Investigation of the Performance of Safety Systems for Protection of the Elderly

Abstract

This study investigates injury occurrence for belted occupants as a function of age. An analysis of NASS/CDS 1997–2003 data was conducted to determine crash involvement rates and injury rates for front seat occupants versus mean occupant age. In frontal and near-side crashes, the average age of MAIS 3+ belted front seat occupants injured in crashes less severe than 15 mph is of the order of 50 years. The average age of the population exposed to crashes less severe than 15 mph is under 40 years old. The crash exposure and frequency if injuries to the elderly were both found to be the highest in low severity crashes. The chest is the most frequent body region injured for the elderly. These findings suggest the need for more benign safety systems to protect the elderly in low severity crashes. Design of safety systems for the elderly should give priority to reducing the chest loading in low severity frontal and near-side crashes.

EARLIER RESEARCH HAS SHOWN a dramatic reduction in injury tolerance as people age. Evans [1991] found that the fatality risk in fatal crashes was 3 times greater at age 70 than at age 20. Zhou [1996] examined the effect of age on injury tolerance to thorax loading by restraint systems and recommended that the belt loading be reduced by a factor of 0.28 for the 66 to 85 year old group as compared to the 16 to 35 group. For frontal blunt loading, the comparative injury tolerance reduction was 0.79 and for side impact loading the reduction was 0.73.. This large difference in injury tolerance poses a dilemma for designers of safety systems. The questions are: should higher chest loading be retained to protect the younger population in the higher severity crashes or should the restraints be softened to better protect the older population at the expense of less efficient energy absorption for the young. In this study, we examine the opportunities for improving the crash protection of the elderly. The NASS/CDS was used to provide insight into the exposure and injuries by age and crash severity.

METHODS

This analysis investigates the average age of occupants who are involved in both injury and non-injury crashes. Relative differences in their crash involvement rates and frequency of injury are examined versus the direction of impact and crash severity. The work of Zhou [1996] examined the change in chest injury tolerance with age and suggested the following age groupings: 15–35; 36–65; 65–85. This paper uses similar groupings for its analysis to be consistent with previous published research. However, to increase the populations, the two younger groupings were combined for some comparisons.

DATA SOURCES

Data from the National Automotive Sampling System- Crashworthiness Data System (NASS/CDS) was used for this analysis. NASS/CDS case data has been collected since 1988 by the National Center for Statistics and Analysis and is a sample of tow-away crashes that occur within the US. The data is used to monitor the effectiveness of traffic safety programs and to provide a resource to understand the relationship between the type and seriousness of crashes and their associated injuries. To qualify for inclusion, the crash must have a police report, be reported to the state, involve a “harmful event” (defined as property damage, personal injury, or both) and occur as a result of a non-stable situation deemed accidental (non-intentional, non-disease related or not due to a natural disaster).

Each investigated crash must involve a motor vehicle in transport on a public roadway and must involve at least one towed vehicle. At each sampling site the research team investigates a subset of police reported crashes. One of 24 teams of crash researchers throughout the country investigates the each crash and collects all relevant data. For this investigation, detailed review of police accident reports, hospital records, out-of-hospital care records, photographs of the vehicles, and the vehicles themselves are conducted. With the sampling process, the data are weighted to represent the nationwide incidence of crashes and resulting injuries. Based on the probability of sampling, a weighting factor is assigned to each case so that its characteristics may be projected to the total population.

CLASSIFICATIONS OF STUDY CASES

Front seat, belted occupants were the focus of this examination. These included front left and front right positions (drivers and right front passengers). Any occupant wearing a manual three point belt or an automatic shoulder belt with lap belt used was considered belted and included within the study. This population included occupants with and without available air bags. Crash mode was categorized using Collision Deformation Classification (CDC) data collected by NASS/CDS investigators. Each mode is categorized as follows:

Frontal: (PDOF≥11 and PDOF≤1, Any Seating Position) or (PDOF=10 or 2 where General Area of Damage is Front)

Nearside: (PDOF≥2 and PDOF≤4, Right Seating Position, General Area of Damage is Right) or (PDOF≥8 and PDOF≤10 and, Left Seating Position, General Area of Damage is Left)

These crash categories were published and applied by NHTSA during the Final Economic assessment of the FMVSS Advanced Airbag Final Rule [1].

Crashes from NASS/CDS 1997–2003 were analyzed to identify the mean age of occupants involved in frontal and nearside impacts by crash severity or deltaV. These mean age values were calculated per 1 mph increment. In order to determine if a general trend existed within these derived means, an 11 point moving average was applied to each resulting dataset. In other words, the mean age for occupants shown in Figures 1–4 below at 10 MPH are an average of age for occupants involved in crashes from 5 mph to 15 mph.

Figure 1.

Frontal crash involvement by age group versus 5 mph deltaV increment (NASS/CDS 1997–2003 data)

Figure 4.

MAIS3+ injury populations for Nearside Crashes by age group versus 5 mph deltaV increment (NASS/CDS 1997–2003 data)

Mean ages were calculated for occupants who sustained AIS3 and higher injuries as well as the total crash involved population. The AIS3 and higher injured (or MAIS3+) are occupants who sustained at least one injury with an AIS severity of 3 or higher. This categorization includes AIS3, 4, 5 and 6 as well as fatally injured occupants who died within 30 days of a crash due to crash related injuries.

