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Accid. Anal. and Prev., Vol. 27, No. 6, 777-783, 1995 pp. Couvrieht 0 1995 Elsevier Science Ltd Prkzin the USA. All rights reserved

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HOW DRIVERS SIT IN CARS

S. PARKIN*, G. M. MACKAY and A. COOPER

Birmingham Accident Research Centre, University of Birmingham, P.0 Box 363, Birmingham B15 2TT, U.K.

(Received 26 August 1993: Accepted 1 January 1994)

Abstract-This paper presents results from a study to measure the separation of the driver’s head and shoulder to various internal features of the car. Drivers were filmed whilst driving in general traffic flow, hence were unaware that they were involved in a study. The results show that certain sub-groups of the driver population are likely to be more at risk for certain impact types. Small females are considerably closer to the steering wheel than the rest of the population, and therefore prone to head strikes in frontal impacts. Large males are likely to interact with the cant rail and B-pillar in side impacts.

Keywords-Drivers, Sitting position, Cars, Video, Hybrid III, Dummy, FMVSS 208, FMVSS 214, Restraint, Airbag, Head restraint

INTRODUCTION

Much design work related to crash performance is predicted on the initial sitting positions of current crash test dummies. In-depth crash reconstruction aimed at evaluating seat belts, steering wheel design and airbag performance is similarly based on assump- tions about sitting position and the posture of drivers. Several studies have attempted to quantify how drivers actually sit in cars, but these have generally been laboratory based, and therefore not necessarily representative of the real-world driver population under real-world driving conditions. The data used to position dummies in the current crash tests came from a NHTSA sponsored study at UMTRI (Robbins et al. 1983) in which the subjects were seated in a “standard- ized driving posture”, with the seat back angle fixed. The subjects were asked to sit upright, with their back pressed against the seat. In a study to determine the driver’s eye position, Meldrum (1966) measured subjects seated in actual vehicles, but under simulated driving conditions, placing the vehicle in front of a large street scene mural. The data from this work led to the driver’s field of vision standard SAE 5941.

Other studies have attempted to measure drivers during, or after, driving. Schneider et al. (1979) compared driver anthropometry in non-driving and after- driving conditions, and found little overall differences in the two situations. This study, however, took no account of seated posture. A study at Jaguar Cars by Wankling ( 1991) measured the driver’s eye position under actual driving conditions, using an electromag-

*Author for correspondence.

netic sensor, but no firm conclusions were reached due to the small sample size. In a recent study, Schneider et al. (1991) compared measured driver eye position to SAE 5941, from a sample of 50 subjects and 6 vehicles after driving on a short test track.

This paper will present results from drivers, driving their own cars, under actual driving conditions, without the subjects being aware that they are being involved in a study.

METHODOLOGY

Drivers were filmed using a video camera equ- ipped with a high speed shutter as they passed a white screen. Three separate locations were used, with the camera being out of sight of the drivers. The camera was pointed at right angles to the traffic flow, facing the screen, as shown in Fig. 1.

Hence, the drivers were silhouetted against the

White screen

rl Video Camera

Fig. 1. Unobtrusive filming of the drivers.

778 S. PARKIN et al.

background. The camera was able to capture the image of the vehicle and driver, even when the vehicle was travelling at around 30 mph. The camera height was set at mid side window height for an average vehicle, and filming took place at between 15 and 20 m from the subjects. The angle from the camera to mid side window and top of the side window was therefore very small, and so parallax was not considered to unduly affect results. The video was played back and measurements taken from a television monitor. Figure 2 shows which measurements were taken: nasion (junction of the forehead and nose) to the steering wheel top (A-B), and to the top of the side window (A-D); the centre of the shoulder to the centre of the B-pillar (I-J); and front or back of the head (whichever was in view) to the centre of the head restraint (A or G-H). The B-pillar height was measured and used as a scaling factor for all the measurements when compared to the known height of the B-pillar. A correction factor was also applied, as explained later in the paper.

After the measurements were scaled and cor- rected, they were converted to the following dimen- sions: nasion to steering wheel centre (horizontal, vertical and direct; A-C); top of head to top of side window (vertical; E-F); back of head to centre of head restraint (horizontal and vertical; G-H); and centre of shoulder to centre of B-pillar (horizontal, I-J). The measurements to the top of the side window and the centre of the head restraint required anthro- pometric data to shift the originally measured point on the head to the required point on the head (A-E, and A-G).

