Conclusion

Researchers have suggested that the ABS anomaly might be explained by the risk compensation (Evans, 1995; Evans and Gerrish, 1996) or by improper operation of ABS-equipped vehicles (Kahane, 1994).  We find evidence strongly supporting the latter.  Specifically, we show that the ABS anomaly is confined largely to drinking riskier drivers.  We apply the Farmer et al. (1997) risk-ratio analysis and find that 1993-1995 fatal crash involvements among drinking drivers in 1992 model year ABS-equipped GM vehicle lines are 64% higher than expected based on exposures of the vehicle lines and the number of drinking drivers in fatal accidents in the pre-ABS versions of the vehicles. In the same years we find that there is a significant reduction in fatal crash involvements for sober drivers—11% fewer fatal crash involvements for the later GM vehicles.  Similar conclusions hold for another set of vehicle lines that adopted ABS as early as the 1985 model year; we find sober drivers continue to have more fatal accident involvements than expected (significantly more involvements resulting in the death of an ABS vehicle occupant) but there is again a striking difference between drinking and sober drivers.

Farmer (2001) and Hertz (2000) suggest that the prospects for antilock brakes are somewhat better given the evidence of attenuation of the ABS anomaly.  For example, Farmer (2001) finds that the GM vehicle lines adopting ABS in 1992 were no longer overinvolved in fatal accidents resulting in the death of an ABS vehicle occupant during the years 1996-1998. We show that the ABS anomaly is related to years of vehicle service, not calendar years:  the attenuation effect is evident in the other set of vehicle lines that adopted antilock brakes as early as 1985. We find that drinking drivers in ABS-equipped vehicles have risk ratios significantly greater than one for the first three to four years of service, and then the risk ratios fall near one.  It is likely that the most important reason for this phenomenon may be increased driver skill with ABS after successive years of vehicle operation.  But risk ratios may also be lower because some drivers who are most prone to taking further risks while driving under the influence of alcohol are out of the sample after the first four years of service: they are dead or no longer in the sample vehicles having had already a serious accident in their ABS-equipped vehicle.  We conclude that the attenuation in the ABS anomaly reported by Farmer (2001) and Hertz (2000) should not lead us to conclude that vehicles adopting ABS in years after 1992 were not subject to these same problems in the first years of vehicle service.

We also find that the changes in fatal accident involvement rates after ABS adoption occur mainly for riskier drivers.  Riskier drivers are also the most likely to drink and drive and hence suffer a much higher involvement rate in ABS-equipped vehicles, but sober riskier drivers have significantly lower accident involvement rates (to the benefit of ABS vehicle nonoccupants).  We conjecture that drinking drivers incur these deadly problems in ABS-equipped vehicles because of their reaction times are dulled and perhaps also because of greater adherence to old habits.  Arguing that older drivers are more likely to suffer these same problems, we find support for our conjecture comparing risk ratios for sober drivers over age 65 to younger drivers.

An immediate implication of our paper concerns the design and implementation of new safety equipment.  Given the extraordinary influence of alcohol use on traffic fatalities, should not active safety appliances be designed and tested considering that the driver may well use, perhaps for the first time, such equipment while under the influence of alcohol?   We find no indication that this question has been considered in the design and implementation of antilock brake systems.  Even the most recent simulator and track tests (McGehee et  al., 2000), while emphasizing the importance of reaction times, fail to consider the impact of alcohol use on driving performance with ABS.