Age Appropriate Restraints for the Right Front Passenger

Abstract

This study applies NASS/CDS, GES and FARS data to examine occupant exposure plus injury and fatality rates for belted occupants in frontal crashes by seating position, age and gender. The NASS data was used to examine the distributions by crash severity. The GES data showed that when two elderly occupants (age 65+) were present, the female occupied the right front passenger position 73% of the time. A paired comparison analysis using FARS data showed that, for elderly occupants (age 65+), the fatality risk for elderly right front passengers is 42% higher than for elderly drivers. The NASS/CDS analysis found 74% of the seriously injured vulnerable passengers with MAIS 3+ injuries were in crashes less severe than 26 mph. This group of injured occupants was made up of 43% aged 50 and older and 42% younger females. The injury rates for the older (age 50+) right front passengers were 1.8 times the rates for the elderly drivers. These results suggest the need for more benign safety systems for the right front passenger that are appropriate for the lower injury tolerance of the predominant occupants of that seating position.

THE CONCEPT OF AGE appropriate safety systems has been applied to children as they grow from one safety seating system to another [Winston, 2003]. This same concept of age appropriate safety systems should also be applied to adult right front passengers. If the right front passenger seat is most frequently occupied by a person more vulnerable to injury than the driver, the safety system for the right front passenger should be more benign than for the driver. The right front passenger has more ride-down distance than the driver for energy absorption during a frontal crash. The use of the added ride-down distance could be used to apply the restraint forces at a lower level but over a longer distance. Consequently, more benign restraint systems should be feasible for the vulnerable right front passengers. The benign systems could include higher technology safety belts - air belts, 4 point belts, and force limiting lap and shoulder belts that adjust appropriately for the passenger weight and crash severity.

Numerous studies have shown the higher vulnerability to injury and death of older occupants involved in motor vehicle crashes [Augenstein, 2001; Fildes, 1991; and Mackay 1994,2001]. Other studies of gender differences indicate that females, like older occupants, are more vulnerable to injury than males of the same age [Lenard 2001, Welsh, 2001]. Evans found that the same physical insult was three times more likely to fatally injur a 70 year old compared with a younger person age 15 to 45. He further showed that women age 15 to 45 were 25% more likely to be killed from a similar physical insult than their male counterparts [Evans 2001, 1991].

Studies of serious injuries by body region for restrained occupants in frontal crashes have shown that for belted occupants age 65+, the chest is the body region most frequently injured at the MAIS 3+ level [Augenstein, 2005]. Similar results were found for belted fatally injured older drivers in frontal crashes [Kent, 2005].

Earlier studies examined changes in chest injury propensity using cadaver testing and found a significant decrease in injury tolerance by age [Zhou, 2001]. For frontal crashes, belt loading was found to more significantly increase the risk of injury compared with loading by air bags. When compared to 16–35 year old occupants or the “young” group, the chest injury threshold for bag loading was reduced to 84% for the 36–65 year old age group and to 79% for the 66–85 year old age group. For belt loading, the reduction was to 47% for the 36–65 year old age group and to 28% for the 66–85 year old age group.

This earlier research supports the thesis that females and older occupants of both genders could benefit from restraint systems that apply forces at lower levels than the force allowable for young males.

METHODS

The National Automotive Sampling System/Crashworthiness Data System, NASS/CDS (1997–2004), was used to compare the relative injury risk of belted drivers and right front passengers during frontal crashes. The analysis considered occupants by gender and age when stratified by crash severity. A comparison of the occupancy of the driver and right front passenger seats by gender and age was made using General Estimates System, GES (2003–2005), data. The Fatality Analysis Reporting System, FARS (1998–2005), paired comparisons were used to determine fatality risk ratios for driver and passenger combinations by age and gender.

