Effect of seat belt use and airbags deployment on mortality in patients with road traffic injury

1. Introduction

Deaths from road traffic injury (RTI) reached 1.19 million annually in 2021, and passenger deaths in 4-wheel vehicle accounted for 30% of RTI fatalities [1]. It is the leading cause of death especially in children and people aged 5–29 years, and low- and middle-income countries bear the greatest burden of road traffic fatalities and injuries [1, 2]. In Korea, although the annual RTI incidence rate has increased slightly, the overall trend of deaths from RTI has been decreasing since 2013. The number of deaths in 2022 reached 2735 and the death rate of 4-wheeler passengers accounted for 49.1% of all RTI fatalities [3]. Most patients who survive RTI have severe disabilities and suffer high economic costs, resulting in a public health burden [4]. Several strategies have been implemented to reduce RTI, including road safety campaigns such as seat belt use; reducing alcohol-impaired driving; use of various safety technologies such as seat belts, airbags, child safety seats, and electronic stability control; and strong law enforcement [5, 6, 7].

Seat belt use is considered the most effective modality for saving lives in RTI events [5, 6]. Unbelted vehicle occupants were 10 times more likely to die, and proper use of seat belts can reduce the risk of death by 45% [8, 9]. However, seat belt use rates vary widely across countries. It increased to 90.4% in 2021 in the United States; however, it remained low in developing countries at <60% [9, 10, 11]. According to the 2021 Report on the Transport Culture Index of Korea, seat belt use rates increased from 73% in 2011 to 87% in 2017, but remained constant at 84% in 2021 [12].

Airbags have been introduced to provide further protection against RTI during severe collisions [13, 14]. Frontal airbags have saved 50,457 lives from 1987 to 2017 in the United States and reduced fatalities by 14% when seat belts were not used [9]. Most airbags are designed to deploy in moderate-to-severe RTI, but are affected by other variables such as speed and direction of impact [9, 14]. And regarding airbag-related injuries, unrestrained drivers in frontal collisions were more likely to sustain more severe injuries [14, 15, 16].

Both seat belts and airbags are well known to reduce fatal outcomes in patients with RTI and have been implemented with a safety device designed to prevent injuries. However, studies comparing the preventive effects of seat belts and airbags on clinical outcomes are limited. This study aimed to compare the effectiveness of these safety devices in reducing mortality of patients with RTI.

2. Materials and methods

2.2 Study population

The study population included all patients who sustained RTI in a vehicle and visited the ED between January 2011 and December 2020. We excluded cases resulting from out-of-vehicle RTI, who died before reaching the hospital from in-vehicle RTI, and those involving vehicles with ≥10 passenger (variables related to driver or passenger injuries were collected in three categories: <10 passenger vehicles; 11–19 passenger vehicles and pickup trucks; and, >20 passenger vehicles, buses, trucks, and large vehicles. The analysis is limited because of the low seat belt use rate in large vehicles and because most airbags are installed in the front seat, making it difficult to analyze airbag deployment in large vehicles). Additionally, we excluded children aged <6 years (they are obliged to use safety car seats as per Korean law) and those with missing data of seat belt use, airbag deployment and clinical outcomes. The flowchart of this study is illustrated in Fig. 1.

Fig. 1.Study population.

3. Results

Of the 2,627,450 injured patients, 429,501 visited the ED due to RTIs. A total of 82,262 patients were included in the analysis, excluding those with out-of-vehicle injuries (n = 88,576), in-vehicle injuries involving vehicles with >10 seats (n = 161,041), children aged <6 years (n = 8209), unknown outcomes (n = 41), seat belt use (n = 22,742), and airbag deployment (Fig. 1).

Table 1 shows the demographic characteristics of the study population that used safety devices. Among the 82,262 eligible patients, 13,929 (16.9%) were in the seat belt use and airbag deployment group; 47,213 (57.4%), seat belt use-only group; 1820 (2.2%), airbag deployment-only group; and, 19,300 (23.5%), non-use group. The airbag deployment-only group was more likely to be younger (median age, 34 years), injured at night (18:00–06:00), consume more alcohol (20.8%), use more EMS (64.0%), and have poor mental status at the ED visit (all p < 0.001).

