U.S. Department

of Transportation http://www.nhtsa.dot.gov 

National Highway

Traffic Safety 

Administration

DOT HS 809 724 March 2004 

NHTSA Technical Report

Evaluation of Rear Window Defrosting and 

Defogging Systems 

This document is available to the public from the National Technical Information Service, Springfield, Virginia 22161.
The United States Government does not endorse products or 

manufacturers. Trade or manufacturers’ names appear only because 

they are considered essential to the object of this report. 
 Technical Report Documentation Page 

1. Report No. 

DOT HS 809 724 

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

March 2004

4. Title and Subtitle 

Evaluation of Rear Window Defrosting and Defogging Systems 

6. Performing Organization Code 

7. Author(s) 

Christina Morgan 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Evaluation Division; Office of Planning, Evaluation and Budget 

National Highway Traffic Safety Administration 

Washington, DC 20590

11. Contract or Grant No. 

13. Type of Report and Period Covered 

NHTSA Technical Report

12. Sponsoring Agency Name and Address 

Department of Transportation 

National Highway Traffic Safety Administration 

Washington, DC 20590

14. Sponsoring Agency Code 

15. Supplementary Notes 

 

16. Abstract 

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. 

17. Key Words 

rear window defrosting and defogging systems; statistical 

analysis; evaluation; State crash files 

18. Distribution Statement 

19. Security Classif. (Of this report) 

Unclassified

20. Security Classif. (Of this page) 

Unclassified

21. No. of Pages 

 49

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

TABLE OF CONTENTS 

TABLE OF CONTENTS..........................................................................................................iii

EXECUTIVE SUMMARY........................................................................................................ v

BACKGROUND ....................................................................................................................... 1 

REAR WINDOW DEFOGGER AND WIPER INFORMATION ............................................ 1 

STATE CRASH DATA............................................................................................................. 9 

ANALYSIS DATABASES ..................................................................................................... 10 

ANALYSIS METHOD............................................................................................................ 14 

HYPOTHESES – ..................................................................................................................... 18

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES IN THE 

PRESENCE OF ANY ONE OR MORE CONDITIONS WHEN THEY ARE MOST

LIKELY USED (but the effect does not increase if there is more than one adverse 

condition) and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

HYPOTHESES – ..................................................................................................................... 24 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES AND THE 

EFFECTS ARE INDEPENDENT AND ADDITIVE FOR THE THREE ADVERSE

CONDITIONS and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

HYPOTHESES – ..................................................................................................................... 25 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES AND THE 

EFFECTS ARE EQUAL AND ADDITIVE FOR THE THREE ADVERSE 

CONDITIONS and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

CONCLUSIONS...................................................................................................................... 26 

APPENDIX A.......................................................................................................................... 29 

iii 
iv
EXECUTIVE SUMMARY 

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 changinglane 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

State 

Changing Lane & 

Backing Crashes Stopped Crashes Total 

Michigan 71,668 250,158 321,826

Florida 44,761 276,357 321,118

Pennsylvania 7,791 174,669 182,460

Maryland 13,121 61,604 74,725

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 highsales make-model that went from zero defoggers in one model year to 100 percent in the next, 

v 
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. 

vi
 

BACKGROUND 

FMVSS 103 requires all passenger cars, multipurpose passenger vehicles, trucks and buses 

manufactured on or after January 1, 1968, to have windshield defrosting and defogging 

systems. However, rear window defrosting and defogging systems, hereafter referred to as 

rear window defoggers, are not required. 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. Rear window defoggers allow the driver to see through the rear window under 

adverse weather conditions. Clear vision is especially important when the driver wants to 

back up or change lanes. 

FMVSS 104 requires all passenger cars, multipurpose passenger vehicles, trucks and buses 

manufactured on or after January 1, 1968, to have windshield wiping and washing systems. 

Similar to rear window defoggers, rear-window wiping and washing systems are not required. 

Rear window wipers have been installed on a smaller group of vehicles for even better vision. 

This study assesses the effectiveness of rear window defoggers in reducing crashes in which a 

driver is backing up, changing lanes, or performing other tasks that would be facilitated by 

vision through the rear window. Typically, studies to assess the effectiveness of a vehicle 

safety standard consist of an analysis of crash data that compare the experience of vehicles 

that meet the safety standard to those that do not meet the standard. For most evaluation 

studies, we can identify exactly what vehicles were equipped with a safety technology, and 

which ones were not, based on the model year (e.g., before/after a standard’s effective date, or 

before/after a date when the technology became standard equipment) or dedicated VIN 

characters (e.g., air bags), or specific mention in the crash data (e.g., child safety seats). 

However, rear-window defoggers and wipers have never been required safety equipment and 

were offered as optional equipment, for many years, on quite a few make-models. The 

presence of rear-window defoggers and wipers generally cannot be deduced from the first 12 

characters of the VIN, and it is rarely (if ever) mentioned on crash data files. Therefore, it is 

necessary to obtain information on the proportion of vehicles with rear-window defoggers or 

wipers from another source. 

REAR WINDOW DEFOGGER AND WIPER INFORMATION 

Rear window defogger and wiper information is published yearly in the Ward’s Automotive 

Yearbook1

. Ward’s Yearbook lists the percentage of factory-installed equipment on 

passenger vehicles by manufacturer and model. Each yearbook contains a list of factoryinstalled equipment for the previous years’ production of vehicles. For example, Ward’s 

Automotive Yearbook 2002 contains a list of factory-installed equipment for vehicles 

produced in calendar year 2001. Ward’s groups the information about factory-installed 

equipment into four different vehicle groups: Domestic Cars, Imported Cars, Domestic Light 

Trucks, and Imported Light Trucks. Light trucks include all pickup trucks, vans, and Sport 

Utility Vehicles (SUVs). Table 1 shows the availability of rear window defogger and rear 

window wiper information contained in Ward’s Automotive Yearbooks by the four vehicle 

groups. 

1

 This book is published by Ward’s Communications, Inc. 

1 
Table 1 

Availability of Rear Window Defoggers and Wipers by Vehicle Group

Vehicle Group Rear Defoggers Rear Wipers

Domestic Cars 1973-later 1983-later

Imported Cars 1975-1998 1983-later

Domestic Light Trucks 1987-later 1987-later

Imported Light Trucks 1976-later 1983-later

Domestic cars include all cars produced in the U.S., Canada and Mexico for the U.S. market. 

They include some models sold by foreign-based manufacturers but assembled in North 

America. For example, the Honda Accord and Civic have been assembled in the U.S. since 

1987 and are listed with domestic cars since 1987. Similarly, the domestic light trucks 

(pickups, vans and SUVs) include all light trucks produced in North America for the U.S. 

market, even some models sold by foreign-based manufacturers. For example, the Toyota 

Tundra, Nissan Quest, and the Honda Odyssey are a few of the models sold by foreign-based 

manufacturers included in the domestic light truck list. 

Imported cars and imported light trucks include all vehicles imported for U.S. sales. 

Consequently, this can include some models by domestic-based manufacturers assembled 

abroad. For example, the Geo Storm, Ford Aspire, and Cadillac Catera are included in the 

imported car list. 

Ward’s classifies factory-installed equipment by calendar year of production. This is not 

exactly the same as model year, since some new model year vehicles are usually produced 

and introduced in the fall of the previous year. For example, vehicles produced in calendar 

year 2001 will include mostly 2001 model year vehicles and a small portion of 2002 model 

year vehicles. In this report, the calendar year production numbers are being used to represent 

the model year numbers (i.e., calendar year 2001 numbers are being used to represent model 

year 2001 model year numbers). This adds a little error to the analysis, but it is the only 

information available on rear window defoggers. 

