Significance of tractor-trailer impacts to the rear of light vehicles

Abstract

Objective

This study determined the type of striking vehicle or object associated with serious injury to at least 1 occupant in the vehicle struck in the rear.

Method

The 1990–2015 NASS-CDS was analyzed for light vehicles involved in rear crashes. The type of vehicle struck in the rear resulting in serious injury to at least 1 occupant was determined. Rear impacts were identified with GAD1 = B without rollover (rollover ≤ 0). Vehicles with serious to maximum injury were identified as VAIS 3 to 6 (VAIS 3+). The type of striking vehicle or object was determined for the rear impacts causing serious injury. The risk and relative risk for serious injury were determined. Review of electronic cases was conducted for light vehicles with serious injury (VAIS 3+) in impacts by tractor-trailers or large trucks.

Results

The highest risk for serious injury to an occupant in the struck vehicle was from a tractor-trailer or large truck (2.71%), followed by a fixed object (1.49%) and van (1.00%). The average risk was 0.33%. The relative risk was 8.2 times for impact with a tractor-trailer or large truck and 4.5 times for impact with a fixed object. The highest risk impacts with a fixed object were rear impacts into a tree/bush (3.57%) and pole (2.90%). Tractor-trailers and large trucks accounted for 16.2% of serious injury in struck vehicles, followed by fixed objects at 12.9%; the 2 accounted for 29.1% of serious injury to occupants of struck vehicles. The case review identified 22 tractor-trailer and 1 large truck crashes involving 31 seriously injured occupants in the struck vehicle. There were 2 general crash scenarios associated with serious injury. One involved intrusion deforming the occupant compartment. The other involved vehicle accelerations sufficient to seriously injure an occupant in the struck vehicle without significant deformation to the occupant compartment.

Conclusion

This study provides insight into the striking vehicle or object associated with serious injury to light vehicle occupants in rear impacts. Tractor-trailer impacts into the rear of a vehicle involved a high risk for serious injury in the struck vehicle. The use of collision mitigation technologies, such as forward collision warning or automatic emergency braking, on the tractor-trailer may support better speed control and lead to lower closing velocities of rear impacts to light vehicles.

Keywords:

• Tractor-trailer crashes
• rear impacts
• serious injury
• collision avoidance

Introduction

There is interest in rear crashes and the causes of injury to front and rear seat occupants (Burnett et al. 2004; Viano and Parenteau 2008a, 2008b). There has been much less focus on the striking vehicle or object associated with serious injury to occupants in the struck vehicle. Many factors influence injury in rear crashes, including vehicle-to-vehicle configuration, vehicle mass, stiffness and geometry, and crash severity (Viano and Parenteau 2010; Padmanaban et al. 2016; Parenteau et al. 2021). Padmanaban et al. (2016) looked at the effect of striking and struck vehicle mass on injury in rear crashes. They reported a higher risk of serious and fatal injuries when the struck vehicle was lightweight.

There is also increasing interest in commercial vehicles having driver assistance technology such as forward collision warning or automatic emergency braking, as well as improved brakes. Early studies focused on improved brakes to reduce stopping distance with emergency braking (Murphy et al. 1971; Haney 1979; Kuehn et al. 2011). More recently, there has been interest in the use of radar, laser, and camera technologies to detect the closing rate between a commercial vehicle and another vehicle on the road and provide warnings to the driver and/or automatically apply the brakes in situations where a driver has not already started braking the tractor-trailer (Jermakian 2012; Woodrooffe and Blower 2012; Woodrooffe et al. 2013; Teoh 2021).

NHTSA collects field accident data on U.S. roadways using several databases depending on the level of injury involved. National Automotive Sampling System-General Estimates System and Crash Report Sampling System data are commonly used for the distribution of police-reported crashes. The databases are representative of a national sample of minor to fatal motor vehicle crashes (www.nhtsa.dot.gov) Analyses of police-reported crashes indicate that more than 25% of light vehicle crashes are rear impacts (Ference and Najm 2005; Najm et al. 2006). Najm et al. (2003) found that more than 59% of cases involved a stopped or decelerating lead vehicle and that most 2-vehicle crashes occurred at intersections.

