Occupant-to-occupant contact injury in motor vehicle crashes

ABSTRACT

Objective: This is a descriptive study of the frequency and risk of occupant-to-occupant contact injury by crash type and occupant age. It focused on rear impacts because of a recent Senate inquiry.

Methods: 1994–2013 NASS-CDS data were used to investigate the effects of occupant-to-occupant contact on the risk of serious-to-fatal injury (Abbreviated Injury Scale [AIS] 3+) by crash type and age group. NASS-CDS in-depth cases were analyzed to identify crash circumstances for AIS 3+ occupant-to-occupant contact injury in rear crashes.

Results: Serious injury (AIS 3+) due to occupant-to-occupant contact was uncommon. It represented only 0.84% of all AIS 3+ injury for all age groups. The overall risk of AIS 3+ occupant-to-occupant contact injury was 0.042 ± 0.007%. The highest incidence was in side impacts (69.8%) followed by rollovers (22.9%). Occupant-to-occupant contact injury risk was lower in rear impacts than in other crash types, at 0.0078 ± 0.0054%.

The highest risk of AIS 3+ injury with occupant-to-occupant was for the <9-year-old age group when compared to other age groups for all crash types. The risk was 0.051 ± 0.026%, representing 2.69% of all AIS 3+ injury in the <9-year-old age group. Only 4.2% of AIS 3+ occupant-to-occupant contact injury occurred to children <9 years old in rear impacts. The corresponding injury risk was lowest in rear impacts, at 0.014 ± 0.014%%, when compared to other crash types.

The analysis of in-depth NASS-CDS cases of occupant-to-occupant contact injury in children< 9 years old involved in rear impacts identified very severe collisions in older model vehicles with deformation of the occupant compartment and yielding front seats as main factors for the contact injury. Front seat occupants injuring rear-seated children was not identified in the in-depth NASS-CDS cases.

Conclusions: AIS 3+ occupant-to-occupant contact injury occurs primarily in side impacts and rollovers. Most contact injury is to adults (89.4% incidence). Occupant-to-occupant contact injury to children is rare in rear impacts.

KEYWORDS:

• Occupant contact
• injury source
• motor vehicle crashes

Introduction

Recently, Senators Markey and Blumenthal (2016) raised questions about front seat performance in rear crashes where there may be a risk of occupant-to-occupant contact injury to rear-seated children. This raises 2 questions: (a) what is known about occupant-to-occupant contact injuries in rear impacts and (b) how rear-seated children are severely injured in rear impacts. This study addresses the first question by evaluating NASS-CDS to determine the incidence and risk of injury from occupant-to-occupant contact by crash type and occupant age. It updates earlier studies with more recent NASS-CDS data and focuses on an in-depth review of NASS-CDS cases of occupant-to-occupant contact injury in rear crashes.

There have been a number of published studies regarding occupant-to-occupant contact injury, which are most common in side impacts (Digges et al. 2005; Ichikawa et al. 2002; Newland et al. 2008). Newland et al. (2008) found an increased injury risk of adult (>12-year-old) near-side occupants with adult far-side occupancy. There was an 8% increase in serious (Maximum Abbreviated Injury Scale [MAIS] 3 + F) injury risk of near-side drivers in cases where a lap–shoulder-belted right-front passenger was present and a 30% increase with an unbelted right-front passenger. Newland et al. (2008) also carried out side pole tests with near and far belted dummies. They observed that the near-sided dummy was subjected to bilateral impacts, one side from the intruding structure and the other side from impact with the far-side dummy. Occupant-to-occupant contacts were a source of injury.

Methods

Crash data source

In this study, NASS-CDS was analyzed. It is a stratified, multiphase, unequal selection probability sample of motor vehicle crashes that are prospectively selected for in-depth investigation (http://www.nhtsa.gov/Data/National-Automotive-Sampling-System-(NASS)/NASS-Crashworthiness-Data-System). Most of the vehicles were towed from the scene because of damage. The data include information based on crash investigation teams that gather information from the crash site, vehicle, medical records, police accident reports, and personal interviews. NASS-CDS data for calendar years 1994–2013 were included for all motor vehicle occupants in light vehicles irrespective of seating position, belt use, or ejection status. Only light vehicles with model year 1994+ were included in the study. The data for calendar years 2009–2013 are representative of model year 2000+ vehicles.

