Motor vehicle accidents are one of the leading causes of mortality worldwide, and the leading cause of death for young adults 18–29 years old [1]. In the United States, mortality from motor vehicle accidents is the 3^rd leading nationwide cause of unintentional injury death, resulting in over 900,000 years of life lost annually as of 2020 [2]. Motor vehicle fatalities rank in the top 3 causes of death for individuals under the age of 34 [3].

The National Highway Traffic Safety Administration (NHTSA) maintains the Fatality Analysis Reporting System (FARS) [4,5], which tracks all traffic crashes in the USA since 1975 that involve at least one fatality. FARS data are used to inform safety decisions at the local, state, and national levels, and provide key insights into the efficacy of changing vehicle and roadway safety standards [4]. To be included in FARS, a crash must occur on a public road and must result in at least one death within 30 days of the crash. Road fatalities in the US continue to decrease as better advanced safety technologies emerge and become standard features across the board. Occupant fatalities involving vehicles manufactured in the last five or ten years have decreased steadily, down significantly since 1975 (Fig 1A). Despite improvements, when compared to 15 peer nations, the United States ranks last in reducing the rate of annual vehicle fatalities, and fatalities in 2021 were the highest since 2005 [6,7].

Overall, evolving crash testing and vehicle standards are estimated to have prevented ~60% of all potential vehicle fatalities in the USA [8,9]. However, the bulk of the fatality reduction is in males (Fig 1B). Qualitatively, female fatality rates have been relatively stable when compared to the overall trend. While historically males have driven more miles per licensed driver than females, that gap is steadily closing, with decreasing differences in licensure rates and driving exposure [10–12]. Driving exposure for females has increased, and female drivers have been reported to display similar behavioral risk profiles to males when behind the wheel [12–14].

Given these trends, it is important to note that no vehicle crashworthiness test in the US involves an anthropomorphic test device (ATD) representative of the average adult female, despite clear evidence of the increase of female drivers over the last 20 years [15]. The current US regulations only require crash testing with the 50^th percentile adult male and 5^th percentile adult female ATD. Prior research has shown that female drivers and vehicle occupants are more likely than males to suffer severe or fatal injuries when involved in a crash severe enough to result in fatality for an occupant involved in the crash [14,16–18]. Female vehicle occupants are at higher risk of Abbreviated Injury Scale (AIS) 2+ (moderate) and 3+ (serious) injuries compared to males when controlling for a variety of factors, particularly to the lower extremities [19,20]. This disparity between men and women in automobile crashes is a major public health issue with implications for automobile design, personnel protection, and governmental regulation. The goal of this paper is to utilize fatal crash data to assess the extent to which a difference in fatality risk exists for male and female occupants under matched crash conditions.

Female vehicle occupants between ages 20 and 40 are ~20% more likely to die in a crash than males of the same age under matched conditions (Figs 2 and 3). This general trend is apparent when isolating vehicle type (passenger car vs. light trucks), seat-belt use (belted vs unbelted), number of vehicles involved (one, two, or three or more vehicles), crash direction (front vs side), and airbag deployment, and persists for females up to age 50. To best illustrate the results produced by the double pair method, consider the case of belted drivers, aged 23–27 years (25-year-old drivers), in passenger cars, using belted front-right seat passengers as a control, matching airbag deployment, in crashes between 2010 and 2020. These results are given in Table 1 for each sex/age cohort under these conditions, giving a relative risk of 4.7% higher for 25 year old, belted female drivers with matched airbag deployment compared to 25 year old male drivers under the same conditions (Relative Risk (R) = 1.047; 95% Confidence Interval (CI) = 0.836–1.168).

Taking the weighted average across the controls presented in Table 1 for each set of subject occupants, we produce Table 2, for 25 year old vehicle occupants. Across cases for these occupants, the overall risk (Table 2) is 11.7% (R = 1.117, 95% CI 1.040–1.207) higher risk for females. This value is plotted as the corresponding value in Fig 2. Repeating this analysis for female occupants at other ages, we find younger females have a higher fatality risk than younger males when driving or sitting in the front passenger seat, regardless of seat-belt use (Table 2). Fig 3 shows the results of repeating this analysis for all crashes from 1975 to 2020. Fig 4 is representative of the confidence interval estimation utilizing the bootstrap method, and shows the distribution of R values.