CALIBRATION

In order to calibrate the system, five subjects of varying age, sex and stature were filmed in two different car types from a number of filming distances. The driver’s head was fixed in a stable position using the car’s head restraint. The actual forehead to steer-

Fig. 2. Measurements taken of the driver to the vehicle.

ing wheel separation was measured and then compared with measurements taken from five filming distances, both static and dynamic. In the dynamic condition, the car was driven at between 15 and 20 mph. Results indicated that the on-screen measurements were all between 1 and 11% greater than the actual measurements. A correction factor of -6% was therefore applied to all subsequent measurements making them accurate to within f5%.

STATISTICAL ANALYSIS

The cumulative frequency plots shown later in the paper have been analysed using the probability density function of normal distributions, and the standard deviations at various percentile levels have been calculated. The calaculated values for twice the standard deviation of a particular percentile value is given underneath each figure (rounded to the nearest mm), with the percentile value it refers to also shown. A line may be drawn horizontally on the cumulative frequency curve at the particular percentile value such that the line is centred on the actual value and extends twice the standard deviation in each direction. The line represents a 95% probability that the percentile quoted lies within its bounds. In general, the line would be so small (usually of the order of lo-20 mm) that it has not been practical to plot it on the curves.

SAMPLE

Nineteen car models were chosen based on the most popular models from 7 manufacturers. Three of the manufacturers had most of their range in the sample, from the smallest to the largest model. The car sample was thus thought to reasonably represent the general car population in terms of size and weight. As each of the sample vehicles passed the camera in the general traffic flow, the measurements were

Table 1. Driver population by sex

Sex Number

Male 742 Female 237 Not known 21 Total 1000

Percent

75.8 24.2

100.0

Table 2. Driver population by age

Age Number

Young 315 Middle 564 Old 78 Not known 43 Total 1000

Percent

32.9 58.9

8.2

100.0

How drivers sit in cars 179

Table 3. Vehicle population by make and model

Make and Model Number

Citroen BX 26 Ford Fiesta MK3 49 Ford Escort MK3 94 Ford Escort MK4 58 Ford Sierra 148 Ford Granada 21 Peugeot205 47 Peugeot 405 33 Rover Metro 38 Rover 200 69 Rover Montego 96 Rover 800 24 Vauxhall Nova 41 Vauxhall Astra 81 Vauxhall Cavalier 63 Vauxhall Carlton 17 Volvo 760 19 Volkswagen Polo 39 Volkswagen Golf 31

Total 1000

Table 4. Vehicle population by wheelbase and size

Wheelbase (m)

2.05-2.40 2.41-2.58 2.599280 Total

Crash 3 size

1 2 3

Number

218 432 350

1000

recorded. In this way the driver population generated itself, and was completely random. One thousand readings were taken. Tables l-4 show how the driver and vehicle population was comprised. In the age category “Young” means the driver looked to be below 35 years old; “Middle”, the driver appeared to be aged 35-55; “Old”, the driver looked to be aged over 55.

The majority of the driver sample was male (75%) and aged between 35 and 55 (60%).

RESULTS

The following graphs (Figs 3-5) are the plotted results for each of the measurements taken. For each graph the sample is plotted as a whole, and also split by sex. Each sub-group has the following percentiles plotted: 1, 5, 25, 50, 75, 95, 99. It should be noted that each percentile plotted is the percentile within the measurements and does not necessarily represent the overall population percentile. Hence, when the paper quotes “50%ile male” it means the 50%ile measurement of the male sample, not the measurement that occurs with the 50%ile male.

Nasion to steering wheel centre Figure 5 shows that females are considerably

closer to the steering wheel than males.

90 -

80 -

--o- ALL (n=lOOO)

__*_ Male (rl=741)

._-__t-._ Female (n&237)

150 200 250 300 350 400 450 500 550 6 Horizontal distance (mm)

Fig. 3. Nasion to steering wheel centre (horizontal).

Twice standard deviation: All: 25%ile (6 mm), 50%ile (5 mm), 75%ile (6 mm) Male: 25%ile (6 mm), 50%ile (5 mm), 75%ile (6 mm) Female: 25%ile (9 mm), 50%ile (8 mm), 75%ile (10 mm)

10

At the 50%ile level, females are 6.2 cm (2.5 in.) closer than the males. The difference between 5%ile female and 95%ile male is 21.5 cm (8.5 in.). 15% of the female population are closer than 40 cm (15.7 in.) to the steering wheel.