The highest proportion of patients was in the non-use group (70.1%). Frontal collision was the most common in the seat belt use and airbag deployment, and airbag deployment-only groups (19.8% and 24.8%, respectively). Regarding the anatomical classification of injuries, the proportion of head and face injuries was higher in the airbag deployment-only and non-use groups (57.0% and 50.6%, respectively). However, neck injury was the most common in the seatbelt use-alone group (42.6%). The airbag deployment-only group had a higher injury severity score (median score, 13) and higher proportion of in-hospital mortality and intracranial injury (3.6% and 5.9%, respectively) (all p < 0.001; Table 2).

Fig. 2 shows the trends in the number of safety devices used by year. The proportion of non-users decreased from 29.3% in 2011 to 15.1% in 2020. The seat belt use rate reached approximately 82% by 2020. Compared to the seat belt use-only group, the seat belt use and airbags deployment group had higher in-hospital mortality and intracranial injury (AORs (95% CIs) were 1.70 (1.31–2.19) and 1.29 (1.13–1.47), respectively). The airbag deployment-only group was 10 times more likely to die (10.24 (7.49–13.99)) and 3 times more likely to suffer from intracranial injury (3.00 (2.42–3.72)) (Table 3).

Fig. 2.Trends in the applied safety devices by year.

For patients with RTI with an EMR-ISS ≥9, the AORs (95% CIs) for in-hospital mortality were similar to those in the main analysis. The AORs (95% CIs) for in-hospital mortality and intracranial injury were 1.65 (1.27–2.15) and 1.18 (1.03–1.35), respectively, in the seat belt use and airbags deployment group. For patients with RTI with EMR-ISS ≥16, there were no statistically significant differences in the proportions of in-hospital mortality and intracranial injury between the seat belt use and airbag deployment group and the seat belt use-only group. Regardless of the severity of the injury, the in-hospital mortality rate was 5–8 times higher and that of intracranial injury was 1.5–2.5 times higher in the airbag deployment-only group (Table 4).

4. Discussion

Based on the EDIIS data, we found that seat belt use had significant preventive effects against clinical outcomes. The proportions of both in-hospital mortality and intracranial injury were higher in the seat belt use and airbags deployment group than those in the seat belt use-only group, but there was no difference in severe RTI between the two groups. Airbag deployment itself did not reduce RTI-related in-hospital mortality and intracranial injury, rather, it showed a 10 times higher mortality rate and 3 times higher intracranial injury rate.

Seat belt use was the most effective modality for reducing RTI fatalities. Among the numerous efforts to increase the rate of seat belt use, mandatory seatbelt legislation is highly effective in promoting the wearing of seatbelts and is a cost-effective measure to reduce the severity and sequelae of traumatic RTI-related brain injuries [18, 19]. In fact, 69% of the countries worldwide have adopted the best practice of mandating the use of seat belts by front and rear seat occupants [1]. However, seatbelt use rates did not increase further from the high 80% in developed countries and were found to be <60% in developing countries [9, 11]. Beyond legislation, various efforts have been made to increase the seat belt use rate, including public campaigns and the development of new technologies such as belt reminders or interlocks [1, 20, 21].

Airbags, in combination with seat belts, are regarded as supplemental safety measures that reduce the risk of injury from RTI. Despite using seatbelts, car occupants are injured when they hit the vehicle’s interior parts such as the steering wheel or dashboard, and airbags reduce the level of contact [9]. Fatalities in frontal collisions, specifically in vehicles with airbag deployment, have been reduced by 22% among belted drivers [22]. The United States has implemented the mandatory installation of airbags; however, in Korea, no such obligation exists. However, vehicle manufacturers have voluntarily installed airbags; the installation rate of airbags in manufactured vehicles in Korea was 88.3% in 2003 [23].

Despite the reduced risk of injury from airbags, airbag deployment had no further protective effect on outcomes even in patients with severe RTI (with EMR-ISS greater than 16) in vehicles that used seatbelts. This finding is inconsistent with the findings of previous studies. Our results differ from those of most previous studies probably because the previous studies evaluated only the effect of frontal airbags in frontal collisions [6, 14, 22]. Stewart et al. [13] reported that airbag deployment reduced the severity of head injuries, but their study population was small (n = 1937) and confined to severely injured drivers (Injury Severity Score (ISS) >12). A meta-analysis reported that frontal airbags reduced accident fatalities among belted drivers in frontal collisions. However, the size of the overall effect was too heterogeneous to draw conclusions [22].

