Evaluation of Rear Window Defrosting and Defogging Systems

Christina Morgan

Rear window defrosting and defogging systems are not required on motor vehicles by any Federal standard. Rear window defoggers became available as optional or standard equipment in most cars during the 1970's or 1980's and are popular with consumers. Today, almost all passenger cars, minivans, and sport utility vehicles have rear window defoggers, but most pickup trucks and full-size vans do not.

The analysis examined whether there were proportionately fewer backing-up and changing-lane crashes involving cars with rear-window defoggers than cars without rear-window defoggers. The database was extracted from State crash files. The analyses did not show a benefit for rear window defoggers. The main analysis found that rear window defoggers have no effect on changing lane and backing crashes in conditions when they are most likely used (when raining or snowing, during the earlier part of the morning, or during winter).

Even though the statistical analyses did not show a significant reduction in actual crashes, we would expect most drivers to like rear-window defoggers because they are quite convenient, and improved rearward vision helps a driver feel more mobile and secure when changing lanes or backing up.

Rear window defrosting and defogging systems, hereafter referred to as rear window defoggers, allow the driver to see through the rear window under adverse weather conditions. They get rid of condensation, frost, ice, and/or snow on the back windows. They are most likely to be used if any one or more of the following occur: it is snowing or raining, during the earlier part of the morning or during winter. A clear window will obviously help a driver who is backing up, and can also help a driver see if it is safe to change lanes.

Rear window defoggers are not required on motor vehicles by any Federal standard. Rear window defoggers became available as optional or standard equipment in most cars during the 1970's or 1980's and are popular with consumers. Today, almost all passenger cars, minivans, and sport utility vehicles have rear window defoggers, but most pickup trucks and full-size vans do not.

The analysis examined whether there were proportionately fewer backing-up and changing-lane crashes involving cars with rear-window defoggers than cars without rear-window defoggers. The basic analytical method was to estimate the overall reduction of rear-impact involvements to cars that had been backing or changing lanes just before the crash relative to a control group of rear-impact involvements to cars that were stopped prior to the crash in adverse conditions when defoggers are more likely to be used.

The analysis databases were initially extracted from State crash files of Florida, Maryland, Michigan and Pennsylvania. Ward’s Automotive Yearbooks supplied information on the proportion of vehicles equipped with defoggers, by make-model and model year. However, we had no information whether specific, individual crash-involved vehicles were equipped with defoggers. The initial databases included 321,118 rear-impact cases from Florida (1986-1999), 74,725 cases from Maryland (1986-2000), 321,826 cases from Michigan (1981-1991) and 182,460 cases from Pennsylvania (1981-2000). But the analysis was limited to the Florida and Michigan data, because the Pennsylvania and Maryland data did not have an adequate number of changing lane and backing-up cases.

Number Of Cases in Initial Databases By Crash Type And State

It was hard to detect a reduction in backing and changing lanes crashes, since rear window defoggers were gradually introduced into the cars. For example, there was not a single high-sales make-model that went from zero defoggers in one model year to 100 percent in the next, or even had close to such a jump. Logistic regression analyses were used to calibrate the ratio of backing and changing-lane crashes to control-group crashes as a function of the percentage of vehicles with defoggers, and also to control for other factors related to the driver, vehicle, crash and environmental circumstances that could affect the mix of relevant to non-relevant crashes and/or increase or decrease the effect of defoggers.

We were unable to conclude that rear window defoggers reduce crashes. The analyses did not show a benefit for rear window defoggers. The main analysis found that rear window defoggers have no effect on changing lane and backing crashes in conditions when they are most likely used (when raining or snowing, during the earlier part of the morning, or during winter).

Even though these data do not show a statistically significant crash reduction, 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.

It should be noted that there are a large unknown number of backing up crashes of minor or no damage that are not reported to the police. For example, a driver backs out of the driveway and hits a pole, mailbox, or a neighbor's vehicle. It is possible that rear window defoggers are effective in reducing these unreported crashes. However, we have no data to evaluate these unreported crashes.