Residual Injuries After Recent Safety Improvements

Abstract

This study examines the residual injuries reported in NASS/CDS 1997–2004 by crash mode, crash severity, body region and occupant age. It examines how serious injuries are distributed in present day crashes and identifies opportunities for further injury reduction.

In planar crashes, approximately 66% of the MAIS 3+ injuries occur in crashes less severe than 25 mph delta-V. Chest injuries predominate in these crashes, particularly among elderly occupants. A reduction in chest injuries to belted elderly occupants during low severity frontal crashes offers a prime opportunity for further improvement of safety systems. Younger occupants could also benefit from improved chest protection.

AN EARLIER STUDIES BY THE AUTHORS found that the average age of occupants with MAIS 3+ injuries decreased with increasing crash severity (Augenstein, 2005). This study investigates the frequency of serious injuries and fatalities by occupant age, crash type, crash severity and body region. The purpose is to identify the largest opportunities for further injury reduction.

METHODS

A descriptive analysis of NASS CDS 1997–2004 was performed to determine the injury distribution and injury rates for belted and unbelted occupants as a function of crash mode, crash severity, body region and occupant age. Weighted NASS/CDS data was used for this study. It was recognized that NASS/CDS contains a very sparse sample of crashes with low severity. Consequently, large groupings of crash severity were necessary. In most cases crashes were grouped by severities above and below 25 mph. The fatality sample in NASS is less robust than the MAIS 3+ sample. As a consequence, fatality analysis was limited to crash modes with more than 5% of the fatalities.

The combined 1997–2004 years of NASS/CDS contain data on 65,000 crash exposed front seat adult occupants with 9,700 injured at the MAIS 3 or greater level. The MAIS 3+ population included any occupant who is fatally injured regardless of the AIS severity of their injuries. After applying the NASS/CDS weighting factors, the sample expands to represent 31,141,000 crash exposed occupants with 814,400 MAIS 3+ injuries. The analysis presented below focuses primarily on planar crashes. When single event rollover crashes where no planar impact occurs are removed, the MAIS3+ injured occupant population becomes 8,274 and 677,700 following weighting.

Recent safety standards for frontal air bags and improved side protection were incorporated in the US light vehicle fleet by model year 1998. The residual injuries in the fleet of vehicles, model year 1998 and later were examined in NASS 1997–2004 to determine opportunities for further improvements. The distribution MAIS 3+ head, chest/abdominal and lower extremity injuries for three age groups of belted occupants was evaluated for frontal and side impacts. All MAIS 3+ injuries to the specified body regions were included in the analysis. The total numbers of weighted injuries were: head - 8,645; chest/abdomen - 22,549; and lower extremity – 16,386.

In order to understand the potential effect of missing delta-V’s on analysis results, damage extent was compared for cases with and without delta-V information. The extent of damage describes vehicle zones that are damaged due to crash. They are coded by crash investigators using the Collision Deformation Classification System (CDC) for damage by direction of impact (i.e. front, side and rear). At a deltaV of 25 MPH, the average extent of damage was approximately 2.7. Approximately 65% of the MAIS3+ injured population was involved in crashes at or below 25 MPH. For the population of occupants with unknown deltaV’s, the extent of damage for 65% of this population is approximately 3.6. This implies that cases with missing deltaV’s result in slightly more severe damage to the vehicle. This difference my result in conservative estimates of the populations injured at each crash severity.

RESULTS

The distribution of exposed occupants and MAIS 3+ injuries by crash mode is shown in Table 1. The table also shows the rate of MAIS 3+ injuries per 100 population exposed to each crash mode, and the fatality percentage for the MAIS 3+ injured populations by crash modes. The population includes both belted and unbelted occupants. The reported belt use rate for the entire population was 87%. For the population with MAIS 3+ injuries and known delta-V, 56% were belted.

Table 1.