The Fatality Analysis Reporting System and NASS-CDS are other NHTSA databases used to assess the distribution of fatal and tow-away crashes. Viano and Parenteau (2008b) analyzed 1996–2005 Fatality Analysis Reporting System and CDS data and found that rear crashes accounted for 3.2% of fatalities and about 8% of exposed occupants in tow-away crashes. The risk of fatal injury was lower in rear crashes than in other crash types. The results are consistent with prior studies (NHTSA 2003; Parenteau and Viano 2021). Parenteau and Viano (2021) analyzed NASS-CDS data from 1999 to 2015 and reported that the risk for serious to fatal injury (Maximum Abbreviated Injury Scale 3 + F) was 0.497% ± 0.120% in rear impacts, which was lower than the risk for other impact types. Tatem and Gabler (2017) analyzed 2000–2015 NASS-CDS data for fatalities in rear-impacted vehicles and found that the number of fatalities was higher when a car was struck by a light truck or van. However, the sample size was small with 10 unweighted cases.

In this study, NASS-CDS was analyzed for the type of struck vehicle in a rear impact and the type of striking vehicle or object. The data were analyzed for the risk and relative risk of serious injury (VAIS 3+) to an occupant in the struck vehicle. The study provides perspective on the relative risk for serious injury by the striking vehicle or object.

Materials and methods

NASS-CDS data

NASS-CDS is a national, statistically sampled vehicle crash database consisting of about 4,000 to 5,000 yearly crashes. The database is used by vehicle researchers to assess crash performance and overall safety of vehicles. NASS-CDS is a stratified sample of crashes that are prospectively selected for in-depth investigation. Most vehicles are towed from the scene. The data collected include information from NASS crash investigation teams, vehicle inspection, medical records, police reports, and interviews.

In this study, the data were obtained from NASS-CDS for calendar years 1990–2015. Crashes selected were vehicles hit in the rear:

• Rear with impact location (GAD1 = B) and excluded rollovers (ROLLOVER < = 0).

Vehicles struck in the rear included the following:

• 2-door passenger cars (BODYTYPE = 2,3)

• 4-door passenger cars (BODYTYPE = 4,5,6,7)

• Other passenger cars (BODYTYPE = 1,8,9,10,11,12,17)

• Utility (BODYTYPE = 14,15,16,18,19) or (BODYTYPE = 13 and year <1992)

• Minivans (BODYTYPE = 20)

• Pickups (BODYTYPE = 30,31,32,33,34, 35,36,37,38,39)

Striking vehicle or object associated with the rear impact of the struck vehicle or object were the following:

• Vehicle in transport (OBJCONT1 = 1,2,3,4,5,6,7,8,9,10, 11) including

  • Passenger cars (OTBDYTYP = 1,2,3,4,5,6,7,8,9)

    • 2-door car (OTBDYTYP = 2,3)

    • 4-door car (OTBDYTYP = 4,5)

  • Other cars (OTBDYTYP = 10,11,12,17) or (OTBDYTYP =13 and year >1991)

  • Utilities (OTBDYTYP = 13 and year <1992) or (OTBDYTYP = 14,15,16,18,19)

  • Vans (OTBDYTYP = 21,22,23,24,25,26,27,28,55)

  • Pickups (OTBDYTYP = 30,31,32,39,74) or (OTBDYTYP = 33 and year >1991)

  • Light trucks (OTBDYTYP = 40,41,42,47,48,49) or (OTBDYTYP = 33 and year <1992)

  • Tractor-trailers/large trucks/buses (OTBDYTYP = 50,58,59,60,61,62,63,64,65,66,67,68,69); because there were no buses in the case sample causing serious injury, the category is referred to as tractor-trailer, large trucks.

  • Other (OTBDYTYP = 70,71,78,79,80,82,88,91,92,93,94,97)

• Vehicle not in transport (OBJCONT1 = 70,71)

• Fixed object including

  • Tree/bush (OBJCONT1 = 41,42,43)

  • Pole (OBJCONT1 = 45,50,51,52,53)

  • Embankment/ditch/ground (OBJCONT1 = 44,60,61)

    • Guardrail (OBJCONT1 = 46,47,48,49). Note: The guardrail option became available in 2008.