 

Ward’s Automotive Yearbook reports the combined percentage of rear window defoggers that 

blow air on the rear window and that electrically heat the rear window for 1976 model year 

domestic cars. For 1977-1981 model year domestic cars, Ward’s separately reports the 

percentage of rear window defoggers that blow air on the rear window and that electrically 

heat the rear window. It appears that rear window defoggers that blew air on the rear window 

were discontinued after 1981. The only 1981 model year domestic car with rear window 

defoggers that blew air was the Chevrolet Caprice. Only 1.6 percent of Chevrolet Caprice had 

rear window defoggers that blew air, but 44.3 percent of them had rear window defoggers that 

electrically heated the rear window. Since the percentage reported was a combination of both 

types of rear window defogger in 1976, the percentage used in the analysis for model year 

2 
1977-1981 will be the combination of both types. For 1982 model year domestic cars, Wards 

also reports the percentage of factory installed rear window washers. 

Today, almost all new cars and imported light trucks sold in the U.S. have rear window

defoggers, but only about half of the domestic light trucks have rear window defoggers. 

Table 2 shows the percentage of factory-installed rear window defoggers by model year and 

vehicle group. 

Table 2 

Percentage Factory-Installed Rear Defogger By Model Year And Vehicle Group 

Model Year

Domestic 

Cars

Imported 

Cars

Domestic 

Light 

Trucks 

Imported 

Light 

Trucks 

1973 16.4%

1974 21.5%

1975 25.7% 72.1%

1976 29.9% 67.4% 0.6%

1977 35.0% 82.4% 0.6%

1978 39.1% 88.0% 0.0%

1979 47.5% 87.6% 0.0%

1980 50.5% 86.1% 0.1%

1981 50.9% 97.6% 0.0%

1982 55.9% 99.5% 0.0%

1983 59.0% 97.2% 6.1%

1984 63.1% 97.5% 11.9%

1985 74.0% 95.2% 11.6%

1986 71.7% 91.8% 13.4%

1987 73.6% 89.9% 17.3% 18.6%

1988 81.7% 87.2% 19.2% 19.2%

1989 83.3% 90.4% 17.0% 29.1%

1990 81.2% 91.2% 25.6% 31.9%

1991 88.6% 87.8% 31.5% 47.8%

1992 90.8% 96.0% 32.5% 44.9%

1993 91.9% 92.0% 37.3% 56.5%

1994 93.3% 93.1% 35.5% 65.0%

1995 94.1% 94.4% 40.2% 71.2%

1996 94.3% 95.7% 47.2% 88.4%

1997 94.7% 96.2% 40.6% 93.7%

1998 95.4% 99.9% 50.8% 98.4%

1999 95.2% 48.8% 100.0%

2000 96.9% 50.0% 99.6%

2001 94.2% 54.0% 99.7%

3 
The percentage of new vehicles sold in the U.S. with factory-installed rear window defoggers 

has steadily increased since Ward’s started reporting rear window defoggers. For domestic 

cars, the rate has gone from 16 percent for model year 1973 cars to 94 percent of model 2001 

cars. For imported cars, the rates have steadily increased like those of domestic cars, but are 

generally higher than those of domestic cars until recently, when almost all new cars sold in 

the United States are equipped with rear window defoggers. The rates for light trucks have 

some of the same general trends as cars. The rates have steadily increased like the car rates 

and the rates for imported light trucks are higher than those of domestic light trucks. The 

main difference between the car and light truck rates is that the domestic light truck rates do 

not catch up to the imported light truck rates in recent model years. Almost all imported light 

trucks have rear window defoggers, but only 54 percent of domestic light trucks have rear 

window defoggers. 

Table 3 shows the percentage of factory-installed rear window defoggers by model year for 

six make-models of cars with high sales volumes. 

In general, the percentage of factory-installed rear window defoggers on models by domesticbased manufacturers have steadily increased by model year, but the percentage on models by 

foreign-based manufacturers was high in the late 1970’s and has remained high. Most of the 

time the percentage of rear window defoggers increases slightly from one year to the next for 

each make-model. 

The percentage has never increased from 0 to 100 percent in consecutive model years for any 

make-model, but in several make-models the percentage has had a big positive jump in 

consecutive model years. The Ford Tempo, Ford Crown Victoria, and Honda Civic shown in 

Table 3 have had large jumps in the percentage of rear defoggers in consecutive model years. 

4 
Table 3 

Percentage Of Factory-Installed Rear Window Defoggers 

By Model Year For Six High-Sales Make-Models 

Model 

Year 

Chevrolet 

Cavalier 

Ford 

Tempo 

Ford Crown 

Victoria 

Honda 

Civic 

Toyota 

Corolla 

Domestic 

Toyota 

Corolla 

Imported 

1975 11.7% 95.0%

1976 12.9% 85.5%

1977 20.0% 85.0%

1978 91.0% 84.5%

1979 37.6% 87.1% 67.4%

1980 39.1% 70.6% 100%

1981 61.2% 79.6% 100%

1982 90.4% 64.5% 100% 100%

1983 51.1% 63.3% 75.3% 90.0% 100%

1984 49.8% 50.6% 63.3% 95.0% 100%

1985 47.9% 53.4% 80.1% 89% 100%

1986 55.2% 60.4% 80.9% 84.0% 100%

1987 62.0% 64.2% 86.7% 100%* 0%

1988 66.5% 68.6% 89.3% 100%* 100%

1989 61.1% 94.9% 87.5% 100%* 100% 100%

1990 61.0% 97.1% 89.4% 100%* 94.0% 100%

1991 70.1% 92.3% 94.0% 100%* 100% 100%

1992 66.8% 90.3% 95.3% 100%* 100% 100%

1993 73.1% 88.5% 97.1% 100%* 100% 100%

1994 75.2% 100% 97.7% 100%* 62.0% 62.3%

1995 77.4% 100% 100%* 68.0% 62.5%

1996 75.5% 100% 100%* 74.0% 78.0%

1997 80.1% 100% 100%* 75.0% 98.0%

1998 78.9% 100% 100%* 74.7%

1999 85.3% 100% 100%* 79.0%

2000 91.8% 100% 100%* 87.0%

2001 100% 100% 100%* 46.8%

 * Percentage for both domestic and imported Civics 

Table 4 shows the biggest positive jumps for several high-sales make-models, ordered by 

amount of gain. The Honda Civic had one of the largest positive jumps by any make-model. 

The percentage for rear window defoggers jumped 71 percentage points in the Civic from

20.0 percent in 1977 to 91.0 percent in 1978. About 30 make-models had an increase of 20 

percentage points or more in one year. In the Tempo, the percentage of rear window 

defoggers jumped from 68.6 percent in 1988 to 94.9 percent in 1989. In the Grand Marquis, it 

jumped from 58.0 percent in 1980 to 83.6 percent in 1981. In the Crown Victoria, it jumped 

from 39.1 percent in 1980 to 61.2 percent in 1981. About 100 make-models had an increase 

of 10 percentage points or more in consecutive years. Many of the high-sales foreign make5 
models had rear window defoggers as standard equipment when these models were first 

introduced. The Honda Accord, Nissan Sentra, and Toyota Camry were introduced with rear 

window defoggers as standard equipment. 