Overall Distribution of Front Seat Occupant Exposure, MAIS 3+ Injuries, Injury Rate and Fatality Content of the MAIS 3+ Injuries

Crash Mode	Exposed	MAIS 3+	MAIS 3+/100	Fatality/MAIS 3+
Frontal	65%	56%	2.7	0.23
Nearside	8%	14%	5.2	0.29
Farside	8%	7%	2.8	0.30
Rear	9%	2%	0.7	0.32
Roll	10%	21%	6.8	0.34
Total/Average	100%	100%	3.1	0.27

The most commonly used metric for measuring crash severity is delta-V. Delta-V is the velocity change that the vehicle was subjected to during the crash. It is based on a calculation of the energy absorbed by the deformed structure of each vehicle and the mass of each vehicle. About 60% of the primarily planar cases have delta-V as a known variable. This large population of missing delta-V’s exists because the accurate investigation and calculation of this parameter is frequently not possible due to the complexities of some crashes.

The data with known delta-V shows that for crashes more severe than about 10 mph, different crash modes have different injury risks for the same delta-V. The injury risk versus crash severity in mph by crash mode is shown in Figure 1. It may be noted that rollover is missing from Figure 1. This omission is due to the different characteristics of rollovers. About one third of the MAIS 3+ injuries in rollovers do not involve a damage producing crash prior to the rollover. Consequently, there is no basis for calculating a delta-V. For the crashes with impacts prior to rollover, a delta-V for the pre-roll impact can be calculated. However, other severity measures associated with the rollover may have more influence on the rollover severity that the calculated delta-V [Digges, 2005].

Figure 1.

MAIS 3+ Injury Rate for Adult Front Seat Occupants versus Crash Severity by Crash Mode

The distribution of MAIS 3+ injuries and fatalities in crashes with known delta-V is shown by crash severity and crash mode in Table 2. The Table also displays the fatality content of each cell. For example for the frontal crash mode and 36+ mph MAIS 3+ injury cell, 0.30 of the 9% in that cell were fatally injured. The percentages of MAIS 3+ for the belted and the unbelted populations each add to 100%. The row labeled ‘% of All’ shows the distribution of MAIS 3+ and fatalities by crash mode. For the belted front seat population, 68% of all the MAIS 3+ injuries and 50% of the fatalities occur in the frontal crash mode.

Table 2.

Distribution of MAIS 3+ Injuries, Fatalities and Fatality Content of MAIS 3+ Injury Cells for Front Seat Adult Occupants by Crash Mode and Crash Severity

	Belted					Unbelted
Delta-V Mph	MAIS 3+ Injuries for Each Crash Mode
	Front	Near	Far	Rear	Total	Front	Near	Far	Rear	Total
0–15	17%	6%	1%	0.6%	25%	24%	2%	5%	0.2%	31%
16–25	29%	11%	2%	0.2%	43%	22%	6%	3%	0.8%	32%
25–35	12%	6%	1%	0.6%	20%	19%	2%	1%	0.1%	23%
36+	9%	1%	1%	0.7%	12%	11%	1%	1%	0.2%	14%
% of All	68%	24%	6%	2%	100%	76%	12%	11%	1%	100%
Delta-V	Fatalities for Each Crash Mode
0–15	3%	1%	0.1%	1.0%	6%	8%	2%	1%	0.5%	11%
16–25	16%	15%	3%	0.0%	33%	27%	6%	4%	0.5%	37%
25–35	12%	12%	5%	0.4%	29%	14%	5%	4%	0.3%	23%
36+	18%	6%	5%	3.3%	32%	23%	3%	2%	0.5%	29%
% of All	50%	33%	12%	5%	100%	71%	16%	11%	2%	100%
Delta-V	Fatalities/MAIS 3+ for Each Crash Mode
0–15	0.03	0.03	0.01		0.03	0.07	0.22	0.02		0.08
16–25	0.08	0.19	0.18		0.11	0.27	0.22	0.37		0.26
25–35	0.14	0.28	0.47		0.20	0.16	0.50	0.63		0.22
36+	0.30	0.77	0.61		0.40	0.47				0.46
Average	0.11	0.20	0.28	0.32	0.14	0.21	0.31	0.22	0.30	0.22

The belted and unbelted populations were separated in this table in order to examine differences. It should be noted that the unbelted population has been decreasing within the tow-away crash population as belt use increases. For the NASS/CDS years examined, the belt use for the MAIS 3+ injured was 54.7%. Below 25 mph the belt use for the MAIS 3+ injured was 56.3%. At the higher speeds the belt use was 51.4%.