  • Barrier/wall (OBJCONT1 = 54,55,56,57,58,59).

The fixed object impacts were typically single-vehicle crashes where loss of control caused the struck vehicle to spin out with a rear-leading impact into a fixed object.

National estimates for the number of involved vehicles in each category were made using the inflation factor (RATWGT). SAS software, version 9.4, was used (www.sas.com). The unweighted and weighted data were imported into Excel to generate the results tabulated in this study.

Injury

• Vehicles with occupants having a maximum known Abbreviated Injury Scale (AIS) score were determined using the VAIS variable using AIS98. Vehicles having an occupant with serious to maximum injury were identified as VAIS 3 to 6 (VAIS 3+).

• Serious injury risk: The risk that a vehicle had an occupant with serious to maximum injury was determined by dividing the number of vehicles with seriously injured occupants (VAIS 3+) by the number of struck vehicles involved.

The risk of serious injury for each subgroup of striking vehicles or object was compared to the overall risk for serious injury for all rear crashes to give a relative risk. The relative risk indicates whether a type of striking vehicle is involved in greater or lesser risk of serious injury for the struck vehicle. The weighted data and risks are provided so other calculations can be made.

Electronic cases were downloaded from the 2004–2015 calendar years for cases with VAIS 3+ in a vehicle struck in the rear by a tractor-trailer, large truck, or bus (www.nhtsa.dot.gov). NHTSA defines a large truck as >4,545 kg (10,000 lb) GVWR, including single-unit trucks and truck tractors. The cases were reviewed to gain an understanding of crash circumstances and factors involved in the serious to maximum injuries.

Results

Struck light vehicles and striking vehicle type or object

Table 1 shows the type of struck vehicle involved in rear impacts on U.S. roads using 1990–2015 NASS-CDS. The 26 years of accident investigation involved 8,917,873 weighted light vehicles struck in the rear. There were 29,545 struck vehicles with at least 1 seriously injured occupant (VAIS 3–6). The greatest number of struck vehicles involved 4-door passenger cars, followed by 2-door cars, pickups, and utility vehicles. The vehicles with the highest risk of serious to fatal injury were 2-door passenger cars, followed by 4-door cars, minivans, and utility vehicles. The relative risk was 1.87 in 2-door cars and 1.06 in 4-door cars compared to the overall risk of serious injury.

Table 1. Type of struck vehicles in a rear impact using 1990–2015 NASS-CDS.

	Struck in rear		Frequency		Risk
Struck vehicle type	Involved	VAIS 3+	Involved (%)	VAIS 3+ (%)	VAIS 3+ (%)	Relative risk
Car, 2-door	1,467,140	9,087	16.5	30.8	0.619	1.87
Car, 4-door	4,098,950	14,415	46.0	48.8	0.352	1.06
Car, other	95,654	91	1.1	0.3	0.095	0.29
Minivan	488,982	1,458	5.5	4.9	0.298	0.90
Pickup	1,433,619	1,190	16.1	4.0	0.083	0.25
Utility (SUV)	1,333,528	3,304	15.0	11.2	0.248	0.75
Total	8,917,873	29,545	100	100	0.331	1.00

Table 2 shows the type of striking vehicle or object associated with a rear impact of a light vehicle. The most common striking vehicle was a 4-door passenger car, followed by 2-door car, pickup, and utility vehicle. For struck vehicles with serious to maximum injury (VAIS 3+), the most common striking vehicle was a 4-door passenger car, followed by tractor-trailer or large truck then pickup and fixed object. Tractor-trailers, large trucks, and buses accounted for 16.2% of struck vehicles with a seriously injured occupant. Fixed objects accounted for 12.9% and utility vehicles accounted for 11.6%. The tractor-trailer, large truck, or bus and fixed object impacts accounted for 29.1% of serious injury in a struck vehicle.

Table 2. Striking vehicle or object involved in a rear impact of a struck vehicle using 1990–2015 NASS-CDS.