Do old people sit closer to the steering wheel? The answer is “yes”, but only by 1 cm, and the

result is not statistically viable. The “Old” driver population was compared to the “Young” plus “Middle” population for their proximity to the steering wheel centre, horizontally, vertically, and directly, and by sex. In almost all the cases the “Old” group at all percentile levels were approximately 1.0 cm closer than the “Young” plus “Middle” group. Although not statistically viable, it would be expected

90 -

80 -

e ‘O- = E 60- B E 50-

A! 40-

30 -

240 260 280 300 320 340 360 360 400 420 Vertical distance (mm)

Fig. 4. Nasion to steering wheel centre (vertical).

Twice standard deviation: All: 25%ile (3 mm), 50%ile (2 mm), 75%ile (3 mm) Male: 25%ile (3 mm), SO%ile (2 mm), 75%ile (3 mm) Female: 25%ile (4 mm), SO%ile (4 mm), 75%ile ( 5 mm)


+ ALL (n=lOOO) __Q_ Male (n=741) . .._.&___ Female (r&37)

“I , . 8 I

300 350 400 450 500 550 600 650 700 Direct distance (mm)

Fig. 5. Nasion to steering wheel centre (direct)


to be explained by virtue of the reduced stature of the older generation. The “Old” group is defined (arbitrarily) as looking over 55 years of age. The onset of increased thoracic spine curvature does not become especially marked until age 70 or so, and thus our definition was unlikely to pick out this factor clearly.

Top of head to top of side window Figure 6 shows that only a small proportion of

the driver population has the top of their head level with or above the top of the side window, and hence the start of the metal structures at roof level. Males are considerably more at risk from a head strike with metal during a lateral impact than females (14.6% compared to 2.6%). More than l/lOth (11.7%) of the whole driver population is similarly exposed.

loo-

90 -

80 -

0 70- =

Female (n=236)

-40 -20 0 20 40 60 80 100 120 Vertical distance (mm)

o! - __...- v -- -100 -50 0 50 100 150 200

Fig. 6. Top of head to start of metal structures (vertical). Fig. 8


All: Male: Female:

U ALL (rk573) --*- Male (h433) __._.&.._ Female (11~140)

0 50 100 150 200 250 300 350 400

Horizontal distance (mm)

Fig. 7. Back of head to centre of head restraint (horizontal)

All: Male: Female:

Twice standard deviation: 25%ile (5 mm), 50%ile (4 mm), 25%ile (6 mm), 50%ile (5 mm), 25%ile (10 mm), 50%ile (8 mm),

75%ile (5 mm) 75%ile (6 mm) 75%ile (8 mm)

Back of head to centre of head restraint Very little difference is apparent between the

horizontal separation of the back of the head to the centre of the head restraint for males and females. 50% of the population were 15.1 cm (5.9 in.) or more from the head restraint, horizontally (see Fig. 7).

The optimum vertical position of the head restraint is for the centre of the head restraint to be level with the centre of the back of the head. The great majority of the drivers had their head restraint set too low, with only 5% of the drivers at or above this level (see Fig. 8). Some 50% of the population had the head restraint 10 cm (4 in.) or more below the centre of the head, probably representing a particularly high risk condition for a rear end collision.

a 70 .z ; 60

; 50

E 40 1 30

20

10 i

Vertical distance (mm)

Back of head to centre of head restraint (vertical).


75%ile (5 mm) 75%ile (5 mm) 75%ile (9 mm)


Centre of shoulder to centre of B-pillar Only male shoulders are likely to

the B-pillar in a purely lateral impact, in Fig. 9. Only 1% of the males had

interact with as illustrated the centre of

their shoulder directly in line with or behind the centre of the B-pillar. The number of males who had the back of their shoulder level with the leading edge of the B-pillar is clearly dependent on the average width of the large male shoulder, and the B-pillar at shoulder height. If these figures are taken to be 11 cm (4.3 in.) and 8 cm (3.1 in.) respectively, then 25% of the males will interact with the B-pillar in a purely lateral impact.