Earlier studies have shown that occupant age is an important factor that influences the risk of injury in a crash [Augenstein 2003]. Table 3 examines the distribution of MAIS 3+ injuries by occupant age and crash severity for belted and unbelted adult occupants. The column labeled ‘MAIS 3+ by Delta-V’ provides the percentage of all MAIS 3+ injuries represented by the combined belted and unbelted populations. For each severity group, the belted and unbelted MAIS 3+ distributions each add to 100%. The belt use rate was as listed in the above paragraph. Cases with manual belt use reported as unknown were removed at this stage of analysis.

Table 3.

Distribution of MAIS 3+ by Crash Severity and Occupant Age for Belted and Unbelted

Delta-V Ranges	MAIS 3+ by delta-V	MAIS 3+ by Occupant Age
		Belted			Unbelted
		16–35	35–65	65+	16–35	35–65	65+
0–15 mph	30%	42%	33%	25%	46%	28%	26%
0–25 mph	66%	42%	37%	22%	41%	39%	20%
25+ mph	34%	47%	40%	13%	48%	37%	14%
All deltaVs	100%	43%	38%	19%	44%	38%	18%

Table 4 shows the fatality content of the MAIS 3+ injury cells by three age groups by crash direction. Cells with less than 1% of the fatalities were considered to contain insufficient data and were omitted from the table. Tables 5 and 6 show the MAIS 3+ injury distribution and injury rates by crash mode for three occupant age groups in crashes above and below 25 mph delta-V. In these tables, the injury rate is shown for any cell with more than 1% of the MAIS 3+ injuries. Cells with less than 1% were considered to contain insufficient data. Only belted occupants are included. Table 7 shows the distribution of fatalities by age groups for belted front seat occupants in frontal and nearside crashes.

Table 4.

Distribution of Fatalities per MAIS 3+ Injured Occupant by Crash Mode for all DeltaV’s (Belted and Unbelted Occupants)

Crash Mode	Belted			Unbelted
	16–35 Yrs	35–65 Yrs	65+ Yrs	16–35 Yrs	35–65 Yrs	65+ Yrs
Frontal	0.06	0.10	0.19	0.17	0.19	0.23
Nearside	0.16	0.18	0.33	0.21	0.29	0.36
Farside	0.19	0.29		0.17	0.49
All	0.10	0.13	0.23	0.18	0.22	0.28

Table 5.

Distribution of MAIS 3+ and Injury Rate by Crash Mode and Occupant Age, Delta-V Planar Crashes Less Than 25 Mph – Belted

Crash Mode	MAIS 3+ Distribution			MAIS 3+ Injuries/100 Exposed
	16–35 Yrs	35–65 Yrs	65+ Yrs	16–35 Yrs	35–65 Yrs	65+ Yrs
Frontal	19%	16%	11%	0.9	1.3	2.6
Nearside	7%	6%	4%	1.8	3.7	8.5
Farside	2%	1%	1%	0.9	0.5	2.6
Rear	0.3%	0.4%	0.2%
All	27%	24%	17%	0.9	1.3	3.1

Table 6.

Distribution of MAIS 3+ and Injury Rate by Crash Mode and Occupant Age, Delta-V Planar Crashes Greater Than 25 Mph – Belted

Crash Mode	MAIS 3+ Distribution			MAIS 3+ Injuries/100 Exposed
	16–35 Yrs	35–65 Yrs	65+ Yrs	16–35 Yrs	35–65 Yrs	65+ Yrs
Frontal	10%	9%	2%	14.5	21.3	18.5
Nearside	3%	2%	0.5%	49.3	65.1
Farside	1%	2%	0.2%	11.1	26.4
Rear	1%	0.7%	0.3%	2.4
All	16%	13%	3%	14.7	24.2	21.0

Table 7.

