shows the evolution of the number of drivers killed in single-vehicle crashes by gender. As expected, the number of fatalities among male drivers involved in single-vehicle crashes is much larger than that for female drivers. Also as expected, shows that the total number of fatal crashes in which male and female drivers are involved tend to follow an upward curve, reflecting the constant incorporation of new drivers to American roads. However, the annual number of male fatalities in this type of crash decreased until the early 1990s, when it resumed its growth. To some extent, the shape of this curve could be explained by the association between single-vehicle crashes and drinking and driving, particularly at nighttime (Heeren et al., 1985
). Impaired driving and the prevalence of alcohol-related crashes in the country were declining for almost 30 years until the early 1990s, when such a decline stalled (e.g., Stewart et al., 2004
). shows the number of male drivers killed with a BAC≥.08 following precisely such a pattern of early decrease and then stalling. It is very likely that the initial decline in alcohol-related crashes translated into a decline in the number of single-vehicle crashes. Once the reduction in impaired driving leveled off, the number of single-vehicle crashes resumed its growth. In an attempt to find more evidence about the association between the decline in impaired driving and the evolution of the number of male drivers shown in , we re-estimated that curve but for daytime single-vehicle crashes only (i.e., between 6 a.m. and 6 p.m.), when the prevalence of impaired driving is minimal compared to nighttime driving. Although not shown because of space limitations, the “dip” in the curve for male drivers shown in almost disappeared when such a curve was based on daytime crashes only.
Number of drivers killed in single-vehicle crashes by alcohol involvement and gender, 1982–2006
Interestingly, both the curve showing the annual number of female fatalities and the curve showing female involvement in BAC≥.08 in , albeit lower than those for males, do not show the “dip” in the curves for male drivers. This result indicates that the role of alcohol on the occurrence of single-vehicle crashes is much less significant among female drivers than among male drivers.
An alternative and illustrative way to display the curves shown in is to make them relative to the year 1982, as Voas and Hause (1987)
did in their study of nighttime crashes. shows how the curves in evolve from their initial 1982 value and that the number of males involved in BAC≥.08 crashes has decreased from its initial 1982 value down to .63 times that value in the early 1990s, but it has stalled since. Further, the total number of male drivers involved in single-vehicle crashes, despite an early dip, has returned to the 1982 counts and beyond.
Evolution of the ratio of the annual number of drivers killed in single-vehicle crashes in 1982 by alcohol involvement and gender, 1982–2006
Perhaps the most illustrative aspect of involves sober drivers, females in particular. shows clearly that while the number of BAC≥.08 fatal crashes has decreased or stalled since 1982 for both male and female drivers, the number of “sober” (BAC=.00) fatal crashes has continuously grown for both of them, but such a growth was larger for female drivers (p<.001) than for male drivers (the curve for “sober female drivers” tops all the others in ).
Comparisons between the involvement of female and male drivers in fatal single-vehicle crashes are further shown in , which depicts the evolution of the CIR/RAIR for the same groups of drivers shown in and . As explained in the previous section, our analytical strategy looked first at estimating the evolution of the overall CIR/RAIR for each year (CIR/RAIRoverall) and then comparing the resultant curve with that for the risk-related driving conditions under study (improper maneuvering, speeding, drinking and driving, and seatbelt nonuse). shows that CIR/RAIRoverall, the odds (compared to males) of female involvement in single-vehicle fatal crashes each year sharply increased until the mid-1990s, when they were more than 1.6 times those of 1982. This sharp increase coincides with all reports showing an increased involvement of female drivers in crashes. Since then, however, the overall odds of female involvement reversed the trend and started to decline slightly, although the 2006 crashes remained about 1.4 times that of 1982.
Evolution of the annual CIR/RAIR statistic representing the odds of female drivers involvement in fatal single-vehicle crashes by type of crash, 1982–2006
, when comparing the curves of the risk-related driving conditions against the overall CIR/RAIRoverall, reveals two types of CIR/RAIR curves. One group of curves includes those for improper maneuvering, and speeding. At the 1% alpha significance criterion (i.e., p≤.01), the shape of these curves (i.e., their year by driving condition interaction term) did not differ significantly from that for CIR/RAIRoverall. The second group includes the CIR/RAIR for the drinking-and-driving and the seatbelt nonuse crash conditions: CIR/RAIRBAC>.00, CIR/RAIRBAC≥.08, and CIR/RAIRseatbelt, respectively. Also under the same 1% alpha criterion, this second group of curves was statistically different (flatter) than for the CIR/RAIRoverall and for the first group. In other words, under this criterion, the odds of female involvement in single-vehicle fatal crashes involving speeding or improper maneuvering have increased over time, but not more than the overall increase in crash involvement of female drivers during that period. On the other hand, the odds of female involvement in alcohol-related or seatbelt nonuse single-vehicle fatal crashes increased less than those for the other types of crashes.
At the 5% alpha significance criterion (i.e., p≤.05) however, the shape of all driving condition curves was statistically different (i.e., at this significance level, the shape of all curves were found significantly different from that for CIR/RAIRoverall). When we re-run our analysis, including a dummy (1,0) variable to account for the observed change in the mid-1990s (i.e., a value of 0 indicated the 1982–1992 period and a value of 1, the 1993–2006 period), we obtained a 1% significance of the interaction term for BAC>.00, BAC≥.08, and seatbelt nonuse, but the other two driving conditions were nonsignificant (p values of .12 for each of them).
Next, we explored if the odds of female involvement vary by age group, with special focus on underage drivers. displays the CIR/RAIRoverall curve (also shown in ) and CIR/RAIR15–20, CIR/RAIR21–24, and CIR/RAIR25+, the curves for drivers aged 15 to 20, 21 to 24, and 25 and older. shows that the curve for drivers aged 25 and older closely follows the CIR/RAIRoverall curve. This was expected as most drivers in the file fall in that age group. For the younger drivers, the picture is different. For those aged 21 to 24, the CIR/RAIR21–24 begins to level off after 1990. The CIR/RAIR15–20 curve keeps growing until 1996, when it levels off at a higher value. Statistical tests concur with this visual evaluation: interaction terms associated to these curves indicate that their shape is significantly different from that of the CIR/RAIR25 and overall curves (p<.001 for the general model; p<.05 for the above-described model with the dummy (1,0) variable to account for the observed change in the mid-1990s). Relevant to this study is the finding that since the 1990s, the odds of underage female involvement in single-vehicle fatal crashes was significantly higher than the overall increase in crash involvement of female drivers of all ages during that period (i.e., the main effect for CIR/RAIR15–20 after the 1990s was significantly higher (p<.01) than that for the other groups).
Evolution of the annual CIR/RAIR statistic representing the odds of female drivers being involved in fatal single-vehicle crashes by age group, 1982–2006
The relatively high vulnerability to crash risk by underage female drivers suggested in induced us to look more closely at this group. We therefore repeated the analysis shown in , but for underage drivers only. The resulting picture, depicted in , is not much different than the one shown in , except the curves in reach higher CIR/RAIR values. Statistical tests for the curves in also yield similar results to those applied to test the curves in . Whereas the shape of the improper maneuvering and speeding curves were not statistically different from that for CIR/RAIRoverall, the BAC and seatbelt nonuse curves were significantly different, in particular after the mid-1990s (p>.01).
Evolution of the annual CIR/RAIR statistic representing the odds of underage female drivers being involved in fatal single-vehicle crashes by type of crash, 1982–2006