	Struck in rear		Frequency		Risk
Striking vehicle or object	Involved	VAIS 3+	Involved (%)	VAIS 3+ (%)	VAIS 3+ (%)	Relative risk
Car, 2-door	1,435,114	1,637	16.1	5.5	0.114	0.34
Car, 4-door	3,474,518	9,006	39.0	30.5	0.259	0.78
Other passenger car	412,788	755	4.6	2.6	0.183	0.55
Other car	4,054	12	0.0	0.0	0.289	0.87
Utility	1,212,137	3,427	13.6	11.6	0.283	0.85
Minivans	357,225	400	4.0	1.4	0.112	0.34
Vans	131,021	1,316	1.5	4.5	1.004	3.03
Pickup	1,286,617	4,219	14.4	14.3	0.328	0.99
Light truck	29,583	0	0.3	—	—	—
Tractor-trailer/large truck/bus	176,078	4,772	2.0	16.2	2.710	8.18
Other	20,647	157	0.2	0.5	0.761	2.30
Unknown	99,250	0	1.1	—	—	—
Vehicle not in transport	22,891	22	0.3	0.1	0.096	0.29
Fixed object	255,949	3,822	2.9	12.9	1.493	4.51
Total	8,917,872	29,545	100	100	0.331	1.00

The highest risk for serious injury was an impact by a tractor-trailer or large truck (2.71%), followed by a fixed object (1.49%) and a van (1.00%). The average risk for serious injury (VAIS 3+) was 0.33% for rear impacts. The relative risk was 8.2 times the average risk for all rear crashes when the vehicle was struck in the rear by a tractor-trailer or large truck. The relative risk was 4.5 times when the vehicle impacted a fixed object.

Table 3 shows the type of fixed object contacted by the rear of the struck vehicle using 1990–2015 NASS-CDS. The 26 years of accident investigation involved 255,949 fixed object impacts to the rear of a light vehicle. There were 3,822 vehicles with at least 1 seriously injured occupant (VAIS 3+). The highest risk for serious injury involved a rear impact into a tree or bush (3.57%), pole (2.90%), or barrier or wall (0.56%). This gives a relative risk of 10.8 times for tree/bush rear impacts and 8.75 times for pole rear impacts.

Table 3. Object impacting the rear of a struck vehicle using 1990–2015 NASS-CDS.

Fixed object	Struck in rear		Frequency		Risk
Rear impact	Involved	VAIS 3+	Involved (%)	VAIS 3+ (%)	VAIS 3+ (%)	Relative risk
Tree/bush	58,206	2,080	22.7	54.4	3.574	10.79
Embankment/ditch/ground	49,011	121	19.1	3.2	0.247	0.75
Guardrail	8,814	0	3.4	0.0	—	—
Pole	35,763	1,037	14.0	27.1	2.900	8.75
Barrier/wall	93,298	520	36.5	13.6	0.557	1.68
Other/unknown	10,857	64	4.2	1.7	0.590	1.78
Subtotal	255,949	3,822	100	100	1.493	4.51
Total	8,917,872	29,545	100.0	100.0	0.331	1.00

Individual case review of tractor-trailer impacts

Tractor-trailers and large trucks as the striking vehicle posed the highest risk for occupants in the vehicle struck by another vehicle on the road. In addition, 2.71% of tractor-trailer crashes caused serious injury to an occupant in the struck vehicle. NASS-CDS electronic cases involving occupants with serious injury (VAIS 3+) in the struck vehicle and a tractor-trailer, large truck, or bus as the striking vehicle were downloaded and reviewed.

There were 23 unweighted cases involving 31 seriously injured (Maximum AIS 3+) occupants in the struck vehicle. Table A1 in the online supplement summarizes the cases. Twenty-two of the crashes involved a tractor-trailer and 1 involved a large truck. Tractor-trailer impacts dominated the category with 98.46% of the weighted cases. Only 1.54% were large truck impacts. Fifteen cases involved multiple impacts, including some secondary impacts with a large truck. Three cases involved a fire and only 1 case involved ejection from the struck vehicle.