EXPOSURE AND INJURIES BY AGE AND CRASH SEVERITY

The population of front seat belted occupants were separated into three sub-groups, based on age. The age groups were 16–35; 36–65; 66–85. The distributions of occupant exposure and MAIS 3+ injuries are shown in Figure 1 and 2 for frontal crashes. Similar plots are shown for near-side crashes in Figures 3 and 4.

Figure 2.

MAIS3+ injury populations for Frontal Crashes by age group versus 5 mph deltaV increment (NASS/CDS 1997–2003 data)

Figure 3.

Nearside crash involvement by age group versus 5 mph deltaV increment (NASS/CDS 1997–2003 data)

Figures 5 and 6 show the average age of the exposed populations in frontal and near-side crashes as a function of crash severity, respectively. These Figures also show the average age of the MAIS 3+ injured.

Figure 5.

Mean occupant age for MAIS3+ injured and non-injured by deltaV for frontal crashes (NASS/CDS 1997–2003 data)

Figure 6.

Mean occupant age for MAIS3+ injured and non-injured by deltaV for nearside crashes (NASS/CDS 1997–2003 data)

Figures 5 and 6 were created based on NASS/CDS crash involved occupant age data from 1997–2003. Crash types considered are frontal and nearside crashes as defined above. An 11 point moving average was used to identify overall age trends for MAIS3+ and non-MAIS3+ injured occupants. Each plotted value is based on the weighted mean ages of occupants involved in crashes 5 mph lower and 5 mph higher than the point plotted. This method filters out any excessive point to point variations to isolate the overall age trend for these crashes.

Figure 7 shows the risk of MAIS 3+ injury as a function of crash severity for belted occupants in frontal crashes. The difference between occupants over 65 and all other adult occupants is shown. This data is derived from a logistic regression model where the risk of MAIS3+ injury is predicted based on age category (i.e. 16–65 or 65+) controlling for crash severity using deltaV in the vehicles longitudinal direction. This model was created using SAS Callable SUDAAN which takes into account the stratified sampling process used for the NASS/CDS dataset. Also, SUDAAN allows for the correct calculation of variances based on the weighted NASS/CDS data.

Figure 7.

Injury Risk vs. Crash Severity for 65+ and Less than 65 YO Populations of Belted Occupants in Frontal Crashes

INJURIES BY BODY REGION AND AGE

Table 1 shows a comparison of the injured body regions for young (15–35) and elderly (65+) belted front seat occupants in frontal and near-side crashes. For people with more than one body region injured at the highest AIS, the head was selected as the most critical, followed by the chest and abdomen. Face and neck injuries were included in the head injury category.

Table 1.

Percent of MAIS 3+ Injured by Body Region and Age for Frontal and Near-side Crashes (NASS/CDS 1997–2003, Belted Front Seat Occupants)

	Frontal 15–35	Frontal 65+	Near-Side 15–35	Near-Side 65+
Head	20%	10%	23%	33%
Chest	30%	40%	35%	48%
Abdomen	6%	17%	6%	4%
Lower Limb	27%	18%	15%	14%
Upper Limb	17%	15%	21%	1%

DISCUSSION

The chest is the most frequently injured body region for elderly occupants in frontal and near-side crashes.

For frontal crashes older occupants tend to be involved in proportionally more low speed crashes. This is apparent in the 5–15 MPH crash severity range as shown in Figure 1. Approximately 37% of the MAIS 3+ injuries to elderly front seat occupants occur at these low crash severities, as shown in Figure 2.

Figures 5 and 7 provide further insights into the exposure to injury of the elderly. The average age of the MAIS 3+ injured is around 50 while the average age of the exposed population is under 40. . At crash severities below 25 mph, the risk of MAIS 3+ injury for the over 65 occupants is about 2.5 times that of the younger population. Figure 5 shows that as the crash severity increases, the injured become younger in average age and approach the age of the crash exposed. This relationship suggests the desirability of reducing the restraint forces as the crash severity is reduced. Such a strategy would provide increased protection for the elderly who are more likely to be in these low severity crashes.

The side impact exposure of the elderly parallels that of the frontal impact. Figure 2 and 4 show an increased exposure of the elderly to crashes less severe than 15 mph and 83% of MAIS 3+ injuries to the elderly in this speed range. Figure 6 shows that the mean age for the 65+ population with MAIS 3+ injuries is around 55, compared with 47 for exposed population. This increased exposure and injury frequency at the lower crash severities suggests the need for special attention to the lower injury tolerance of this population in the lower speed crash events.

The new standard for advanced air bags includes a moderate severity (40 kph) crash test into a offset deformable barrier with a 5% female dummy driver. The test is intended to reduce the force of the air bag in this crash mode. However, the injury allowables in this test are the same as those proposed for 56 kph rigid barrier, a much more severe crash. A new standard is also being proposed that would increase the number of tests required for side impact protection. None of these tests addresses the lower injury tolerance of the elderly. At present, there appears to be no test that addresses the needs of the elderly – that of reduced loading in low severity crashes.

Possible safety designs for the elderly might include safety belts with lower force limits, harness-type belt systems that distribute the load on the chest more evenly and side impact padding and air bags that permit softer contacts in low severity crashes.

CONCLUSIONS

This study found that older occupants are more often involved in lower severity frontal and near-side impacts than younger occupants. At the same time, their likelihood for injury is significantly higher than their younger counterparts. This trend suggests that occupant protection systems should consider the lower injury tolerances of the elderly in their design and reduce the forces in low severity crashes. Crash test procedures and priorities should consider this higher risk elderly population in low severity crashes as well as the younger population in higher severity crashes. Chest protection with reduced forces in low severity crashes should be a priority in designing safety systems for the elderly.