Index | Table of contents | Technical Report | Executive Summary | Background | Rear Window Defrogger and Wiper Information | State Crash Data | Analysis Databases | Analysis Method | Hypotheses | Conclusions | Appendix A
The data do not show a benefit for rear window defoggers. The main analysis found that rear window defoggers have little or no effect on changing lane and backing crashes in conditions when they are most likely used (when it is raining or snowing, during the earlier part of the morning, or during winter).
Our inability to know whether a defogger was in use complicated but did not preclude our ability to measure their effectiveness. Our analytical approach accounts for this inability by comparing relevant to non-relevant crashes that are not affected by the use of rear window defoggers. For example, NHTSA’s evaluations of antilock brake systems and air bags used a similar approach (i.e., the data did not indicate if ABS was activated or an air bag deployed) and found statistically significant effects.
It may be that rear windows defoggers, although convenient and helpful for drivers, are not that essential for safety. Perhaps, when drivers do not have the defoggers, they somehow compensate for their absence by relying more on their outside mirrors, using their turn signals more, making their lane changes more gradual, or simply not backing up or changing lanes when they cannot see what is behind them. Whatever the reason, we have not shown that rear window defoggers significantly reduced changing lane and backing crashes. Nevertheless, we would still expect most drivers to like rear window defoggers because they improve rearward vision. They can reduce or eliminate the need to scrape the rear window from outside the vehicle after a frost, a snowfall, or freezing rain, and these systems can eliminate the need for periodic stopping while driving to scrape accumulating snow or wipe condensation from the rear window. Even if drivers can successfully compensate for the lack of defoggers by avoiding lane changes and backup moves unless they are absolutely necessary, they would undoubtedly feel more confident, mobile, and secure if they had defoggers to give them a better view of the traffic behind them.