For the NASS data analysis, frontal crashes were defined as any crash where the principal direction of force (PDOF) was 1, 11, or 12 o’clock or the PDOF was 10 or 2 o’clock with the highest deformation location coded as front (F). Typically, crash severity is determined using the coded change in velocity or deltaV. For those cases where delta-V was missing, the NASS researcher supplied estimated delta-V was used. These estimates are available for those cases where delta-V cannot be accurately computed during crash reconstruction. Estimated delta-V values can be partitioned into 3 categories which are 0–15 MPH, 15–25 MPH and 25+ MPH. For this analysis, calculated and estimated delta-V’s were separated into the two categories including crashes lower than 25 MPH and those over 25 MPH.

The NASS database provided 23,718 raw cases of belted front seat occupants 15 and older exposed to frontal crashes with known or estimated delta-V. Of these occupants, 1,857 suffered MAIS 3+ injuries. When weighted, these cases expanded to 11,606,000 occupants with 167,652 MAIS 3+ injured. In the analysis to follow, only weighted analysis results are presented.

FARS 1998–2005 contains 35,731 vehicles involved in frontal crashes with both the driver and right front passenger seats occupied and both occupants reportedly belted. For the FARS analysis, frontal crashes were defined as those with a PDOF of 1, 11 or 12 o’clock.

GES 2003 – 2005 contains 148,084 unweighted vehicles representing 15,387,000 weighted vehicles exposed to frontal crashes. These are crashes with a damage region defined as front, including front right corner and front left corner damage areas.

NASS ANALYSIS

The NASS 1997–2004 was examined to determine the distribution of exposure and injuries for belted drivers and right front passengers by age, gender and crash severity.

Table 1 shows the percentages of front seat belted occupants exposed to frontal crashes by gender, age and seating position. In the table, drivers and passengers are treated as separate populations. Drivers made up 82.5% of the entire group of front seat occupants.

Table 1.

Crash Exposure of Belted Front Seat Occupants in Frontal Crashes by Gender, Age and Seating Position

	Driver			Right Front Passenger
Gender	Exposed	Exposed	Exposed	Exposed	Exposed	Exposed
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	42%	11%	54%	34%	4%	38%
Female	35%	11%	46%	48%	14%	62%
All	78%	22%	100%	83%	17%	100%

The NASS 1997–2004 belted right front passengers were present in 17.8% of the crashes and they accounted for 19.6% of the MAIS 3+ injuries. To better understand the factors that contribute to the higher frequency of injuries for the right front passenger the distributions of injuries by occupant seating position, age and gender were examined. The results are shown in Table 2. The table also shows the rate of MAIS 3+ injury per 100 belted occupants exposed to frontal crashes.

Table 2.

Distribution of MAIS 3+ Injuries and MAIS 3+ Injuries per 100 Exposed Belted Front Seat Occupants in Frontal Crashes by Gender, Age and Seating Position

Drivers
Gender	MAIS 3+	MAIS 3+	MAIS 3+	3+ Rate	3+ Rate	3+ Rate
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	33%	15%	48%	1.27	2.08	1.44
Female	36%	17%	52%	1.65	2.57	1.86
All	69%	31%	100%	1.44	2.32	1.64
Right Front Passengers
Gender	MAIS 3+	MAIS 3+	MAIS 3+	3+ Rate	3+ Rate	3+ Rate
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	19%	10%	29%	1.02	4.65	1.38
Female	39%	32%	71%	1.46	4.23	2.07
All	58%	42%	100%	1.28	4.32	1.81

Table 3 shows the percentages of occupants injured at crash severities less than 26 mph by gender, age and seating position. Drivers accounted for 82.1% of the occupants exposed to the crashes less severe than 26 mph and were 79.3% of the MAIS 3+ injured.

Table 3.

MAIS 3+ Injured Belted Front Seat Occupants in Frontal Crashes, Percentage of MAIS 3+ Injured in Crashes Less than 26 mph by Gender, Age and Seating Position.

	Driver			Right Front Passenger
Gender	MAIS 3+	MAIS 3+	MAIS 3+	MAIS 3+	MAIS 3+	MAIS 3+
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	56%	73%	60%	60%	61%	61%
Female	72%	84%	75%	78%	80%	79%
All	64%	79%	69%	72%	76%	74%

Table 4 shows the distributions of MAIS 3+ injuries by occupant seating position, age and gender for crashes less severe than 26 mph. As shown in Table 3, this injured population includes 69% of the drivers and 74% of the right front passengers. The table also shows the rate of MAIS 3+ injures per 100 belted occupants of the class injured that were exposed to frontal crashes.