Table 4 

Biggest Positive Jumps For Several High-Sales Make-Models

 

Make-Model Model year Percentage Model Year Percentage Gain 

Honda Civic 1977 20.0% 1978 91.0% 71.0%

Ford Tempo 1988 68.6% 1989 94.9% 26.3%

Mercury Grand Marquis 1980 58.0% 1981 83.6% 25.6%

Ford Crown Victoria 1980 39.1% 1981 61.2% 22.1%

Ford Escort 1985 42.2% 1986 60.0% 17.8%

Chevy Celebrity 1986 64.5% 1987 79.5% 15.0%

Chevy Chevette 1977 14.0% 1978 28.4% 14.4%

Pontiac Grand Am 1993 78.0% 1994 89.3% 11.3%

Chevy Cavalier 1990 61.0% 1991 70.1% 9.1%

Ford Taurus 1990 91.5% 1991 100% 8.5%

Olds Ciera 1990 74.5% 1991 82.8% 8.3%

Toyota Corolla 1999 79.0% 2000 87.0% 8.0%

Chevy Caprice 1985 63.5% 1986 70.5% 7.0%

Table 5 shows the percentage of factory-installed rear window defoggers by model year for 

eight popular make-models of light trucks. In general, the percentage of minivans and sport 

utility vehicles with rear window defoggers is much larger than pickup trucks. Recall in 

Table 2 that the rates for imported light trucks were generally higher than those of domestic 

trucks, but domestic light trucks did not catch up to the imports (Table 2). In other words, the 

difference in percentage of domestic and imported light trucks with rear defoggers in recent 

model years is larger than earlier model years. The difference appears to be caused by: 

• Almost all recent model years of minivans and SUVs both domestic and imported 

have rear window defoggers, 

• Very few recent model years of pickup trucks and full-sized vans have rear window 

defoggers, and 

• A shift to assemble all pickup trucks in North America, even pickup trucks by 

foreign-based manufacturers. 

This causes the overall percentage of rear window defoggers for imported light trucks to be 

much larger than that of domestic light trucks. In model year 2001, all imported light trucks 

had rear window defoggers but only half of the domestic light trucks had them. 

6 
Table 5 

Percentage Of Factory-Installed Rear Window Defoggers 

By Model Year For Eight Popular Light Truck Make-Models 

Pickup Trucks Minivans Sport Utility Vehicles 

Model 

Year 

Chevrolet 

C/K 1500 

Silverado 

Ford 

F150 

Sierra 

Toyota 

Pickup - 

Tacoma 

Dodge 

Caravan

Chevrolet

Lumina - 

Venture 

Ford 

Explorer

Chevrolet 

Blazer 

“S” 

Toyota 

4Runner 

1987 3.4% 0% 0% 67.4% 61.5% 66.8%

1988 2.6% 0% 0% 75.9% 55.9% 82.4%

1989 1.5% 0% 0% 79.5% 0% 100%

1990 1.3% 0% 0% 86.1% 51.4% 0%

1991 1.4% 0% 0% 93.1% 72.5% 83.7% 78.5% 97.0%

1992 0.8% 0% 0%* 89.1% 78.9% 96.7% 86.4% 96.0%

1993 1.0% 0% 0%* 86.9% 74.5% 98.3% 93.9% 96.9%

1994 1.6% 0% 0%* 87.3% 92.2% 99.0% 96.8% 99.0%

1995 2.5% 0% 0%* 85.8% 94.0% 99.3% 100% 99.0%

1996 2.5% 0% 0% 92.5% 92.6% 99.3% 100% 97.0%

1997 2.7% 0% 0% 92.1% 97.6% 99.4% 100% 98.0%

1998 2.9% 0% 0% 93.8% 97.8% 92.8% 97.1% 100%

1999 3.0% 0% 0% 89.0% 100% 100% 98.0% 100%

2000 12.6% 0% 0% 92.1% 100% 100% 96.7% 100%

2001 18.4% 13.6% 0% 93.4% 100% 100% 97.2% 100%

* Percentage for both domestic and imported 

Table 6 shows the percentage of factory-installed rear window wipers by model year and 

vehicle group. The percentages of factory-installed rear window wipers for domestic cars are 

fairly consistent for the last 20 model years. The percentages for imported cars are also fairly 

consistent but only for the last 10 model years. The rates for imported cars are also slightly 

larger than those of domestic cars, but neither of the rates for domestic or imported cars is 

very large. 

By contrast, the percentage of new domestic and imported light trucks with factory-installed 

rear window wipers has greatly increased since Ward’s started reporting rear window wipers 

and are much larger than those for cars. Also note that the percentages of light trucks with 

rear window wipers are very similar to the percentages of light trucks with rear window 

defoggers. In general, almost all light trucks with rear window defoggers also have rear 

window wipers. For model year 2001 light trucks, almost all minivans and SUVs had rear 

window wipers, but no pickup trucks or full-sized vans had rear window wipers. 

7 
Table 6 

Percentage Factory-Installed Rear Wipers By Model Year And Vehicle Group 

Model Year

Domestic 

Cars

Imported 

Cars

Domestic 

Light 

Trucks 

Imported 

Light 

Trucks 

1982 3.2%

1983 2.6% 33.3% 2.6%

1984 3.0% 31.4% 8.8%

1985 3.5% 25.2% 13.2%

1986 4.6% 27.8% 10.9%

1987 5.0% 34.8% 10.6% 8.8%

1988 4.2% 23.0% 16.3% 11.7%

1989 2.8% 17.2% 18.2% 26.5%

1990 4.8% 15.2% 24.4% 21.2%

1991 3.6% 14.1% 27.3% 40.6%

1992 3.8% 11.6% 32.1% 44.9%

1993 5.7% 13.3% 38.4% 55.2%

1994 4.4% 11.1% 35.3% 63.5%

1995 5.1% 9.7% 38.6% 70.2%

1996 2.8% 11.6% 44.6% 86.7%

1997 4.3% 10.3% 37.8% 91.9%

1998 2.8% 10.1% 49.4% 94.9%

1999 2.8% 11.6% 46.4% 94.7%

2000 5.4% 10.9% 47.6% 97.0%

2001 4.4% 13.1% 51.5% 98.0%

Rear window defoggers were already standard equipment on most make-models of minivans 

and SUVs when these models were introduced, so the sample of vehicles without rear window 

defoggers is too small for meaningful results. Conversely, defoggers are so rare on pickup 

trucks and full-sized vans that the sample of pickup trucks and full-sized vans with rear 

defoggers is also too small for meaningful results. Similarly, except for those make-models 

that were always equipped with them, rear-window wipers are so rare that the samples of 

vehicles with the wipers will be too small for meaningful results. Therefore, the analysis 

estimates the effectiveness of rear-window defoggers for cars only, where there are numerous 

make-models that experienced a substantial or at least moderate year-to-year increase in the 

proportion equipped with defoggers. 

A rear-window defogger file was created with the necessary information needed from the 

Ward’s Automotive Yearbooks. The file contains the percentage of rear-window defoggers 

for cars by make, model, and model year for model years 1973-2001. 

8 
STATE CRASH DATA 

A large sample of crash-involved cars is needed to maximize the possibility of detecting any 

effect of rear-window defoggers. Defoggers may reduce crashes where the driver is backing 

up and hits something with the rear of the car or when the driver is changing lanes and the car 

gets hit in the rear. 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. The goal is to detect if there are 

proportionately fewer backing up and changing lane crashes involving cars with rear-window 

defoggers than cars without rear-window defoggers. Since about 50 percent of the model year 

1980 domestic cars had rear-window defoggers, the data needed to include a large sample of 

cars without rear-window defoggers – i.e., 1975-1985 model year vehicles. The data also 

needed to include specific make, model and model year of vehicles so that the percentage of 

factory-installed rear window defogger information available in Ward’s Automotive 

Yearbooks can be linked to the crash data. Therefore, the analyses require a large sample of 

crash data that includes vehicle maneuver (changing lanes or backing up vs. other), impact 

location (rear damage), and a large range of model years and VINs or other codes to identify 

the specific make, model, and model year of cars. State data files are the only data source 

available to NHTSA that can furnish an adequate number of cases for statistical analysis. 

Although the State data files are the best source of data available for this analysis, the State 

data files may not include all backing up crashes. Backing-up crashes that cause little or no 

property damage may not be included in a State’s crash file either because it was not reported 

to the police or it did not meet the State’s minimum-reporting threshold. (As we shall see 

later on, Pennsylvania has very few backing-up crashes.) 