COMPARISON OF THIS STUDY’S OBSERVATIONAL DATA WITH CRASH

TEST DUMMY POSITIONING

Figure 10 shows the 5%ile female, SO%ile and 95%ile male positions relative to the steering wheel, top of side window and centre of B-pillar, as illustrated earlier in the paper. The nasion (junction of forehead and nose) position of the three equivalent Hybrid III crash test dummies is included for comparison, and is shown as a dot. The data for the dummy positions was extracted from a study by Bacon (1989) in which top of head trajectories were measured for the dummies during frontal impacts in a current model car. In that study the centre position of the driver’s seat was used for all the dummies. The vertical position of the nasion was scaled from a figure within the paper for each dummy, with the measurements being taken from the steering wheel centre. The horizontal position was scaled as above, using the 50%ile male dummy position, and assuming a stan-

90 -

80 -

al 70-

P ALL (nz971)

__*_ Male (rl=736)

._.. _e__. Female (n=235)

-50 0 50 100 150 200 250 300 350 400 450 500 HorizonhI distance (mm)

Fig. 9. Centre of shoulder to centre of B-pillar (horizontal),

All: Male: Female:


75%ile (9 mm) 75%ile (8 mm)

75%ile (21 mm)

0 10 20 30 40 50

Fig. 10. Crash test dummy nasion position compared to observed driver positions (black dots are the dummy nasion positions).

dard fore-aft adjustment of 20 cm for the driver’s seat, and that the posture of all three dummies was the same, that is, the seat back angle was constant. In all three cases the dummy positioning is further from the steering wheel than the 5%ile female, 50%ile and 95%ile measurements observed in this study. The differences were greatest for the small female at 9.2 cm (3.6 in.) but were also significant for the other two dummies at 4.7 cm (1.9 in.) and 4 cm (1.6 in.) respectively. It is likely that these differences in the context of the results of crash tests would have a profound influence on assessing head injury risk to the population actually exposed.

DISCUSSION

The main current crash performance standards are, in essence, predicted on single-point global crash tests, as specified under FMVSS 208 and 214, using 50%ile male dummies. In actuality, however, the car designer is faced with the problem of optimizing protection for two populations. The first population is that of the collisions, with many occurring at relatively low speeds, but having a skewed distribu- tion, leading to a tail of collisions at higher severity (Ricci 1980). The second population is that of the actual people sitting in cars. Surprisingly little atten- tion has been directed at the characteristics of this second population.

Many insights have been provided into variations into biomechanical response to impact forces, which reflect the population at risk, but in the context of restraint system design, side impact protection and rear end collision performance, the simple question of size, initial sitting position and posture can have a profound effect on how the car should be designed for optimum protection.

Thus, this research program. We limited our observations and analysis to the most popular, main- stream cars on European roads. We thus excluded


sports cars, utility vehicles and vans, which probably have their own specific characteristics relating to driving positions. We examined literature which describes the anthropometry of the British, North American and other populations which have been surveyed adequately. Broadly speaking, these differences, with the exception of the Japanese, who tend to be shorter stature and have long torsos in relation to leg length, are not great. The North American population is slightly taller than the European, and has an incidence of obesity which is two to four times greater. With these reservations, however, it is likely that our results have general applicability.

There is also the question of how representative the 19 models of cars are of other vehicle populations. It is commonly suggested that a straight arm driving position modelled after Grand Prix cars (making it like Nuvoarli) is a fashionable posture, at least for the young male European driver. This may be an element, but our results indicate a significant proportion of drivers in fact sitting especially close to the steering wheel, particularly females. Only a quarter of our driving population was female, in North America the proportion is about one third or higher.

With airbag technology, sitting closer than the design position to the steering wheel carries risk of increased injury to the brain, neck and chest, especially with the large volume North American bag needed to meet FMVSS 208 passive requirements. The data in this study may well be useful to the designer, therefore, in optimizing specific desigtls to minimize the conflicting requirements of the various elements within the population. It suggests that the ultimate “smart” restraint system would be pro- grammed for the sex and age of the occupant.

When the crash begins to occur it would monitor both the crash severity and the sitting position of the occupant, and time the airbag deployment and seat belt tension for optimum protection of that particular occupant in that particular crash.

For lateral collisions our results suggest that interior head contacts with the side roof rail are relatively uncommon, as is born out by crash investi- gation studies (Morris et al. 1993). The data shows, however, the obvious corollary of partial ejection through side windows and the need to review current thinking on tempered glass or other possible glazing constructions. B-pillar interaction will occur for some quarter of the male population and is, in fact, a particular characteristic of small, four door cars. This condition is not addressed in either the U.S. or proposed European side impact test procedures.

Head to head restraint geometry was the third aspect considered in this study. Experimental studies have illustrated the disadvantages of large separation

distances between the head and the head restraint (Svensson and Lovsund 1992). Our data show that vertical adjustment is a major factor in increasing the potential for neck injury, but that also illustrate a wide range of horizontal separation distances. The small female, who is particularly susceptible to neck injury (Larder et al. 1985) is the very person with the head restraint positioned inappropriately. Perhaps the inclusion of horizontal adjustment in head restraint design should be examined as a design possibility.