Table 4.

Distribution of MAIS 3+ Injuries Less than 26 mph and MAIS 3+ Injuries per 100 Exposed Belted Front Seat Occupants in Frontal Crashes Less than 26 mph by Gender, Age and Seating Position

Drivers
Gender	MAIS 3+	MAIS 3+	MAIS 3+	3+ Rate	3+ Rate	3+ Rate
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	26%	15%	41%	0.67	1.42	0.83
Female	38%	21%	59%	1.15	2.06	1.37
All	65%	35%	100%	0.89	1.73	1.08
Right Front Passengers
Gender	MAIS 3+	MAIS 3+	MAIS 3+	3+ Rate	3+ Rate	3+ Rate
	Age 15–49	Age 50+	Age All	Age 15–49	Age 50+	Age All
Male	15%	9%	23%	0.56	3.00	0.79
Female	42%	34%	77%	1.13	3.22	1.59
All	57%	43%	100%	0.89	3.17	1.29

For this analysis, it was necessary to group occupants into 2 broad age categories (age 15–49 and age 50+) due to the limited data in NASS CDS for occupants over 65. The analysis of FARS and NASS GES do not suffer as much from this issue of small numbers for older occupant demographics.

FATALITY ANALYSIS

An analysis of the Fatality Analysis Reporting System (FARS) data from 1998–2005 was performed in order to understand differences in fatality risk by seating position, age and gender. The analysis applied the matched pair analysis technique to determine differences in fatality rate between groups of occupants in the same vehicle involved in the same crashes. Using this approach, differences in fatality rates can be attributed to factors which differ between the pairs.

As an example, if we consider pairs of occupants in the same vehicle where one occupant was a male and one was a female, we can compare the rate of male deaths to the rate of female deaths to estimate risk ratios. If all other conditions are constant, we can attribute differences in death rate between the two equally sized groups to gender. Using this approach, matched pairs will experience the same crash vector, impacts with identical severities and restraints with similar characteristics (including load limited or pretensioned belt systems).

Within the sample, 35,731 pairs are available for analysis. These are crashes where a driver and a right front passenger were both present during the collision where one or both occupants were fatally injured. Overall, there were 18, 514 driver fatalities and 19,078 right front passenger fatalities present. Safety belt usage, occupant age and gender are known for both occupants of all vehicles in the sample.

Figure 1 below shows the distribution of driver versus passenger fatality for pairs available.

Figure 1.

Counts of Driver and Right Front Passenger Matched Pairs Available for Analysis (FARS 1998–2005).

As mentioned above, the risk of fatality for each category of interest was compared resulting in a risk ratio. The data shown in Table 5 is shown as an example of how calculations are made. The table shows the number of fatalities sustained for driver and passenger pairs where one or more occupants were fatally injured. These pairs include only those where the driver was unbelted and the passenger was belted. All other pairs were discarded for the calculation.

Table 5.

All Unbelted Drivers versus Belted Passenger Pairs (all others excluded).

		Passengers
		Not Fatal	Fatal
Drivers	Not Fatal	131	365
	Fatal	1,222	316

Equation 1 below shows the calculation of the risk ratios. The upper and lower 95% confidence limits for the estimates are calculated based on Equation 2.

$$\begin{array}{l}\text{Risk\hspace{0.17em}}{\text{Ratio}}_{(\text{belted\hspace{0.17em}passenger\hspace{0.17em}}:\hspace{0.17em}\text{unbelted\hspace{0.17em}driver})}=\\ [(365+316)/(131+365+1,222+316)]/\\ [(1,222+316)/(131+365+1,222+316)]\end{array}$$ Equation 1

$$95\%\hspace{0.17em}\text{CL}=\text{exp}[\text{ln}(\text{Risk\hspace{0.17em}Ratio})\pm 1.96\hspace{0.17em}(\text{Standard\hspace{0.17em}Error})]$$ Equation 2