The agency currently receives data from 17 States and maintains these data files for calendar 

years 1989 and onward for analysis. However, data from 13 States were not used in the 

analysis: North Carolina, because NHTSA does not have their files prior to 1992; California, 

Georgia, Illinois, Kansas, New Mexico, Ohio and Washington, because they do not specify 

the impact location (rear impact); Virginia, because it does not have vehicle make and model; 

Texas, because it does not have “changing lanes” as a possible vehicle maneuver; Missouri, 

Utah and Indiana, because earlier years of crash data were not easily available. Florida, 

Maryland, Michigan and Pennsylvania files were initially considered for the analysis. (As we 

shall see later on, only the Florida and Michigan data turned out to be suitable for the 

analysis.) 

Previous calendar years of crash data for these four States were obtained to supplement the 

years currently available at NHTSA. Design Research Engineering supplied 1986–1988 

Florida crash data files, the 1986-1988 Maryland crash data files, and the 1981-1988 

Michigan crash data. The Pennsylvania Department of Transportation supplied the 1980-

1988 Pennsylvania crash data files. Table 7 shows the calendar years of crash data used in the 

analysis by State. Michigan’s 1992-2000 data files could not be used in the analysis because 

the make, model and model year were not available. Florida’s 2000 data file was not used in 

the analysis because it does not contain low-severity non-injury crashes. Pennsylvania’s 1980 

file was not used because it would have contributed fewer than 50 cases to the main analysis. 

 

9 
Table 7 

Calendar Years Of Crash Data By State 

State Calendar years 

Florida 1986-1999

Maryland 1986-2000

Michigan 1981-1991

Pennsylvania 1981-2000

 

ANALYSIS DATABASES 

 

The analysis databases are combinations of the rear-window defogger file and the State crash 

files. Linking the rear-window defogger file to the State crash files created the analysis files. 

The files were linked by matching the vehicle make, model, and model year for cars as 

defined in Ward’s. In Florida, Maryland, and Michigan State data files, vehicle make and 

model was decoded from the Vehicle Identification Number (VIN); model year was not 

decoded from the VIN, but taken directly from the variable available on the State data files. 

In Pennsylvania for calendar years 1988-2000, make and model was decoded from the VIN; 

model year was not decoded from the VIN, but taken directly from the variable available on 

the State data file. For calendar years 1980-1987, very few VINs were included on the 

Pennsylvania data files, so make, model and model year were taken directly from the 

variables available on the State data files. The resulting analysis databases are vehicleoriented files, with one record for each car that was involved in a crash. Since the rearwindow defogger file only contains cars with 1973-2001 model years, the analysis files also 

only contain cars with 1973-2001 model years. The Michigan analysis file only has 1973-

1992 model years and Florida only has 1973-2000 model years. 

We believe rear window defoggers may have the potential to reduce crashes in which a driver 

was backing up or changing lanes before the crash. These tasks can be facilitated by vision 

through the rear window. Only cars with rear damage were included in the analysis. 

Consequently, rear impacts in which the vehicle maneuver was backing up or changing lanes 

were considered the only crashes that should be reduced as a result of rear window defoggers 

(i.e., treatment group). Vehicles with rear damage that were stopped prior to the crash were 

considered the control group. Since these vehicles were not moving at all and did nothing to 

cause the crashes, we can be relatively certain that rear-window defoggers would not have 

prevented them. Therefore, the critical parameters besides the presence or absence of rear 

window defoggers that must be defined in each State file are rear damage and vehicle 

maneuver (backing up and changing lanes vs. stopped). 

Every State has its own unique ways of coding a vehicle’s impact location. “Rear damage” 

cannot be defined exactly the same in each State, but at least the definitions can be made as 

similar as possible. Rear damage was defined to include any impact into the rear portion of 

the car where the driver might at least have had a chance to see the other vehicle behind it, 

either indirectly (via rear view mirror) or directly through the rear window. This includes 

rear-corner impacts and in some cases even impacts resulting in damage to the back portion of 

the side of the car, because sometimes when a car changes lanes into the path of another 

10 
vehicle traveling in the same direction damage to the side of the car is possible. Given this 

extended definition of “rear damage,” the following cars were included in the analysis: 

 Percent of Cars*

State Definition that have Rear Damage 

Florida Impact = 6, 7, 8, 9, 10 26.6 

Maryland 1986-1992 Impact = 7, 8, 9 26.7 

Maryland 1993-2000 Impact = 7, 8, 9, 10, 11,12 25.7 

Michigan Impact = 4, 5, 6 24.4 

Pennsylvania Impact = 4, 5, 6, 7, 8 20.8 

* 1973 – 2001 model year domestic cars with rear defogger information. 

Every State also has its own unique ways of coding “backing up,” “changing lanes,” and 

“stopped” maneuvers. Florida, Maryland, and Pennsylvania have a variable called Veh_man1 

that indicates the vehicle’s maneuver just prior to impact. Michigan has a “driver intent” 

variable that indicates the driver’s intended maneuver just prior to impact. The following 

shows the definition for “backing up,” “changing lanes” and “stopped” by State: 

State Maneuver Definition 

Florida Changing lanes Veh_man1 = 6 

Florida Backing Veh_man1 = 4 

Florida Slowing, stopped/stalled* Veh_man1 = 2 

Maryland Changing Lanes Veh_man1 = 7 

Maryland Backing Veh_man1 = 11 

Maryland Stopped in Traffic Lane Veh_man1 = 6 

Michigan Changing lanes Drintent = 3 

Michigan Backing Drintent = 11

Michigan Stopped on Road Drintent = 12 

Pennsylvania Changing Lanes to left and to right Veh_man1 = 2,3 

Pennsylvania Backing Up Veh_man1 = 1 

Pennsylvania Stopped Veh_man1 = 24 

*Includes some vehicles that were slowing rather than stopped. Both “slowing” and 

“stopped” vehicles may be considered part of the control group, since vision through the rear 

window is not an issue. 

11 
If rear window defoggers were effective, they would reduce crashes in which a driver is 

backing up and changing lanes – relative to the number that would have been expected if the 

cars had not been equipped with rear window defoggers. There is no direct way to count the 

crashes that were prevented, nor is there any way to determine if the rear window defogger 

was activated during the pre-crash maneuver. The basic approach was to study the change in 

proportion of crashes that were relevant, assuming that the presence or absence of defogger 

does not affect the occurrences of non-relevant crashes. The analytical methodology chosen 

for this study controls for some demographic characteristics of the drivers along with 

environmental and vehicle factors. 

Ideally, only make-models that did not offer rear window defoggers as optional equipment in 

an earlier model year but offered them as standard equipment on the next model year should 

be analyzed. But there were no make-models where this happened. Another ideal group to 

analyze would have been make-models that had a big increase in the percentage of defoggers 

in consecutive model years. But there were very few make-models that had an increase of 50 

percentage points or more and only about 30 make-models had an increase of 20 percentage 

points or more in consecutive model years. 

In order to have a large sample of cars, the sample included make-models with a large volume 

of crashes: make-models that had at least 500 cases in Pennsylvania where the cars were 

changing lanes, backing up or stopped just prior to impact and had rear damage. 

Pennsylvania was used because there were 20 calendar years of data. This ensured the sample 

of vehicles would include a wide variety of model years that have various percentages of 

defoggers. If Florida or Maryland had been used then the sample would have been mostly 

more recent model years that have a high percentage of cars with defoggers. If Michigan had 

been used then the sample would have been mostly earlier model years that had very few 

make-models with defoggers. 

But since only 1981-1991 CY of data were available in Michigan, the mix of cars with a large 

volume of crashes was different in Michigan than in Pennsylvania, so Michigan’s database 

included 34 additional make-models not included in the other States’ databases. These 

additional make-models had at least 750 crashes in Michigan. Appendix A contains a list of 

the make-models included in the analysis. The list includes for each make-model the 

percentage of rear window defoggers by model year and the number of crashes in each State. 

Appendix A also contains a list of additional make-models included in Michigan’s database. 

