Differences in male and female injury severities in sport-utility vehicle, minivan, pickup and passenger car accidents

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Abstract

This research explores differences in injury severity between male and female drivers in single and two-vehicle accidents involving passenger cars, pickups, sport-utility vehicles (SUVs), and minivans. Separate multivariate multinomial logit models of injury severity are estimated for male and female drivers. The models predict the probability of four injury severity outcomes: no injury (property damage only), possible injury, evident injury, and fatal/disabling injury. The models are conditioned on driver gender and the number and type of vehicles involved in the accident. The conditional structure avoids bias caused by men and women’s different reporting rates, choices of vehicle type, and their different rates of participation as drivers, which would affect a joint model of all crashes. We found variables that have opposite effects for the genders, such as striking a barrier or a guardrail, and crashing while starting a vehicle. The results suggest there are important behavioral and physiological differences between male and female drivers that must be explored further and addressed in vehicle and roadway design.

Introduction

Consumers are increasingly purchasing larger vehicles, including sport-utility vehicles (SUVs), pickup trucks, and minivans. These light trucks and vans (LTVs) comprised over 34% of the US vehicle fleet and between 40 and 50% of new vehicle sales in 1996 (NHTSA, 1998). The changing composition of the vehicle fleet is having a considerable effect on accident types and injury severity. Studies have shown that the accident characteristics of LTVs are different from those of passenger cars (Malliaris et al., 1996, NHTSA, 1997). LTVs have been shown to have a higher rate of rollovers than passenger cars, particularly SUVs. In fact, the rollover rate of SUVs is the highest of all vehicle types, with 36% of fatal accidents in SUVs involving a rollover compared to just 15% of fatal accidents in passenger cars involving a rollover (NHTSA, 1997).

Consumer Reports (1998) point out that occupants in LTVs may be safer in accidents involving smaller vehicles but are less safe in single-vehicle accidents with fixed objects because of the LTVs’ greater rigidity. This is corroborated by an Insurance Institute of Highway Safety study (Consumers’ Research, 1998), which found that for comparable weights, the fatality rate of SUVs and pickups in single-vehicle accidents is more than twice that of passenger cars. This study also reported that in two-vehicle accidents involving an LTV and a passenger car, the passenger car occupants are much more likely to be fatally injured than the LTV occupants, and that this probability increases as the passenger car weight goes down relative to the LTV weight. Adding to this, NHTSA (1998) reports that the weight difference between LTVs and passenger cars has increased steadily in the 1990s.

Although vehicle size and weight have an effect on occupant safety (Evans and Frick, 1993, Gattis et al., 1996, Chang and Mannering, 1999), it can be problematic to isolate the significance of this effect due to a number of confounding factors. For example, Mateja (1995) points out that, of the previous studies that have concluded that larger passenger cars were safer, many ignored that such passenger cars tended to be driven by older and safer drivers. The inability to untangle the effect of vehicle size from the effect of driver attributes can shed doubt on a study’s fundamental findings. This underscores the need to perform multivariate statistical analyses that include variables accounting for the characteristics of the driver, roadway design, and environmental conditions, as well as the type and size of the vehicle.

One key element in such multivariate analyses is the gender of the driver and the effect that behavioral and average physiological differences between males and females may have on accident-injury severity. Physiological differences can arise from average differences in male/female size and weight and their interaction with vehicle safety design (location of and operation of airbag, crash zones and even safety belt design), as well as differences in the body to withstand impacts. Behavioral differences may arise from different responses to driving conditions, particularly when operating LTVs. This could include male/female differences in risk compensation such as driving more aggressively to compensate for the perceived increased safety provided by LTVs because of their size, weight and higher driving position.

Numerous studies have found differences in accident rates between males and females (Laberge-Nadeau et al., 1992, Mannering, 1993, Massie et al., 1995). Few have explored male/female differences in accident severities. Among those that have, Evans, 1988, Evans, 2001 found that females have a higher probability of dying relative to males in similarly severe accidents in the same vehicle type. Also, Abdel-Aty and Abdelwahab (2001) found that female drivers were more likely to suffer severe injury than males.

The objective of this research is to estimate statistical models to examine the differences between male and female driver-injury severity in passenger cars, pickup trucks, sport-utility vehicles and minivans. To do this, single-vehicle accidents and two-vehicle accidents involving an LTV and a passenger car are examined separately. For single-vehicle accidents, separate models for drivers of passenger cars, pickup trucks, and SUVs/minivans are estimated. For two-vehicle accidents, driver-injury severity for the passenger car and the LTV driver are examined separately. In all cases, separate models are estimated for male and female drivers and these gender-specific models are then compared.

Section snippets

Methodology

The methodological framework is formed by first identifying variables that can be expected to be explanatory when predicting injury severity, and variables that are likely to capture any remaining unobserved effects. Statistical models predicting the probability of injury severity outcomes for males and females by number of vehicles in the accident (single-vehicle, car versus pickup, car versus SUV/minivan) and vehicle type are then developed, estimated, and analyzed. The following subsections

Results

Table 1 shows the number of observations and percentage distribution across injury severities for all drivers, male drivers, and female drivers, for all accident types considered. Note, because the random sampling was conditional on model type the frequency distribution across rows in Table 1 does not represent the universe. This has no effect on the models because they are conditioned in the same way and we account for this in our tests of joint male/female models. The descriptive statistics

Conclusions

The estimation results show there are significant differences between males and females with regard to how various factors affect injury severity. Differences in levels of significance, magnitude and even the direction of the effect that individual variables have on driver-injury severity are observed between male and female drivers. It is not surprising that differences in the magnitude of effects are found between the genders. We show where these differences are and their magnitude. What is

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