Though the delta-V for the struck vehicle was unknown or not determined in the 23 cases, photos of the struck vehicle demonstrated severe impacts to the rear of the struck vehicle. There were 2 types of damage to the struck vehicle associated with serious injury. One type involved rear crush of the occupant compartment. Figure 1 shows examples of struck vehicles impacted by a tractor-trailer with crush of the occupant compartment. The weight of the tractor-trailer and closing speed caused a severe impact, deforming rear structures of the struck vehicle and intruding into the rear occupant compartment. In some cases, the intrusion was severe enough to support the front seatback from rearward rotation in the collision.

Figure 1. Tractor-trailer impacts with crush of the rear occupant compartment in the struck vehicle.

The second type involved rear crush without significant occupant compartment involvement. Figure 2 shows examples of a struck vehicle impacted by a tractor-trailer without significant crush of the rear occupant compartment. The tractor-trailer closing speed accelerated the struck vehicle forward with occupant loading of the seat. In these cases, the vehicle acceleration and forces on the occupant were sufficient to cause serious injury.

Figure 2. Tractor-trailer impacts without significant crush of the occupant compartment in the struck vehicle.

Discussion

This study provides insight into serious injury in rear impacts of light vehicles by the type of striking vehicle or object. NASS-CDS data were queried from 1990 to 2015. The 26 years of weighted accident investigation found that the highest risk for serious injury to occupants of the struck vehicle was 2.71% in rear impacts by a tractor-trailer or large truck. This category accounted for 16.2% of struck vehicles with a seriously injured occupant. The NASS-CDS electronic case review showed that the closing speed between the tractor-trailer and the struck vehicle was severe enough in many cases to crush rear structures of the vehicle and intrude into the occupant compartment. The relative risk was 8.2 times for impact with a tractor-trailer, large truck, or bus as the striking vehicle. For these crashes, reduction of the closing speed between the tractor-trailer and the struck vehicle is the primary means of lowering the risk for injury (Evans 1991). The use of collision mitigation technologies, such as forward collision warning or automatic emergency braking, on the tractor-trailer may support better speed control and lead to lower closing velocities in rear impacts to light vehicles.

NHTSA (2015) granted a petition for rulemaking “to establish a safety standard to require automatic forward collision avoidance and mitigation systems on certain heavy vehicles”; however, no such rule has yet to be been promulgated. NHTSA has also researched forward collision mitigation technology on large vehicles, including forward collision warning and automatic emergency braking systems. Petitioners cited safety benefit estimates from Woodrooffe and Blower (2012). A University of Michigan Transportation Research Institute study evaluated the performance and effectiveness of current and future forward collision avoidance and mitigation systems comprising forward collision warning and automated vehicle braking applied to large (heavy) trucks, including single-unit and semi-tractor-trailers (Woodrooffe et al. 2013). They found that if collision mitigation braking systems were fitted on all semi-tractor-trailers, the reduction in fatalities would be 24% to 57% depending on the sophistication of the technology. Teoh (2021) evaluated effectiveness of automatic emergency braking systems in real-world crashes.

Camden et al. (2017) prepared a report for the AAA Foundation for Traffic Safety. They identified 5 studies that estimated the effectiveness of large truck automatic emergency braking systems in preventing large trucks from impacting the rear of light vehicles as 16% to 52.3%. The range of effectiveness was the result of variations in performance capabilities with braking systems achieving 0.3 to 0.6 g deceleration (Every et al. 2014). Grove et al. (2016) reported the results of a test fleet with 169 drivers operating 150 trucks with crash mitigation systems. In over 3 million miles, no rear crashes were identified, with 6,000 activations of the collision mitigation system. They concluded that the systems can be improved and some activations were less reliable than others.

The reduction in number and/or severity of tractor-trailer and large truck impacts into the rear of passenger vehicles is a priority. NHTSA (2022) traffic safety facts indicates that more than 5,000 people died in 2019 in crashes involving large trucks, and most were passenger vehicle occupants. Impact by the front of the truck was the most common scenario in fatal and injury crashes. The Insurance Institute for Highway Safety (2020) noted that crashes involving large trucks involve high risks of injury and death because of the weight of large trucks, which can be 20 to 30 times heavier than passenger vehicles. Crash testing of tractor-trailers impacting the rear of passenger vehicles has shown the effect of weight mismatch and incompatibility, with the majority of deformation by the passenger vehicle (Figure A1 in the online supplement).