Table 8 shows the percentage of defoggers by model year for six make-models. Some makemodels that met the above criteria were excluded from the analysis because the percentage of 

defoggers jumped all over the place over the span of several model years. The fluctuating 

percentages are unusual, since for most cars the percentage gradually increases by model year. 

Intuitively, it appears that some of the reported percentages could be erroneous, but without 

another source of percentages it is hard to know. So these make-models were excluded from

the analysis. For example, the Chevrolet Camaro was excluded from the analysis for this 

reason (see Table 8). From model year 1982 to 1987, about 60 percent of Camaros had rear 

defoggers. Then the percentage fluctuated between model year 1988 and 1997. During that 

time, it dropped into the 50’s; plummeted to a low of 23.4 percent in model year 1990; rose to 

the 56.7, 62.3, and 78.4; and dropped again into the 60’s before it was consistently in the 70’s. 

12 
Table 8 

Percentage Of Rear Window Defoggers By Model Year For Some Make-Models 

 

Model 

Year 

Ford Escort 

(1981-90 

design) 

Chevrolet

Caprice 

(1977-96 

design 

Ford Crown 

Victoria 

(1979 - design)

Ford 

Taurus 

(1986-95 

design) 

Chevrolet 

Citation 

Chevrolet 

Camaro 

(1982 - design)

1977 32.0%

1978 34.5%

1979 40.6% 38.5% 43.7%

1980 45.8% 39.1% 45.8%

1981 36.5% 45.9% 61.2% 45.0%

1982 38.3% 50.1% 64.5% 47.2% 61.8%

1983 35.9% 57.0% 75.3% 53.4% 63.6%

1984 35.8% 60.7% 63.3% 63.6% 65.8%

1985 42.2% 63.5% 80.1% 61.1% 66.3%

1986 60.0% 70.5% 80.9% 91.1% 68.9%

1987 67.5% 76.7% 86.7% 89.2% 62.5%

1988 80.6% 72.6% 89.3% 93.2% 55.0%

1989 86.5% 77.1% 87.5% 89.7% 53.8%

1990 88.0% 39.4% 89.4% 91.5% 23.4%

1991 81.9% 94.0% 100% 56.7%

1992 87.6% 95.3% 99.6% 62.3%

1993 88.8% 97.1% 100% 78.4%

1994 91.7% 97.7% 100% 69.7%

1995 88.3% 100% 100% 63.6%

1996 98.2% 100% 65.4%

1997 100% 64.0%

1998 100% 78.8%

1999 100% 73.9%

2000 100% 73.2%

2001 100% 89.1%

Increase 51.5 66.2 61.5 8.9 17.4 27.3

 

 

Model years where the percentages were much higher or lower than expected relative to 

previous and subsequent percentages were excluded from the analysis. These data may be 

correct or erroneous, but since there is no way to know, they were excluded from the analysis. 

Model year 1990 for the Chevrolet Caprice was excluded from the analysis for this reason 

(see Table 8). The percentage of defoggers steadily increased from 32.0 percent in 1978 to 

98.2 percent in 1996 except for a severe drop to 39.4 percent in 1990. 

Table 9 shows the number of cases in the analysis databases by crash type and State. At this 

point, the analyses were limited to Florida and Michigan because they have larger samples of 

total cases and far more changing lane and backing cases. Maryland has fewer than 75,000 

13 
total cases. Although Pennsylvania has almost 175,000 total cases, only 4 percent are 

changing lane and backing cases. 

Table 9 

Number Of Cases in Analysis Databases By Crash Type And State

State 

Changing Lane & 

Backing Crashes Stopped Crashes Total 

Michigan 71,668 250,158 321,826

Florida 44,761 276,357 321,118

Pennsylvania 7,791 174,669 182,460

Maryland 13,121 61,604 74,725

ANALYSIS METHOD 

To detect if rear window defoggers are effective, we considered and tested several different 

hypotheses. The first is that rear window defoggers reduce relevant crashes all the time. In 

other words, just the presence of rear window defoggers reduces backing up and changing 

lanes crashes. This hypothesis is probably unrealistic since rear window defoggers only 

improve rearward vision under certain conditions: when drivers turn them on because there is 

condensation, frost, ice, and/or snow on the back window. Nevertheless, this hypothesis was 

tested. (In part, this hypothesis had to be included in each model for methodological reasons, 

as will be explained later on.) 

The second hypothesis is that rear window defoggers reduce relevant crashes when they are 

used. There is no way to determine if the rear window defogger was in use prior to the crash 

on the analysis databases, but we surmise that they are most likely used if any one or more of 

the following adverse conditions occurred: snowing or raining, during the earlier part of the 

morning, or during winter. 

A third hypothesis is that rear window defoggers have independent (and additive) effects of 

reducing relevant crashes under the three individual conditions when they are most likely 

used: raining or snowing, during early morning, or during the winter. This hypothesis 

suggests the overall effect of rear window defoggers is the sum of the effects when it is 

raining or snowing, during early morning, and during the winter. 

The fourth hypothesis is that the more adverse the conditions, the more effective the 

defoggers are at reducing relevant crashes but that the effect of each type of adverse condition 

is equal. In other words, when it is raining, morning, and winter, the effect of defoggers is 

three times as large as when just one of these conditions is present. This hypothesis is very 

similar to the third hypothesis in that is assumes that the effect is additive, but it differs in that 

it assumes each type of adverse condition has the same effect. 

This additive effect assumed by the third and fourth hypotheses is unlikely since there is 

probably not an additional benefit when there is more than one adverse condition. We believe 

rear window defoggers should have a benefit when they are being used and they will be used 

14 
if any one adverse condition is present. For example, if it is raining then they are most likely 

being used. We do not believe that they are even more likely to be used if it is raining and it 

is also during the early morning. The third and fourth hypotheses were also tested (for the 

sake of completeness) even though we believe the second hypothesis is the most reasonable. 

Logistic regression was used to estimate the effect of defoggers on the probability that the 

crash was relevant (as opposed to being a control group crash), while controlling for other 

factors. Estimating the impact of defoggers in reducing relevant crashes could be confounded 

by factors related to the driver, environment, vehicle, crash or other circumstances. To 

accurately estimate the impact of rear window defoggers, variables were included in the 

logistic regression to control for those factors, other than defoggers, which could influence the 

proportion of relevant crashes. For example, if rear window defoggers are in newer cars, or 

are more likely to be driven by older drivers than by other segments of the driving population, 

then driver and vehicle characteristics could confound estimating the impact of rear window 

defoggers. As a result, the age and sex of the driver, whether or not the crash occurred during 

adverse weather conditions (while raining or snowing when rear window defoggers are more 

likely to be used), whether or not the crash occurred during the morning or winter (also when 

rear window defoggers are more likely to be used), the age of the vehicle, vehicle makemodel, and calendar year of the data were chosen for inclusion in the logistic regression 

model. 

The following regression model was run on the Florida data set to test our second hypothesis 

– defoggers reduce relevant crashes in conditions when they are most likely to be used: 

MODEL CLBKSTOP = DEF_USED DEF USED ADWEA WINTER MORN VEHAGE 

VEHAGE2 CY86 CY87 CY88 CY89 CY90 CY91 CY92 CY93 

CY94 CY95 CY96 CY97 CY98 DRVMALE M14_30 M30_50 

M50_70 M70+ F14_30 F30_50 F50_70 F70+ CAVALIER1

ESCORT1...TERCEL;

CLBKSTOP is a flag that indicates whether the crash-involved car was changing lanes or 

backing (failures) or stopped (successes). All records where the crash-involved car was 

changing lanes or backing have CLBKSTOP = 1. All records where the crash-involved car 

was stopped have CLBKSTOP = 2.2

 

DEF is the percentage of rear window defoggers for that make-model in that model year. It is 

a continuous variable with values from 0 to 100. 