Moreover, the deployment-only group had worse outcomes: a higher proportion of poor mentality in the ED and higher injury severity scores were observed. The proportions of in-hospital mortality and intracranial injuries were higher. Airbag deployment alone did not reduce the severity of injury, rather, it increased the risk of lower extremity injury when the seatbelt was not used [24, 25]. Airbags are generally designed to inflate in moderate-to-severe car crashes according to the direction and severity of their impact [9]. Unrestrained occupants are more likely to be positioned along the deployment path of an airbag during a collision, leading to higher lethality from the airbags [14, 16]. These results imply that evidence for the effectiveness of airbags is still lacking with respect to all crashes, and additional research is needed to consider various conditions such as the type of airbag embedded in the vehicle, collision direction, and severity of injury.

There is an overall reduction in the number of fatalities in frontal collisions, mainly owing to the reduced risk of serious head and neck injuries [16, 26]. However, most studies have included only car occupants wearing seatbelts in airbag-equipped vehicles. In this study, the head and face injury rate was 57% in the airbag deployment-only group, but those in the seat belt use and airbag deployment group and seatbelt use-only group were significantly lower (38.1% and 37.2%, respectively). These results reiterate that airbags are complementary safety devices rather than alternatives to seat belts. Considering that they are designed to be deployed during serious RTIs, we ensured that all occupants were properly seated and wore seatbelts to reduce the risk of injury.

Regarding the characteristics of the airbag deployment-only group, the airbag deployment-only group was more likely to be younger, injured at night, consume more alcohol, and use EMS at a greater rate. A higher risk of injury is associated with occupant physique or specific positions of occupant seating, specifically those who are unrestrained or improperly restrained [14, 27]. Our results showed that unrestrained occupants were younger, had more passengers, consumed more alcohol, and had poorer clinical outcomes (Supplementary Table 1). Unrestrained drivers are more likely to use cell phones while driving, drive at excessive speed limits, attempt to pass other vehicles, have alcohol-impaired driving, and not follow traffic rules [28]. Drivers have the greatest influence on passenger seat belt use, and driver restraint use is a significant predictor of restraint use, specifically among child passengers in the RTI [29]. Therefore, public efforts are required to prevent fatal RTIs by increasing traffic safety awareness and implementing a desirable driving culture for car occupants.

This study has several limitations. First, this was a retrospective cohort study and due to the large number of participants, there were noticeable differences between characteristics of the four groups, which may lead to treatment selection bias. And unmeasured potential confounders may have influenced the exposure and outcomes. Injury-related data, which can influence outcomes, such as speed at the time of collision, counterparts of the RTI, and passengers’ seating positions, were not available in the EDIIS registry. Second, seatbelt use and airbag deployment, the main exposure variables, were ascertained only through face-to-face interviews with patients and guardians. Therefore, we excluded >80,000 patients from the analysis. Compared to the target population, missing values for mental status at the ED and injury-related data (especially the type of road and collision direction) accounted for a large proportion in the missing group. In addition, the missing data group had higher proportions of in-hospital mortality and intracranial injuries (Supplementary Table 2). This might have been subject to over- and under-estimation, which could have resulted in bias. Furthermore, we only had information on whether the airbags were deployed. Airbag-related data, such as the type, number, and location of airbags embedded in the vehicle, were limited and could not be used for the analysis.

5. Conclusions

Seat belt use showed preventive effects against in-hospital mortality and intracranial injury due to RTI. Airbag deployment with and without seat belt use had no additional preventive effects on the clinical outcomes. These results suggest that airbags are not a substitute for seatbelts, but act as a supplementary device to reduce RTI. Public health efforts are needed to increase the proper use of safety devices and implement a good driving culture for car occupants, which can help reduce the health burden of RTI.

Availability of data and materials

The data presented in this study are available upon reasonable request from the corresponding authors.

Author contributions

JHK and GJP—designed this study. GJP and SCK—analyzed the results. JHK—drafted the manuscript. YMK, HSC, SCK, HK and SWL—reviewed and edited the manuscript. All the authors have read and approved the final version of the manuscript.

Ethics approval and consent to participate

This study was approved by the Institutional Review Board (IRB) of Chungbuk National University Hospital (IRB No. 2022-10-013). The requirement for informed consent was waived, and patient information was anonymized prior to analysis.

Acknowledgment

Thanks to all the members of participating institutions for helping collect data.

Funding

This research received no external funding.

Conflict of interest

The authors declare no conflict of interest.

Supplementary material

Supplementary material associated with this article can be found, in the online version, at https://oss.signavitae.com/mre-signavitae/article/1798889647069118464/attachment/Supplementary%20material.docx.