Repeating the analysis for light truck occupants, we find a similar trend as in passenger cars, as shown in Fig 5. Female occupants in passenger cars and light trucks are at increased risk to males in one and two vehicle crashes (Fig A-D in S1 Appendix). There is no difference in fatality risk for females compared to males in crashes involving three or more vehicles (Fig E and Fig F in S1 Appendix). Females are at increased risk in frontal impacts and passenger side impacts, but not in driver side impacts (Fig G-I in S1 Appendix).

Despite significant advances in vehicle safety since 1975, female vehicle occupants involved in fatal crashes have a higher risk of death compared to males in matched circumstances. These results show that when in a fatal crash, a younger female occupant is approximately 20% more likely to suffer a fatal injury than a male occupant of the same age, regardless of seating position, airbag deployment, or seat-belt usage.

We have investigated several potential covariates that might explain these findings, including number of passengers, number of vehicles involved, and drug/alcohol use. There are no significant differences in the number of occupants in a vehicle between male and female drivers (Mann-Whitney U-test, p>0.90; See Fig J in S1 Appendix). While there are slight differences in the proportion of male and female fatalities identified as using drugs or alcohol, the magnitude of these differences is insufficient to account for the 20% increase in risk shown by young female occupants.

The distribution of vehicles driven by females is similar to that for males, with a slightly higher proportion of female drivers using cars with vehicle masses of ~2,500 lb, but both sexes use heavier vehicles at the same rate. While male drivers are more frequently involved in single-car crashes compared to female drivers, the proportion of crashes involving multiple vehicles is similar across sex. The distribution by sex of the number of occupants in a vehicle are similar as well. The lack of a large qualitative difference in these covariates would imply limited effect on the relative risk to drivers. To verify this conclusion, the above analyses were applied to subsets of vehicle occupants involved in one-, two- and multi-vehicle crashes, as well as cases with one, two, or several vehicle occupants.

When examining single vehicle crashes, we continue to find a higher relative risk to female occupants compared to males despite the increased proportion of crashes involving males. This is true for both passenger cars and light trucks. Similarly, there is an increased risk to female occupants in crashes with two vehicles when compared to males, but not in crashes with three or more vehicles. While females are not at higher risk than males in crashes with 3+ vehicles, one- and two-vehicle crashes account for the majority of cases in the FARS dataset (Fig K in S1 Appendix).

The results presented here are similar to those presented by Evans in 2001 for FARS data through 1998 [17]. Notably, the inclusion of data from before 2010 in the analysis presented in Fig 3 mainly reduces the error assessment, without qualitatively altering the overall characteristics of the risk curve. While analyses from 1975–2020 include second-row passengers, we excluded this subject class from analyses for 2010–2020 due to small n. The small group size (total n = 662 for belted and unbelted male and female rear passengers) resulted in large subgroup error estimates, which overly influence the error estimates. This limits the generalizability of the results for recent crash cases. However, the overall trend is apparent for all analyses with substantial group sizes. As overall there are more male fatalities per year than female fatalities, a sufficiently large sample size may be necessary to observe the differential risk presented here.

Physiological/anatomical differences are difficult to explore within the FARS dataset, but may provide an explanation. Current vehicle crashworthiness tests do not use a crash dummy representative of the average adult female [15]. The current US regulations require crash testing primarily with the 50^th percentile Hybrid III (HIII) adult male ATD, with a few tests adding in the 5^th percentile adult female as a passenger. Since the 5^th percentile female ATD is primarily a dimensionally scaled version of the HIII male [31], the physiological and anatomical differences between the sexes may not be completely reproduced in testing methods. For example, a recent study identified differences in positioning female cadaver specimens compared to male specimens during reclined frontal impacts, requiring a forward shift of the belt anchor points to produce comparable kinematics and injuries for specimens of both sexes [32].

These results are not completely consistent with other well-described trends related to sex differences in injury. Females are more likely than males to suffer fractures past the age of 60 due to osteoporosis and to experience bone loss at an earlier age [33], and females tend to have greater age related bone density loss than males [34]. Increased fracture risk in elderly females, therefore, cannot explain the observed trends. Without detailed injury report data (not available within FARS), the cause of death cannot be determined for each case. This information would be valuable in parsing the differences between male and female crash survivability. We posit that there may be unobserved trends in the injury patterns, and therefore outcomes, between male and female occupants. These trends may be the result of unintentional vehicle design issues, or a potentially unexplained lack of biofidelity in the ATDs used in testing.