DEF_USED is an interaction variable that expresses the probability that the rear window 

defoggers were most likely being used prior to the crash. It has the value of DEF if it is 

snowing or raining, during the earlier part of the morning or during winter. DEF_USED = 

DEF * USED (defined below). For example, if 1989 Ford Taurus (89.7 percent had rear 

window defoggers) had a collision when it was raining, then DEF = 89.7 and DEF_USED = 

89.7. DEF_USED has the value of zero if it is unlikely that the rear window defogger was in 

use. DEF_USED = 0 when it is not snowing, not raining, not during the earlier part of the 

 2

 SAS/STAT® User’s Guide, Version 6, Fourth Edition, Volume 2, SAS Institute, Cary, NC, 1989, pp.1071-

1126. The LOGISTIC procedure in SAS prefers values of 1 for failures and 2 for successes. 

15 
 

morning and not during winter even if the car is equipped with rear window defoggers. For 

example, if a 1989 Ford Taurus had a crash that occurred on a bright, sunny afternoon in May, 

then DEF_USED = 0. DEF_USED also equals zero when the car does not have rear window 

defogger even if it is snowing or raining, during the early morning or during winter. 

USED indicate if the conditions when rear window defoggers are most likely being used. It 

has the value one if it is snowing or raining, during the earlier part of the morning or during 

winter. It equals zero, otherwise. Specifically, USED = 1 if ADWEA = 1 or MORN = 1 or 

WINTER = 1 (defined below). 

ADWEA has the value 1 if the crash occurred when it was raining or foggy, 0 otherwise. 

WINTER has the value 1 if the crash occurred during January thru April or November or 

December, 0 if it occurred in May-October. 

MORN has the value 1 if the crash occurred during 6:00 am to 9:59 am. 

The model included both a linear and non-linear variable to account for vehicle age. The 

linear variable (VEHAGE) is age of the vehicle when the crash occurred (CY - MY). The 

non-linear variable (VEHAGE2) is VEHAGE * VEHAGE. 

All the “CY” variables are indicator variables for calendar year, they have the value 1 if true 

otherwise the value of 0. For example, CY86 has value 1 if the calendar year is 1986, 0 

otherwise. 

The model includes one dichotomous and eight continuous variables to express driver age and 

gender. Kahane in the Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 

1991-99 Passenger Cars and Light Trucks3

 used this approach. The dichotomous variable is 

DRVMALE. It has the value 1 if the driver is male and 0 if the driver is female. 

“Driver age is expressed as a 4-piece linear variable, separately for males and females 

(eight variables in all): four connected straight-line segments, one from age 14 to 30, 

another from 30 to 50, another from 50 to 70, and the last from 70 and up. The eight 

variables are:

M14_30 = 30 – DRVAGE for male drivers age 14-30, otherwise it is 0 for male 

drivers age 30+ and all female drivers. 

M30_50 = 50-DRVAGE for male drivers age 30-50; = 20 for male drivers age 14-30; 

or =0 for male drivers age 50+ and for all female drivers. 

M50_70 = DRVAGE –50 for male drivers age 50-70; = 20 for male drivers70 +; =0 

for male drivers age 14-50 and all female drivers. 

3

 Kahane, C.J., Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 1991-99 Passenger Cars 

and Light Trucks, NHTSA Technical Report No. DOT HS 809 662, Washington, 2003, pp. 67-74. 

16 
 

M70+ = DRVAGE –70 for male drivers age 70+; =0 for male drivers age 14-70 and 

all female drivers. 

F14_30 = 30 – DRVAGE for female drivers age 14-30, otherwise it is 0 for female 

drivers age 30+ and all male drivers.

F30_50 = 50-DRVAGE for female drivers age 30-50; = 20 for female drivers age 14-

30; or =0 for female drivers age 50+ and for all male drivers. 

F50_70 = DRVAGE –50 for female drivers age 50-70; = 20 for female drivers70 +; 

=0 for female drivers age 14-50 and all male drivers. 

F70+ = DRVAGE –70 for female drivers age 70+; =0 for female drivers age 14-70 

and all male drivers. 

For example, a 40-year-old male driver would have M30-50 = 10, and the other 

variables set to zero. A 25-year-old male driver would have M30_50 = 20, M14_30 = 

5, and the others set to zero. Conversely, a 60-year-old female driver would have 

F50_70 = 10 and the others set to zero. A 75-year-old female driver would have 

F50_70 = 20, F70+ = 5, and the others set to zero. 

The rationale for defining the variables that way is that it treats 50 years as the 

baseline age. Each year that a driver is younger than 50 has some effect (usually 

increasing) on fatality risk, and each year that a driver is older that 50 has another 

effect (also usually increasing).”

4

 Given any specific age and gender, there is exactly 

one combination of the nine variables that will indicate a driver of that age and gender. 

These variables allow different linear relationships between age and crash risk in 

different age/gender groups. 

Indicator variables for each make model were included in the model. These indicator

variables are needed because some make-models are driven differently than others. For 

specific make-models, there may be two or more indicator variables included in the model 

depending on whether or not the car had a major redesign. For example, the Chevrolet 

Cavalier has two indicator variables: CAVALIER1 and CAVALIER2. The Chevrolet 

Cavalier was redesigned in model year 1995. So CAVALIER1 has the value 1 if the car was 

a 1983-1994 model year Chevrolet Cavalier, 0 otherwise and CAVALIER2 has the value 1 if 

the car was a 1995-2000 model year Cavalier, 0 otherwise. Similar indicator variables were 

made for all the other cars included in the model. The reference car was the 1975-1978 

Volkswagen Rabbit. 

4

 Kahane, C.J., Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 1991-99 Passenger Cars 

and Light Trucks, NHTSA Technical Report No. DOT HS 809 662, Washington, 2003, pp. 69-70. 

17 
HYPOTHESES – 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES IN THE 

PRESENCE OF ANY ONE OR MORE CONDITIONS WHEN THEY ARE 

MOST LIKELY USED (but the effect does not increase if there is more than 

one adverse condition) and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

This model contained two independent variables that indicate if rear window defoggers are 

effective. DEF_USED indicates if the presence of rear window defoggers when they are most 

likely to be used affects the rate of changing lane and backing crashes. DEF indicates if the 

mere presence of rear window defoggers affects the rate of changing lane and backing 

crashes. In other words, DEF will indicate if rear window defoggers are effective all of the 

time and DEF_USED will indicate if rear window defoggers are effective in the conditions 

when they are most likely to be used. Thus, the DEF_USED coefficient in the model will test 

our hypothesis that defoggers reduce relevant crashes when they are most likely used. But the 

DEF coefficient must also be included in the logistic regression (because interaction terms 

such as DEF_USED should not be included without also including the main effects DEF and 

USED). DEF will indicate if defoggers when present affect relevant crashes. We believe that 

just the presence of rear window defoggers should not have an effect on relevant crashes. If 

our hypothesis is correct that defoggers reduce relevant crashes when used and they have no 

effect when they are not used, then the DEF_USED coefficient should be negative and 

significant and the DEF coefficient should be close to zero. In fact, even if the DEF 

coefficient were substantial, we suspect it more likely indicates a possible bias in the model or 

data than a real effect of defoggers. We believe DEF_USED is the more important 

coefficient. 

Table 10 shows the results of the logistic regression for the 1986-1999 Florida data. The 

coefficient for DEF_USED is –0.00002, in the “right” direction but not statistically significant 

(Chi-Square = 0.002). 

Technically, the DEF_USED coefficient represents the change in the log odds ratio of 

relevant (changing lanes and backing) to non-relevant (stopped) crashes when defoggers are 

most likely used for a 1 percent increase in the percentage of cars with rear window 

defoggers. A negative coefficient represents a reduction that is associated with the presence 

of rear window defoggers when most likely used. Thus, a 100 percent increase in the 

percentage of cars with rear window defoggers is associated with 0.002 reduction in the log of 

relevant crash rate when defoggers are most likely used. The coefficient can be translated 

into the percentage change in the expected number of relevant crashes in the following way: 

Expected percentage effectiveness = 100*[1-exp (DEF coefficient*100)]. 