Crash types

Impacts were defined as:

•	Frontal impacts: Towaway vehicles with frontal impact location (GAD1 = F) and no rollover (rollover ≤0).
•	Side impacts: Towaway vehicles with side impact location (GAD1 = L or R) and no rollover (rollover ≤ 0).
•	Rear impacts: Towaway vehicles with rear impact location (GAD1 = B) and no rollover (rollover ≤0).
•	Rollovers: Towaway vehicles with a rollover (rollover >0), irrespective of impact location.

Age groups

The age data were divided into children under the age of 9, 9 to 14 years old, and adults 15 years old or older. The data are provided for all occupants, irrespective of age, and for the occupants with known age information.

Injury severity

Injury severity of the occupant was assessed using the MAIS and the “TREATMNT” and “INJSEV” variables. MAIS represents the assessment of life-threatening injuries at the time of first medical evaluation and not long-term consequences. It ranges from MAIS 0 to 6 and 9, where MAIS 9 is an injury with unknown severity. Fatality was also used to determine the injury severity. The following variables were used to define a fatality (F):

•	Treatment: TREATMNT = 1: The occupant was fatally injured and not transported to the hospital.
•	Police injury severity: INJSEV = 4: A fatality according to police ratings.

Injuries were grouped as minor (Abbreviated Injury Scale [AIS] 1) and as serious-to-maximum (AIS 3 to AIS 6 or AIS 3+). The AIS 1 occupant-to-occupant contact injuries are provided to indicate the exposure to occupant-to-occupant contacts. AIS 2 injuries were assessed but not discussed, to provide a complete picture of crash outcomes.

Occupant-to-occupant contact

Occupant-to-occupant injury source was identified using the NASS-CDS-coded INJSOU variable (INJSOU = 46 or 160).

Weighted data

National estimates for the number of occupants and injuries in each category were made using the Ratio Weight (ratwgt) variable in NASS-CDS. All calculations were based on weighted values. Standard errors were determined using the SAS procedure “SURVEYFREQ” accounting for PSU and Ratio Weight factors. SAS software (Version 9.4, SAS Institute Inc., Cary, NC) was used.

Analysis

•	Risk: The risk that an occupant experiences a severe injury was determined by dividing the number of serious injuries (AIS 3+) by the number of occupants with known injury status, MAIS 0–6 or F (MAIS 0 + F). Risks are provided as ±1 SD and were determined for each age group and crash type.
•	Incidence by crash type: The incidence of occupant-to-occupant contact injury was determined by age group and crash type. For each age group, the number of serious injuries (AIS 3+) by occupant-to-occupant contact for a specific crash type was divided by the total number of injuries with a known crash type (front, side, rear, rollover).

NASS-CDS in-depth cases involving occupants seriously injured due to occupant-to-occupant contact in rear crashes were selected for the analysis. The cases were downloaded from NHTSA (www.nhtsa.gov). For each case, the crash circumstances, occupant age, height and weight, injury type and severity, and injury source were summarized and photographs of the vehicle and interior were reviewed.

Results

Table 1 shows the incidence of occupants with known injury severity (MAIS 0+F), AIS 1 injury, and AIS 3+ injury. The incidence is also given for occupant-to-occupant contact injuries. The data are given by crash type and age group. Complete data with unweighted cases and standard errors are provided in Tables A1–A4 (see online supplement). In the 20 years of NASS-CDS data, there were 45,579,040 involved occupants, 40,011,274 were 15+ years old, 1,760,509 were 9–14 years old, and 2,979,900 were <9 years old. The majority (95.7%) of AIS 3+ injuries were to adults and occurred in frontal impacts, side impacts, and rollovers.

Table 1. Incidence of AIS 1 and AIS 3+ injuries with subgroups of occupant-to-occupant contact (NASS-CDS 1994–2013 weighted data).