Various factors influence the risk of occupants in light vehicles struck in the rear by tractor-trailers, including the mass ratio between the struck and striking vehicle and stiffness of structures and large profile of tractor-trailer fronts. Some differences involve greater energy in the striking tractor-trailer. The combination of crash energy and structure stiffness leads to significant crush with modest closing speeds in relation to all severe rear crashes. In this study, 22 NASS-CDS cases involving a tractor-trailer and 1 case with a large truck were reviewed. Many of the cases involved significant intrusion into the rear occupant compartment, sometimes supporting and pushing the front seat forward (Figure 1).

The effect of occupant compartment intrusion has been assessed on vehicle and occupant responses in high-speed rear impact tests. Parenteau et al. (2020) reported 2 tests involving a medium/large truck striking the rear of a passenger car. One test involved the front of a 1979 International Transtar COE tractor striking the rear of a 1984 Renault Alliance. The second test involved a 1998 GMC C6500 striking a 2002 Hyundai Accent. The mass ratio was 7.0 in the test with the Renault and 8.7 in the test with the Hyundai. Figure A1 shows the test setup, vehicle kinematics at maximum engagement and corresponding driver kinematics. The delta-V of the Renault was 53 km/h (33 mph) and the delta-V of the Hyundai was 49 km/h (30.4 mph). The 2 crash tests provide insight on rear impacts by a tractor-trailer or large truck. Deformation and intrusion of rear structures occurred early in the collision. The second-row seatback deformed, intruded forward, and contacted the front seat. This supported and bolstered the driver seat as the seatback, head restraint, and second-row seatback loaded the driver. The effect of intrusion on occupant responses in NHTSA rear crash tests has been discussed (Viano et al. 2008).

The second type of crash circumstance associated with a tractor-trailer/large truck impact involved rear crush without significant occupant compartment involvement (Figure 2). The struck vehicle acceleration was a significant factor in the injury. Though there have been improvements in seat strength, high and more forward head restraints, and pocketing of the occupant in the seatback, there are challengers to improve safety with severe vehicle accelerations, even as rear structures have improved with the revision of FMVSS 301 (Viano and Parenteau 2016; Parenteau et al. 2020). A reduction in the severity of collision forces is essential.

The primary means of lowering risks to the occupants of the struck vehicle in both types of collisions involves mitigating rear crash severity (accelerations and delta-V) by reducing closing speed. Various driver assistance systems such as collision mitigation have been developed and have shown promise in reducing crash severity and the incidence of collisions. The technologies often rely on radar, laser, and camera vision systems and include warning systems such as forward collision warning, as well as intervention technologies such as automatic emergency braking. For example, Teoh (2021) found that driver assistance technologies could reduce impact speed by >50% and the rate of rear crashes by large trucks by 44%. Jermakian (2012) and Kuehn et al. (2011) reported that front crash mitigation with driver assistance technologies would reduce the number of crashes involving large trucks. Collision mitigation technologies continue to be evaluated (Poe 2018). The U.S. Congress (2021) passed the Infrastructure Investment and Jobs Act directing the Department of Transportation to require automatic emergency braking on large trucks.

Limitations

The NASS-CDS study found that the rear of a light vehicle impacting a fixed object accounted for 12.9% of crashes with serious injury. The risk of VAIS 3+ injury was 1.49% and the relative risk was 4.5 times the average rear impact causing serious injury with impact with a fixed object. Fixed object crashes are primarily related to the driver of the struck vehicle, where aggressive driving, loss of control, and yaw rotation at high speed can result in a rear-leading impact into a fixed object. The highest relative risk for serious injury was impact into a tree/bush at 10.8 times and impact into a pole at 8.75 times the average overall risk of rear impacts. The benefit of additional controls and technologies on the struck vehicle and driver than that provided by automatic braking and electronic stability control is beyond the scope of this study. Finally, there may be issues of compatibility to consider with the front of the tractor trailer deforming the rear of the struck vehicle with override or underride. These areas deserve further attention.