In other words, cars with rear window defoggers when they are most likely used have 100 * 

[1- (exp (.002))] = 0.2 percent reduction in changing lanes and backing crashes relative to 

crashes where the vehicle was stopped. Rear window defoggers when most likely used have 

no effect on relevant crash, since the chi-square value is 0.002 and chi-square needs to be at 

least 3.89 for statistical significance at the 0.05 level. 

18 
The coefficient for DEF is 0.00162, in the “wrong” direction but also non-significant (ChiSquare = 3.184). Thus, the Florida data do not show any statistically significant effect for 

defoggers. 

Table 10 

1986-1999 Florida 

Cars With Rear Impacts That Were Changing Lanes, Backing Up, Slowing,

 Stopped, Or Stalled 

Parameter Estimate 

Standard 

Error 

Wald 

Chi-Square Pr > ChiSq 

Intercept -1.6579 0.1343 152.2927 <.0001

DEF 0.00162 0.000909 3.184 0.0744

DEF_USED -0.00002 0.000435 0.002 0.9647

USED -0.1511 0.0416 13.1992 0.0003

ADWEA -0.7571 0.0234 1051.1949 <.0001

WINTER 0.0518 0.0233 4.9489 0.0261

MORN -0.0985 0.0196 25.165 <.0001

VEHAGE 0.0173 0.00427 16.3514 <.0001

VEHAGE2 0.000608 0.000225 7.3093 0.0069

CY86 0.0273 0.0448 0.3715 0.5422

CY87 -0.1325 0.0426 9.6577 0.0019

CY88 -0.1503 0.0399 14.2253 0.0002

CY89 -0.1073 0.0373 8.2893 0.004

CY90 -0.1011 0.0361 7.83 0.0051

CY91 -0.0766 0.0347 4.8868 0.0271

CY92 -0.0459 0.0327 1.9701 0.1604

CY93 -0.0979 0.0319 9.4068 0.0022

CY94 -0.1121 0.0306 13.4317 0.0002

CY95 0.013 0.028 0.2165 0.6417

CY96 -0.2027 0.0284 50.9083 <.0001

CY97 -0.0186 0.0264 0.4957 0.4814

CY98 0.0373 0.0254 2.1556 0.142

DRVMALE -0.0285 0.0296 0.9264 0.3358

M14_30 0.0502 0.0022 521.6789 <.0001

M30_50 0.0175 0.00151 134.1712 <.0001

M50_70 0.0276 0.00168 269.659 <.0001

M70_96 0.0996 0.00282 1243.4684 <.0001

F14_30 0.0549 0.0024 520.9121 <.0001

F30_50 0.000669 0.0016 0.1753 0.6755

F50_70 0.0471 0.00182 671.5062 <.0001

F70_96 0.0886 0.00359 609.8991 <.0001

CAVALIER1 -0.8086 0.1146 49.7464 <.0001 

ESCORT1 -0.8556 0.1179 52.6484 <.0001

CAPRICE1 0.0319 0.1123 0.0805 0.7766

TAURUS1 -0.6526 0.1212 28.979 <.0001 

TEMPO -0.6629 0.1172 31.9908 <.0001

19 
Table 10 – Continued 

Parameter Estimate

Standard 

Error

Wald 

Chi-Square Pr > ChiSq 

CELEBRITY -0.7663 0.1196 41.0775 <.0001

GRANDAM -0.8848 0.1185 55.708 <.0001

CIERA -0.8502 0.12 50.1573 <.0001

CUTLASS1 -0.5913 0.1166 25.7121 <.0001 

CHEVETTE -0.4266 0.1243 11.7865 0.0006

CENTURY1 -0.845 0.1217 48.2322 <.0001

MUSTANG -0.6024 0.1145 27.6735 <.0001

RELIANT -0.8532 0.1263 45.6675 <.0001

CORSICA -0.6775 0.1247 29.5101 <.0001

SUNBIRD -0.8745 0.1272 47.2302 <.0001 

ESCORT2 -0.8156 0.1234 43.6733 <.0001

MONTE1 -0.5853 0.1183 24.4601 <.0001

CROWNVIC 0.3289 0.1175 7.8379 0.0051 

LUMINA -0.6828 0.1272 28.8221 <.0001

CITATION -0.5525 0.141 15.3483 <.0001 

ARIES -0.8317 0.1248 44.4337 <.0001

FAIRMONT -0.4133 0.12 11.8572 0.0006

MALIBU -0.5068 0.124 16.6945 <.0001

ACCORD1 -0.8565 0.1264 45.8754 <.0001

REGAL -0.5897 0.1188 24.6408 <.0001

P6000 -0.638 0.1287 24.574 <.0001

DELTA1 -0.1761 0.1193 2.1787 0.1399

GMARQUIS -0.5362 0.1215 19.4728 <.0001

OMNI4DR -0.7307 0.163 20.0868 <.0001

SABLE -0.7579 0.1309 33.5421 <.0001 

LESABRE1 -0.9783 0.1276 58.7781 <.0001 

TOPAZ -0.6658 0.1337 24.7831 <.0001

SHADOW -0.8883 0.1365 42.3473 <.0001

SUNDANCE -0.8107 0.1395 33.7796 <.0001

DEVILLE1 -0.3088 0.1219 6.4152 0.0113

DEVILLE2 -0.4892 0.1261 15.0633 0.0001

BERETTA -0.5711 0.1292 19.5558 <.0001

THUNDER1 -0.6749 0.1272 28.1715 <.0001

COUGAR1 -0.6792 0.1297 27.4176 <.0001

SKYLARK1 -0.592 0.138 18.399 <.0001

DEVILLE3 -0.6975 0.1208 33.3294 <.0001

HORIZON -0.6784 0.1704 15.8464 <.0001

SATURN -1.1415 0.1323 74.4713 <.0001

CAVALIER2 -1.107 0.1426 60.27 <.0001

SKYLARK2 -0.943 0.1324 50.7292 <.0001

CALAIS -0.9328 0.1331 49.0969 <.0001

DELTA2 -0.8705 0.1318 43.6156 <.0001

GRANDPRIX1 -0.8488 0.1348 39.6177 <.0001

20 
Table 10 – Continued 

Parameter Estimate

Standard 

Error

Wald 

Chi-Square Pr > ChiSq 

CIVIC1 -0.9198 0.1254 53.8029 <.0001

LESABRE2 -0.3507 0.1229 8.1429 0.0043

GRANADA -0.1958 0.1293 2.2906 0.1302

TOWNCAR -0.4348 0.1223 12.6429 0.0004

ACCORD2 -1.0276 0.1292 63.2644 <.0001

ACCORD3 -0.791 0.1275 38.4732 <.0001

SENTRA1 -1.1258 0.127 78.6326 <.0001

CAMRY1 -0.8883 0.1268 49.0939 <.0001

LTD 0.0808 0.1248 0.4191 0.5174

VOLARE -0.2067 0.129 2.5661 0.1092

CAPRICE2 -0.0625 0.1284 0.2372 0.6262

CIVIC2 -1.1626 0.1349 74.2177 <.0001

ACCLAIM -0.7761 0.1489 27.1552 <.0001

GRANDPRIX2 -0.339 0.1216 7.7791 0.0053

CUTLASS2 -0.0547 0.1281 0.1827 0.6691

SUPREME1 -0.6846 0.1463 21.8879 <.0001

SKYHAWK -0.7723 0.156 24.5135 <.0001 

ELECTRA -1.14 0.1385 67.7382 <.0001

HORNET -0.0827 0.1386 0.3563 0.5506

MONTE2 -0.1587 0.136 1.3605 0.2435

BONNEVILLE -0.1353 0.1315 1.0581 0.3037

PARISIENNE -0.3587 0.1503 5.6926 0.017

NOVA1 -0.2251 0.1257 3.2076 0.0733

CAMARO -0.4467 0.1218 13.4537 0.0002

PINTO -0.019 0.1439 0.0175 0.8948

LEBARON1 -0.8419 0.1371 37.7211 <.0001 

CAMRY2 -0.6545 0.1312 24.881 <.0001

ASPEN -0.3034 0.129 5.5308 0.0187

TAURUS2 -0.7342 0.1473 24.8572 <.0001 

ALTIMA -0.921 0.1314 49.1283 <.0001

DAYTONA -0.8438 0.1475 32.7431 <.0001

THUNDER2 -0.3195 0.1313 5.9238 0.0149

CHEVELLE 0.1859 0.1463 1.6149 0.2038

SPIRIT -0.7976 0.1496 28.432 <.0001

LEGACY -1.1856 0.2229 28.2828 <.0001

PROBE -0.8096 0.1453 31.0575 <.0001

OLDS98 -0.9531 0.1377 47.9346 <.0001

LEBARON2 -0.6259 0.139 20.2716 <.0001 

DYNASTY -0.819 0.1529 28.7071 <.0001

MONZA -0.4225 0.1687 6.2762 0.0122

EAGLE -0.1943 0.338 0.3304 0.5654

OMEGA -0.4624 0.1804 6.5732 0.0104

NEONPLY -0.7177 0.1925 13.9012 0.0002

21 
Table 10 – Continued 

Parameter Estimate

Standard 

Error

Wald 

Chi-Square Pr > ChiSq 

NEONDOD -0.8636 0.1769 23.8328 <.0001

ZEPHYR -0.4222 0.1492 8.0099 0.0047