Table 6 below shows the results of the risk ratio calculation for pairs of interest. It should be noted that the populations meeting each condition vary in number and are a subset of the occupants shown in Figure 1. As calculated by Equations 1 and 2, the risk ratio for unbelted drivers compared with belted passengers is 0.44 with a lower and upper confidence limit of 0.35 and 0.56 respectively. This suggests that the risk of fatality during frontal crashes (11, 12 and 1 o’clock PDOF) is 56% percent lower for passengers who are belted compared to drivers who are not belted. Similarly, we find that passengers are 11% more likely to die during frontal crashes compared to drivers regardless of restraint use, age or gender. Belted passengers show an increased risk of fatality compared to belted drivers however this observation is very close to being non-statistically significant. All estimates shown in bold are considered statistically significant and those estimates not shown in bold are not significant.

Table 6.

Fatality Risk Ratios for Occupants Pairs Involved in Frontal Crashes (FARS 1998–2005)

	Risk Ratios (all MY’s)
Belted vs. Unbelted	0.44 (0.35, 0.56)
Passengers vs. Drivers	1.11 (1.03, 1.19)
Belted Passengers vs Belted Drivers	1.10 (1.00,1.21)
Unbelted Passengers vs. Unbelted Drivers	1.08 (0.95,1.22)
Young Belted Passengers vs. Young Belted Drivers	0.93 (0.80,1.08)
Old Passengers vs. Old Drivers, All Belted	1.42 (1.18,1.71)
Young vs. Old, All Belted	2.47 1.79, 3.41)
Female Passengers vs. Male Drivers, All Belted	1.17 (0.99,1.37)
Old Female Passengers vs. Old Male Drivers, All Belted	1.19 (1.10,1.28)
Young Female Passengers vs. Young Male Drivers, All Belted	1.15 (0.88,1.49)

Other noteworthy observations include the fact that there is no significant difference in fatality risk for unbelted passengers versus unbelted drivers. No significant difference in belted occupant fatality risk is observed for the young (drivers and passengers less than 65 years old). However, when both drivers and right front passengers are elderly and belted, passenger fatality risk is 42% higher when compared to drivers. This may be due to the fact that more frail occupants who are often women tend to ride in the passenger seat rather than the driver seat.

GENERAL ESTIMATES (GES) SEAT OCCUPANCY

Figures 2 and 3 show the distribution of occupants by seating position based on NASS GES data. This data was reviewed to understand frequency of front seat occupancy by age and gender. The subpopulation reviewed includes police reported, crash involved vehicles where the driver and passenger seat were occupied. The analysis considers both injured and non-injured occupants. There were 57,726 unweighted pairs of occupants included representing 5,837,000 drivers in crashes from 2003–2005. Figure 2 shows the percent of crashes where a female driver or male driver was accompanied by a female passenger or male passenger. This data suggests that overall, female passengers occupied the passenger seat during 56% of the crash events.

Figure 2.

All Occupants by Seating Position (GES 2003–2005).

Figure 3.

Elderly Occupants by Seating Position (GES 2003–2005).

Figure 3 indicates that, when both front seat occupants were 65 and older, a female occupied the right front passenger seat 73% of the time. Both driver and passenger were elderly males in 10% of these cases.

DISCUSSION

The analysis of GES (Figures 2 and 3) indicates that, when all ages are considered, 56% of the right front passengers are female. In NASS CDS, 52% of the right front passengers in tow-away crashes are female. However, when examined by age, the female occupancy rate increases dramatically for older occupants. GES (Figure 3) shows that when both front seat occupants were 65 and older, a female occupied the right front passenger seat 73% of the time. This result is consistent with NASS data that shows that among right front passengers age 65 and older, 81% are female. For age 50 and older, the percentage of female passengers in NASS remains about the same.

Our examination of individual injury cases in NASS suggests that in many cases when an elderly female was driving, the elder male passenger was older and possibly more vulnerable to injury. This result is illustrated by the data in Table 4. The injury rate for older (age 50+) male right front passengers is more 2 times as high as for male drivers of the same age group (3.0 vs. 1.42). In comparison, the ratio for females is 1.56 (3.22 vs. 2.06). One reason for the higher rate for older males may be that they tend to occupy the passenger seat when they are less agile than the female driver.

