD = 12.2) for the women and 77 kg (SD = 11.6) for the men. On average, women sat 4 to 5 cm closer to the top or center of the steering wheel than did men (Table 1), and the 5th-percentile female driver sat between 12 and 15 cm closer to the steering wheel than the 50th-percentile male driver (see Figure 2). The difference was greatest for the SC measurement.
The distance between the driver and the steering wheel increased as car size decreased; however, the difference was statistically significant only between the small and large cars (Table 1). Once various anthropometric measurements and the size of the car were taken into account, the driver's gender was no longer a significant determinant of where drivers sat in cars (Table 2). The height of the driver was the most important determinant for each of the distances from the steering wheel. On average, for every 10-cm increase in height, the driver sat at least 3 cm farther from the steering wheel.
Both the height of the driver and the size of the car determined how far the head and chest were from the center of the steering wheel. These distances increased with the height of the driver and decreased with increasing size of the car. Drivers of large cars sat approximately 2 cm closer to the steering wheel than did drivers of small cars.
Height, age, and weight were all significant determinants for XB. Taller drivers sat farther from the steering wheel. Independent of their height, older and heavier drivers sat closer to the steering wheel.
This study confirmed that women sit closer to the steering wheel than men do (see Table 1 and Figure 2). The actual differences in sitting distance between men and women were remarkably similar to those reported in an earlier United Kingdom study, which used hidden cameras to film drivers while they were driving in traffic (Parkin et al., 1995). In the current study, at the 50th percentile, women sat 5 cm closer than did men to the steering wheel, compared with 6 cm closer in the study by Parkin et al. Similar results were found by De Leonardis et al. (1998) and Segui-Gomez et al. (1999). In both the present and Parkin et al. studies, the 5th-percentile female driver was 13 cm and 21 cm closer to the steering wheel than the 50th-and 95th-percentile male driver, respectively.
When body dimension was taken into account, gender was no longer a significant determinant of where the driver sat in the car. The height of the driver was the most important determinant of where drivers sat. Once height was included in the models, body measurements other than weight were no longer significant. Height probably acts as a surrogate measure for the length of the lower limb and upper and lower arm. The optimum seating position allows the driver to reach the pedals and have the steering wheel a comfortable distance from the body.
Although there were few heavier or older drivers in the sample, both age and weight had some effect on XB. As girth tends to increase with both age and weight, it was not surprising that the xiphisternum was closer to the steering wheel as age and weight increased. Similarly, Parkin et al. (1995) found that older people sat slightly closer to the steering wheel.
The present study also found that drivers sit farther from the center of the steering wheel in small cars than in large cars (Tables 1 and 2). This finding was consistent with that of De Leonardis et al. (1998), who attributed this difference to the distance between the steering wheel and the accelerator of small and large cars. They found this distance was, on average, about 5 cm longer in large cars than in small cars.
There were limitations to this study. First, it was not possible to standardize the data to the driving population, as data on the physical dimensions of the driver population were not available. Second, younger drivers were over-represented. In this study, the proportion of drivers younger than 25 years was about three times that recorded in the 1995 Australian Bureau of Statistics motor vehicle survey (Australian Bureau of Statistics, 1997). This may have made it more difficult to observe any effect of age on sitting position.
Finally, the results have limited application to those concerned with air bag deployment and sitting distance. The Federal Office of Road Safety (1996) recommends that drivers sit no closer than 30 cm from the steering wheel, and the National Highway Traffic Safety Administration (2000) recommends a distance of at least 10 inches (25.4 cm; NHTSA, 2000). In this study, only 1 in 300 drivers sat within 30 cm of the steering wheel (sternum to center of steering wheel). This may be related to the lack of truly short participants in the sample.
In conclusion, although women sit closer to the steering wheel than men do; this appears to be attributable to their shorter stature rather than to a gender difference per se. We therefore suggest that body size -- especially height -- should be taken into account when designing and setting safety standards for new vehicles. Allowing drivers to maximize their sitting distance from the steering wheel should result in reduced injury severity from frontal crashes. Current design tends to fix the distance between the floor pedals and the steering wheel while allowing the seating position to vary. Consideration should be given to vehicle design that allows independent adjustment of the relative distance among the driver's seat, the steering wheel, and the floor pedals.
This finding may also be of relevance to future research. There are practical limitations to determining a driver's sitting position and distance from the steering assembly once a crash has occurred. The results of this study suggest that driver height may be a good indicator of sitting distance from the steering wheel and could constitute a more easily accessible variable for future research.
The authors thank the two students, Vicki McCloskey and Rachel Stevens, who collected the data. We also thank the people who participated in the study. The opinions of the authors are not to be construed as those of the Australian Transport Safety Bureau.
Michael McFadden is director of National Statistical Coordination and Analysis at the Australian Transport Safety Bureau. He received a Ph.D. in psychology from the University of New South Wales in 1994.
Jennifer Powers is a statistician at the Research Institute for Gender and Health, University of Newcastle. She received a master's in medical statistics from University of Newcastle in 1999.
Wendy Brown is director of the Research Institute for Gender and Health, University of Newcastle. She received her Ph.D. in human physiology from the University of Newcastle in 1984.
Michelle Walker is the research manager for National Statistical Coordination and Analysis at the Australian Transport Safety Bureau. She received her Graduate Diploma in ergonomics from LaTrobe University in 1994.
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Mean Distance (cm) from the Steering Wheel by Car Size and Driver Gender Small Cars Medium Cars Large Cars Distance Men Women Men Women Men Women NT 49.1 45.2 48.1 43.8 48.2 43.2 NC [a] 51.7 47.7 51.1 46.3 50.2 45.3 SC [a] 51.1 46.6 50.3 45.0 48.9 43.8 XB [ab] 31.1 29.1 30.9 26.4 29.6 26.8 F Values Distance Gender Car Size NT 56.7 [***] 2.3 NC [a] 69.0 [***] 4.1 [*] SC [a] 73.1 [***] 6.3 [**] XB [ab] 26.1 [***] 3.5 [*]
Note. NT = distance between nasion and top of steering wheel; NC = distance between nasion and Center of steering wheel; SC = distance between sternum and center of steering wheel; XB = distance between xiphisternum and base of steering wheel.
(a.)Distances greater in small cars than in large cars with Bonferroni .017.
(b.)Value missing for one male driver.
(*.)p [less than].05, (**.)p[less than].01, (***.)p[less than].001.
Coefficients (95% confidence interval) for Car Size and Driver Characteristics Significantly Associated with Where Drivers Sit in Cars Significant Variables NT NC SC Height (cm) 0.32 0.33 0.36 (0.27; 0.38) (0.29; 0.38) (0.31; 0.41) Size of car Small [a] 0 0 Medium -0.91 -1.18 (-2.05; 0.23) (-2.38; 0.03) Large -1.58 -2.14 (2.72; -0.43) (-3.35; -0.93) Age (years) Weight (kg) Adjusted [R.sup.2] 32.7% 40.3% 42.0% Significant Variables XB Height (cm) 0.30 (0.23; 0.36) Size of car Small [a] Medium Large Age (years) -0.08 (-0.12; -0.04) Weight (kg) -0.05 (-0.10; -0.01) Adjusted [R.sup.2] 34.5% (a.)Reference category.
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|Title Annotation:||implications for serious injury risk for men and women drivers|
|Author:||McFadden, Michael; Powers, Jennifer; Brown, Wendy; Walker, Michelle|
|Article Type:||Statistical Data Included|
|Date:||Dec 22, 2000|
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