	Crash type
Age
(yr)	Front	Side	Rear	Rollover	Total w/other & unk
	Occupants (MAIS 0+F)
All	19,511,109	8,857,015	2,996,601	3,478,830	45,579,040
15+	17,289,117	7,753,158	2,527,409	3,050,301	40,011,274
9–14	701,181	402,637	150,057	184,876	1,760,509
<9	1,246,646	593,256	273,572	232,386	2,979,900
	AIS 1 injuries
All	20,386,588	8,884,304	2,489,438	6,381,034	43,669,494
15+	19,222,195	8,382,116	2,323,797	6,024,231	41,195,355
9–14	538,731	265,706	88,078	190,659	1,209,636
<9	614,807	233,037	77,516	165,982	1,247,832
	AIS 3+ injuries
All	792,856	613,811	35,708	534,949	2,261,166
15+	758,999	595,174	30,968	511,315	2,162,868
9–14	14,211	10,716	1,766	8,481	41,753
<9	19,646	7,921	2,974	15,153	56,532
	AIS 1 injuries w/occupant contact
All	47,047	105,249	9,674	67,735	244,376
15+	26,287	89,987	7,839	59,929	196,623
9–14	4,210	8,703	1,634	4,099	19,147
<9	16,550	6,559	201	3,707	28,607
	AIS 3+ injuries w/occupant contact
All	964	11,404	235	3,743	18,957
15+	552	10,649	197	3,614	16,940
9–14	214	228	—	—	498
<9	198	527	37	129	1,519

Overall, there were 244,376 ± 20,789 AIS 1 and 18,957 ± 3,189 AIS 3+ occupant-to-occupant contact injuries in 20 years of NASS-CDS data. In rear impacts, only 235 AIS 3+ occupant-to-occupant contact injuries occurred, or only 12 cases per year. There were only 37 occupant-to-occupant contact injuries in the <9-year-old group, representing about 2 cases per year.

Table 2 shows the risk of serious (AIS 3+) injury per involved occupant. The overall risk of AIS 3+ injury was 5.0 ± 0.8%. The risk was highest in rollovers (15.4 ± 3.0% overall), irrespective of age groups. The overall risk of AIS 3+ was greatest in the 15+ age group (5.4 ± 0.9%) and lowest in the <9 age group (1.9 ± 0.4%).

Table 2. Risk and incidence of AIS 3+ injuries with subgroups of occupant-to-occupant contact (NASS-CDS 1994–2013 weighted data).

	Crash type
Age
(yr)	Front	Side	Rear	Rollover	Total w/other & unk
	Risk AIS 3+ injury per MAIS 0 + F
All	4.1%	6.9%	1.2%	15.4%	5.0%
15+	4.4%	7.7%	1.2%	16.8%	5.4%
9–14	2.0%	2.7%	1.2%	4.6%	2.4%
<9	1.6%	1.3%	1.1%	6.5%	1.9%
	Risk AIS 3+ injury w/occupant contact
All	0.0049%	0.129%	0.0078%	0.108%	0.042%
15+	0.0032%	0.137%	0.0078%	0.118%	0.042%
9–14	0.0032%	0.057%	—	—	0.028%
<9	0.0158%	0.089%	0.014%	0.056%	0.051%
					w/o other & unk
	% Incidence AIS 3+ injury
All	40.1%	31.0%	1.8%	27.1%	100%
15+	40.0%	31.4%	1.6%	27.0%	100%
9–14	40.4%	30.5%	5.0%	24.1%	100%
<9	43.0%	17.3%	6.5%	33.2%	100%
	% Incidence AIS 3+ occ-to-occ contact injury
All	5.9%	69.8%	1.4%	22.9%	100%
15+	3.7%	70.9%	1.3%	24.1%	100%
9–14	48.5%	51.5%	—	—	100%
<9	22.2%	59.2%	4.2%	14.5%	100%

Table 2 also shows the risk of AIS 3+ occupant-to-occupant contact injury by crash type and age group. Serious injuries due to occupant-to-occupant contact were generally uncommon and represented only 0.84% of all AIS 3+ injury. The overall risk was 0.042 ± 0.007% for AIS 3+ occupant-to-occupant contact injuries. The highest incidence of AIS 3+ occupant-to-occupant contact injury was in side impacts (69.8%), followed by rollovers (22.9%). The risk of AIS 3+ injury from occupant-to-occupant contact was highest for the <9-year-old age group at 0.051 ± 0.026%. It represented 2.69% of all AIS 3+ injuries in children less than 9 years old. There were no serious injuries due to occupant-to-occupant contact in the 9–14 age group in side, rear, or rollover crashes. For all age groups, occupant-to-occupant contact injuries were lowest in rear impacts with a risk of 0.0078 ± 0.0054%. For the <9-year-old group, only 4.2% of occupant-to-occupant contact injury occurred in rear impacts.