This combined data suggests that in 80% to 90% of the cases when the right front passenger seat is occupied by an elderly occupant, that occupant is either female or a male who is older or more vulnerable to injury than the driver.

Paired comparison analysis of FARS for belted drivers and right front passengers showed that when all ages of occupants were considered, the passenger position had a slightly higher risk of being fatally injured. There was no difference in the risk when both occupants were young. However, when both occupants were 65 and older, the odds ratio for the passenger was much higher – 1.42. This result further supports the hypothesis that when elderly occupants occupy both front seats, the most vulnerable to injury is the passenger, not the driver.

It is interesting to examine the injuries and fatalities that occur to belted elderly occupants in the lower severity crashes. An earlier study found that 58% of the frontal crash fatalities among belted front seat occupants aged 65+ occurred in crashes less severe than 25 mph [Augenstein, 2006]. The earlier paper showed that, for an elderly occupant, sustaining an MAIS 3+ injury \carries a much higher risk of death compared with the same injury in a younger occupant. The paper suggested that reductions of chest injuries in lower severity frontal crashes offered a large opportunity for improvement. The opportunity is further examined by seating position and gender in Table 3. The Table shows that for the population under consideration, 69% of all MAIS 3+ injured drivers and 74% of all MAIS 3+ injured passengers are in crashes less severe than 26 mph. For females of all ages and occupant locations, over 75% of the MAIS 3+ injured are in the lower severity crashes.

Table 4 shows the injury distributions and injury rates for belted front seat occupants in lower severity frontal crashes. For younger occupants, the injury rate for females is about 2 times that of males (0.56 vs. 1.13 for the right front passenger). This difference suggests an opportunity exists for reducing injuries to females of all ages in lower severity crashes. This observation reinforces need for gender and age appropriate safety systems for both the driver and right front passenger.

When both genders are combined, the injury rates for the driver and right front passenger shown in Table 4 are the same (0.89). However, for the older occupants, there is a large difference (1.73 vs. 3.17). It is also interesting to note that the right front occupant is rarely an elderly male (4% occupancy and 9% of the MAIS 3+ injured). The relatively high injury rate for both the male and female right front occupants suggests the need for the restraint systems for the right front passenger to be more benign than for the driver.

The FARS and GES databases do not provide information on the height and weight of right front passengers. Consequently, these variables were not included in the analysis. However, our review of NASS/CDS cases with MAIS 3+ injured elderly right front passengers in low severity crashes suggests an overrepresentation of obese and overweight occupants. This population may require restraints that are tuned for both occupant weight and injury tolerance.

These results suggest that an opportunity exists for providing age, weight and gender appropriate restraint systems for the right front passenger. These restraint systems could be more focused on reducing the forces on the body in low severity crashes while maintaining the current level of safety in high severity crashes.

CONCLUSION

Paired comparisons of fatal frontal crashes show that for all ages and genders, belted passengers carried a 10% higher fatality risk than belted drivers. There was no significant difference in the fatal injury risk of belted young drivers and young right front passengers in frontal crashes. When both driver and right front passenger were elderly, the passenger risk was 42% higher than for the driver. The difference in fatality risk is principally due to the presence of vulnerable elderly occupants in the passenger location.

Among belted adult front seat passengers with MAIS 3+ injuries who were exposed to frontal crashes, 74% were in crashes less severe than 26 mph. Of these injured occupants, 42% were young females and 43% were males and females age 55 and above. Both male and female elderly right front passengers had much higher injury rates than the equivalent groups as drivers. Restraint systems in the right front passenger position that could be tailored to reduce injuries in lower severity crashes would be beneficial to this entire population. Such systems would be particularly beneficial to the 43% that are elderly and the 42% that are younger females and who have much higher injury rates than young males. The benign systems could include higher technology safety belts - air belts, four point belts, age and weight appropriate force limiting belts, etc.