Distribution of Fatalities by Crash Mode and Occupant Age, Delta-V Planar Crashes Less than and Greater Than 25 Mph – Belted

Crash Mode	Less than 25 mph				Greater than 25 mph
	16–35	35–65	65+	All	16–35	35–65	65+	All
Frontal	6%	7%	11%	24%	10%	16%	8%	34%
Nearside	3%	7%	10%	20%	9%	7%	6%	22%
Total	9%	14%	21%	44%	19%	23%	14%	56%

Tables 8 and 9 show the distribution of MAIS 3+ injured by body region and occupant age groups for belted occupants in frontal and nearside crashes. This data is for vehicle model year 1998 and later. Table 8 contains data for crashes less than 25 Mph. These lower severity crashes cause 62% of the MAIS 3+ injuries. Table 9 contains data for the 38% of the MAIS 3+ injuries above 25 mph. Table 10 shows the overall distribution of MAIS 3+ injuries by age groups.

Table 8.

Distribution of MAIS 3+ by Body Region and Occupant Age, Belted Front Occupants in Frontal and Nearside Crashes Less Than 25 Mph in Vehicles Model Year 1998 and Later

Body Region	Frontal				Nearside
	16–36	35–65	65+	All	16–36	35–65	65+	All
Chest/Abd	26%	61%	56%	49%	49%	50%	55%	51%
Head	39%	6%	22%	21%	11%	9%	19%	12%
Lower Ex	35%	33%	22%	31%	40%	40%	26%	36%

Table 9.

Distribution of MAIS 3+ by Body Region and Occupant Age, Belted Front Occupants in Frontal and Nearside Crashes Greater Than 25 Mph in Vehicles Model Year 1998 and Later

Body Region	Frontal				Nearside
	16–36	35–65	65+	All	16–36	35–65	65+	All
Chest/Abd	33%	34%	54%	37%	38%	77%	70%	57%
Head	8%	12%	5%	9%	39%	14%	22%	27%
Lower Ex	58%	54%	41%	54%	23%	9%	8%	16%

Table 10.

Distribution of MAIS 3+ for Belted Front Occupants in Frontal and Nearside Crashes Greater Than and Less Than 25 Mph in Vehicles Model Year 1998 and Later

Crash Severity	Frontal and Nearside
	16–36 Yrs	35–65 Yrs	65+ Yrs	All
LT 25 mph	19%	26%	16%	62%
GT 25 mph	16%	16%	6%	38%
All	35%	43%	22%

DISCUSSION

The distribution of MAIS 3+ injuries and the associated injury rates are shown by crash mode in Table 1. Both belted and unbelted adult front seat occupants are included. The belt use rate for the MAIS 3+ injured was 56%. The belt use for the entire exposed population was reported to be 87%. However, some overstatement of belt use is likely. The table also shows that the injury rate for near-side crashes and rollovers is much higher than for the other crash modes and these also carry a high percentage of fatalities. On the average, 27% of the MAIS 3+ injuries result in fatalities.

The relationship between crash severity (delta-V) and injury risk for planar crashes is shown in Figure 1. The rollover crash mode can not be adequately represented by delta-V and it has been excluded from consideration in all subsequent tables and discussion. The chart shows that frontal and far-side crashes have generally similar risks by delta-V. Frontal crashes carry a slightly lower risk than the average and far-side carries a slightly higher risk. Near-side carries a much higher risk and rear carries a much lower risk.

Table 2 shows the distribution of MAIS 3+ injuries and fatalities by crash mode and crash severity for belted and unbelted occupants. The ratio of fatalities to MAIS 3+ injuries for each cell is also shown. This ratio illustrates the need to examine the fatality content of the cells with MAIS 3+ injuries. It may be noted in Table 2 that some cells of MAIS 3+ injuries contain more than 50% fatalities while others contain less than 2%.

In recent years, safety regulatory priorities have focused on frontal crash tests at severities above 25 mph and near-side crash tests in the 16 to 25 mph delta-V range. For belted occupants approximately 12% of the MAIS 3+ injuries and fatalities are in the frontal crash mode at 25 to 35 mph. There is an even larger population of frontal crash injuries and fatalities in the 16–25 mph severity range. These are 29% and 16% respectively. This suggests continued opportunities for safety improvements in lower severity frontal crashes.

For belted occupants in nearside impacts, 11% of the MAIS 3+ injuries and 15% of the fatalities occur in the 16–25 mph delta-V range. As shown in Figure 1, the near-side crashes carry higher injury risks than frontal crashes for delta-V ranges above 10 mph. The 16–25 mph range continues to offer a large opportunity for safety improvement.