Table 3 shows the occupant-to-occupant contact injuries in rear crashes from NASS-CDS. There were 5 unweighted cases of AIS 3+ occupant-to-occupant contact injury in rear crashes. Appendix B (see online supplement) provides information on each case. The cases involved older vehicles from the 1994–2001 model years. Three of the cases involved substantial deformation of the occupant compartment and 2 involved yielding of the front seats with interaction between adjacent occupants. Four cases were with occupants 15 years or older. Two of the cases involved 3 impacts, one with 2 impacts and one with a single rear impact. One of the cases with 3 impacts involved the driver and the right-front occupant. Both occupants had at least one AIS 3+ injury from occupant-to-occupant contact. The injuries to the right-front passenger included one to the head and one to the lungs. The injuries occurred from contact with the driver. The driver sustained an injury to the right shoulder from contact with the right-front passenger.

Table 3. NASS-CDS cases with AIS 3+ occupant-to-occupant contact injuries in rear crashes.

NASS ID			Crash type			Injured occupant						AIS 3+ Contact			Striking occupant
Year	PSU	Caseid	Case vehicle	# impacts	Max Crash severity	Seating location	Restraint	Age (yr)	Gender	Height (in)	Weight (lb)	# AIS 3+ occ contacts	Occupant contact injury location	AIS	Seating location	Age (yr)	Gender	Height (in)	Weight (lb)
15+ yo
1997	75	85J	1994 Chev.Geo Metro	3	40	RF	auto belt	38	M	76	220	3	Head Lungs	43	D	38	M	74	201
						D	auto belt	38	M	74	201	1	Shdr	3	RF	38	M	76	220
2007	42	60B	1996 Honda Accord	3	Sev	LR^*	None	20	F	65	179	1	Liver	4	RR	36	F	66	146
2007	41	205B	1999 Toyota Tacoma	2	Mod	RF	L-S	65	F	62	104	1	Ribs	4	D	74	M	68	201
2010	41	31B	2001 Honda Prelude	1	Sev	D	Unk	21	F	67	130	1	Brain	4	RR	55	M	69	192
<9 yo
2003	11	193B	1995 Saturn SC	1	44	LR	L-S CRS	5 yo	F	36	35	1	Brain	3	RR	1.5	F	26	26

D: Driver, RF: Right-front, LR: Left-rear, RR: Right-rear; Shdr: Shoulder

^* compl ejection through right rear glazing

The rear impact data for children <9 years old was based on one unweighted case (2003-11-193B). The crash involved a 1995 Saturn SC impacted in the rear by a 2003 Ford Ranger Super cab. The rear delta V for the Saturn was 27.3 mph. The intrusion pushed the left second-row child in a booster seat into the front seatback leading to a crushed chest (AIS 4) and death. One of the serious injuries involved a head injury from occupant-to-occupant contact with the second-row infant seated to her right.

Discussion

AIS 3+ occupant-to-occupant contact injury occurs primarily in side impacts and rollovers. Most contact injury is to adults (89.4% incidence). Occupant-to-occupant contact injury to children is rare in rear impacts. The NASS-CDS cases of AIS 3+ occupant-to-occupant contact injury in rear impacts involved older vehicles from model years 1994–2001. These vehicles were in production before improvements in rear structures for the revision of FMVSS 301 and the introduction of stronger front seats of a high retention design. Three of the cases involved substantial deformation of the occupant compartment and 2 involved yielding of the front seats with interaction between adjacent occupants. Modern vehicles and seats have improved since 2001. This may be a reason why newer model vehicles were not in the sample. The combination of improved rear structures and stronger front seats seem to have resulted in a safety improvements (Viano and Parenteau 2015, 2016).

As time proceeds, other safety systems will further reduce the risk of serious injury in rear impacts. For example, the phase-in of automatic radar braking systems offers the possibility of significantly reducing rear impacts related to driver distraction and lack of attention to traffic stopped or braking ahead. The issue raised by Senators Markey and Blumenthal (2016) seems moot with modern rear structures and seats. Even as automatic braking technology phases in, there is not an issue of occupant-to-occupant AIS 3+ contact injury in rear impacts with modern vehicles and it was rare in older models.

Limitations

This study assessed the incidence and frequency of occupant-to-occupant injury using NASS-CDS data. Occupant-to-occupant contact is determined by a team of field researchers. It may be challenging to distinguish between occupant-to-occupant-induced injury and vehicle structure–induced injury. The level of confidence associated with the injury source was not assessed.