This study has been limited to drivers of main- stream cars on U.K. roads. In the future we would like to extend the work to some other populations in North America or Japan, for example. The actual data collection takes little time, and thus useful com- parisons with other environments could be made easily. It would also be useful to examine the front passenger population. Drivers are naturally limited in choice of position by the very act of driving; passengers have the freedom to slouch, sit turned or indeed put their feet on the passenger side airbag module or out of the window, and do so, as any casual observation of an expressway in summer will confirm. In addition, children represent a small, but significant part of the front seat population. Quantitative data on the front seat population sitting position should be a necessary component part of the designer’s knowledge if future restraint systems and other aspects of the vehicle crash protective package are to be optimized for the real world, rather than the conditions prescribed by current dummies.

CONCLUSIONS

(4

(b)

64

(d)

(4

76% of the driver population was male and 58% appeared to be between 35 and 55 years old. 25% of the total population are within 45.4 cm (18 in.) of the centre of the steering wheel. Females are considerably closer than males to the steering wheel by 6.2 cm (2.5 in.) at the 50%ile level. The 5%ile female is 21.5 cm (8.5 in.) closer than the 95%ile male, at 38.5 cm (15 in.) from the centre of the steering wheel. “Old” people, as defined in this study, are only 1 cm closer to the steering wheel than the rest of the population. This though was not found to be statistically viable. The specified positions of the Hybrid III dummies appear to be markedly different from their equivalent percentiles in the actual population, particularly for the small female, where the difference is 9.2 cm (3.6 in.).


(f 1

(8)

(h)

14.6% of the male population is likely to suffer a head strike with the metal structures at roof level during a lateral impact, as compared to 2.6% of females. Overall, more than l/l0 th of the driving population is similarly compromised. 50% of the driving population have the head restraint positioned greater than 15 cm (6 in.) from the back of their head, horizontally. Only

5% have the head restraint correctly positioned vertically, with the great majority having it positioned too low. 50% of the population had the head restraint positioned 10 cm (4 in.) or more below the centre of the head. Only male shoulders are likely to interact with

the B-pillar in purely lateral impacts. Dependent on the size of a particular vehicle’s B-pillar, this

figure is likely to be approximately 25% of the male driving population.

Acknowledgements-Grateful thanks are extended to the David R. Foust Memorial Fund which partly sponsored the work for this study. The fund was established in 1988, and is administered by the Association for the Advancement of Automotive Medicine. The fund is designed to help young researchers in their research into impact biomechanics.

REFERENCES

Bacon, D. G. C. The effect of restraint design and seat position on the crash trajectory of the Hybrid III dummy.

Proc. 12th ESV Conf.; 29 May-l June 1989: 451-457.

Larder, D. R.; Twiss, M. K.; Mackay, G. M. Neck injury to car occupants using seat belts. Proc. AAAM Conf., Des Plaines, IL; October 1985: 153-165.

Meldrum, J. L. Automobile driver eye position. SAE Transactions Paper no. 650464. 74: 599-609; 1966.

Morris, A.; Hassan, A.; Mackay, G. M.; Hill, J. Head injuries in lateral impact collisions. Proc. IRCOBI Conf.; Eindhoven; September 1993: 41-55.

Robbins, D. H.; Schneider, L. W.; Snyder, R. G.; Pflug, M.; Haffner, M. Seated posture of vehicle occupants. 27th Stapp Car Crash Conf. Proc. P-134, Warrendale; 1983: 199-224.

Ricci, L. National crash severity study, passenger cars. NHTSA Contract Report DOT-HS-8-01994. Washing- ton; June 1980.

Schneider, L. W.; Anderson, C. K.; Olson, P. L. Driver anthropometry and vehicle design characteristics related to seat positions selected under driving and non-Driving conditions. SAE Technical Paper Series, Paper no. 790384; 26 February-2 March 1979.

Schneider. L. W.; Reed, M. P.; Manary, M. A. Accommodat- ing the driving population. UMTRI Research Review. Vol. 21, No 4; Jan-Feb 1991.

Svensson, M. Y.; Lovsund, P. A dummy for rear end collisions. Development and validation of a new dummy- neck. Proc. IRCOBI Conf., Verona, Italy; Sept. 1992: 299-310.

Wankling, J. J. Measurement of eye points dynamically in a vehicle. Proc. Ergonomics Society Annual Conf.; April 1991. Contemporary Ergonomics. ISBN O-74840-007- 9.

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