Far-side crashes contribute about 6% of the MAIS 3+ injuries but 12% of the fatalities among belted front seat occupants. Unbelted occupants in far-side crashes represent about 11% of unbelted MAIS 3+ injuries. To date little regulatory attention has been devoted to this crash mode. Opportunities for improvement exist to address 8% to 10% of the fatalities that occur at the lower crash severities.

Table 3 examines the relationship of occupant age to the distribution of MAIS 3+ injuries by crash severity ranges. The belted and unbelted populations are treated separately and the injury percentage for each group adds to 100%. The table shows that the oldest population has a decreasing share of the MAIS 3+ injuries as the crash severity increases. Further, serious injuries (regardless of impact severity) more often result in fatality for the elderly. As shown in Table 4, 10% of young belted MAIS3+ injured occupants die due to their injuries. For those 65 years old and older, 23% with MAIS3+ injuries die. Safety improvement in low severity crashes could be particularly beneficial to seniors.

Table 5 provides insights into the MAIS 3+ injury distribution by crash mode and age group for belted occupants. It includes crashes less severe than 25 mph. In frontal and nearside crashes, the highest injury rates are for the 35–65 and the 65+ age groups. These two age groups and crash modes contribute about 37% of all the MAIS 3+ injuries. Elderly occupants exposed to low severity frontal and nearside impacts constitute about 15% of the MAIS 3+ injuries and carry a high injury rate.

Table 6 shows the distribution of the 32% of the MAIS 3+ injury that occur to belted occupants in crashes with delta-V greater than 25 mph. The largest opportunity for injury reduction is for the younger occupants. Older occupants in the more severe crashes comprise only 3% of the MAIS 3+ injuries.

Table 7 contains data on the distribution of fatalities in frontal and nearside crashes. For the younger age groups there are more fatalities above 25 mph than below 25 mph. However, for the 65+ group, there are more fatalities below 25 mph than above. The largest fatality cells below 25 mph are the 65+ group in frontal and nearside crashes. Each cell contains about 10% of the fatalities. This area is offers a large opportunity for improving safety.

Tables 8 and 9 address the distribution of MAIS 3+ injuries among belted occupants in vehicles model year 1998 and later. In each column, the injuries for the three body regions add to 100%. The tables show that for older adults, the chest stands out as the major body region injured. In contrast, the younger populations are more prone to lower extremity injuries. In lower severity frontal crashes, chest/abdominal and abdominal injuries to the older belted population offer the largest opportunity for reduction. For nearside crashes less than 25 mph, chest/abdominal injuries offer the largest injury reduction opportunity.

CONCLUSIONS

In planar crashes, approximately 66% of the MAIS 3+ injuries occur in crashes less severe than 25 mph delta-V. For occupants 65 and older, 75% of the MAIS 3+ injuries are in these lower severity crashes. In the lower speed crashes, the highest numbers of belted occupant fatalities are sustained by people 65+ years old in frontal crashes. Approximately 20% of the belted front seat occupants with fatal injuries in frontal and nearside crashes are 65 and older and in crashes less severe than 25 mph.

For belted occupants in crashes more severe than 25 mph, the largest injured populations are the age groups 15 to 35 in frontal crashes. This group accounts for 16% of the serious injuries received by belted occupants. However, one third of these injuries are in crashes of severity greater than 35 mph. The population 65+ years old represented a much smaller percentage – about 3%.

In passenger vehicles model year 1998 and later, occupants 65+ in low severity frontal and nearside crashes account for 16% of the MAIS 3+ injuries to belted occupants. Chest injuries predominate among this group. In the lower speed frontal crashes, 56% of their MAIS 3+ injuries are to the chest/abdomen. In nearside crashes, the fraction is 55%. These injuries carry a much higher risk of death than similar injuries to younger people.

The 65+ group is likely to increase as the population ages. A reduction in chest injuries to elderly belted occupants in low severity frontal and nearside crashes is a prime opportunity for further improvement of safety systems. Improvements for the elderly could be expected to apply to the younger populations that suffer 45% of all MAIS 3+ injuries in these lower severity crashes.