FIREBIRD -0.4837 0.1298 13.8907 0.0002

CENTURY2 -0.3467 0.1546 5.0307 0.0249

THUNDER3 -0.855 0.1361 39.4743 <.0001

INTREPID -0.5406 0.1558 12.0399 0.0005

MARQUIS -0.5115 0.1399 13.37 0.0003

COUGAR2 -0.753 0.1433 27.5981 <.0001

LEBARON3 -0.2955 0.1508 3.8413 0.05

CORDOBA -0.2765 0.1558 3.1491 0.076

NEWYORKER -0.899 0.1519 35.033 <.0001

DART -0.1866 0.1528 1.4905 0.2221

DIPLOMAT 0.3079 0.1245 6.111 0.0134

CONTOUR -0.8677 0.1639 28.0303 <.0001

FIRENZA -0.7187 0.1702 17.8207 <.0001

SUPREME2 -0.5465 0.1229 19.7705 <.0001

LEGACY1 -0.6543 0.3883 2.839 0.092

NOVA2 -0.8272 0.1522 29.5268 <.0001

COUGAR3 -0.1681 0.143 1.382 0.2398

VALIANT -0.2049 0.2075 0.9752 0.3234

GREMLIN -0.3648 0.1803 4.094 0.043

JETTA -1.0479 0.1388 56.9669 <.0001

COROLLA -1.0354 0.1256 67.916 <.0001

LOYALE -0.9161 0.1702 28.9677 <.0001

ACCORD4 -0.9514 0.1339 50.4566 <.0001

VOL240 -1.068 0.1382 59.7267 <.0001

EXCEL -0.9987 0.1326 56.6941 <.0001

MAXIMA1 -0.8257 0.1376 36.0173 <.0001

SUBARU -0.8949 0.1716 27.2023 <.0001

SPECTRUM -1.0584 0.1526 48.0771 <.0001 

GOLF -1.0393 0.161 41.6642 <.0001

TERCEL -1.0464 0.195 28.7966 <.0001

SENTRA2 -0.9259 0.1374 45.4432 <.0001

RABBIT -1.1188 0.1591 49.4222 <.0001 

MAXIMA2 -0.8316 0.1391 35.7504 <.0001

CELICA -0.6559 0.1372 22.8457 <.0001

The regression coefficient (0.0173) for VEHAGE shows that changing lanes and backing 

crashes increase relative to stopped involvements, as cars get older. Changing lanes and 

backing crashes increase 2 percent for every year a car gets older. The negative regression 

coefficient for almost all of the CY terms implies that changing lanes and backing crashes 

were less common in the past than in recent years. VEHAGE, VEHAGE2, and most of the 

CY terms are included in the model because they are significant. 

22 
The coefficients of the other independent variables seem reasonable. The positive coefficients 

for M14_30, M70+, F14_30, and F70+ show that the youngest and oldest drivers are 

especially prone to backing up or changing lane crashes. ADWEA and MORN are negative, 

indicating that changing lanes and backing crashes decrease relative to stopped involvements 

during adverse weather and early morning. USED is also negative, indicating the relevant 

crashes decrease during conditions when rear window defoggers are most likely used. 

The make-model indicator variables are used only as control variables in the model. Some

will have high or low coefficient values by chance alone indicating more or fewer changing 

lane and backing crashes than the average. We could reduce the number of these variables, if 

we grouped several make-models together. But there is no basis to group them. For example, 

not all small cars are driven in such a manner that they have fewer (or more) relevant crashes 

than large cars. Thus, all the individual make-model terms are included in the model and it is 

irrelevant if certain make-model terms are significant and others are not. 

A similar model was run on the Michigan database. The adverse weather indicator variable 

for Michigan also had a value of 1 if the crash occurred when it was snowing, sleeting, hailing 

or freezing rain. Michigan’s model included 34 additional make-models not included in the 

Florida model that had at least 750 crashes in Michigan. 

 

The only other difference in the model was the number of individual CY terms. Since 1981-

1991 Michigan data were analyzed, the model had 10 individual CY terms: CY81, CY82, ..., 

CY90. 

Table 11 shows the DEF_USED and DEF coefficients, percent reduction, and significance for 

the model by State. All of the results are non-significant indicating that rear window 

defoggers have no effect on changing lane and backing crashes, in all conditions and in 

conditions when they are most likely used. The DEF_USED coefficient in Michigan is in the 

“wrong” direction, corresponding to a 5 percent increase in changing lane and backing 

crashes, but this effect is not statistically significant. 

Table 11 

DEF_USED And DEF Coefficients And Percent Reduction By State 

DEF_USED 

DEF 

State Coeff 

Percent 

Reduction Coeff 

Percent 

Reduction 

Michigan 0.00047 -5% 0.00015 -2%

Florida -0.00002 0.2% 0.00162 -18%

23 
HYPOTHESES – 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES AND THE 

EFFECTS ARE INDEPENDENT AND ADDITIVE FOR THE THREE 

ADVERSE CONDITIONS and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

The analysis was repeated to see if the individual adverse conditions when defoggers are most 

likely used reduce relevant crashes. This analysis tests three hypotheses: 

1. rear window defoggers reduce relevant crashes when raining or snowing,

2. rear window defoggers reduce relevant crashes during the early morning, and 

3. rear window defoggers reduce relevant crashes during the winter. 

The model included three individual interaction variables, DEFADW, DEFMORN, and 

DEFWIN, instead of DEF_USED and USED. 

DEFADW has the value of DEF if it is snowing or raining. DEFADW = DEF * ADWEA. 

For example, if 1989 Ford Taurus (89.7 percent had rear window defoggers) had a collision 

when it was raining, then DEF = 89.7 and DEFADW = 89.7. DEFADW has the value of zero 

if it was not raining or not snowing. It also has the value of zero if the make-model was not 

equipped with any defoggers in that model year. 

DEFMORN has the value of DEF if it is during the early morning, particularly between 6:00 

am and 9:59 am. DEFMORN = DEF * MORN. DEFMORN has the value of zero when the 

crash occurred before 6:00 am or after 10:00am. 

DEFWIN has the value of DEF if it is during winter: November through April. DEFWIN = 

DEF * WINTER. DEFWIN has the value of zero when the crash occurred in the months May 

through October. 

DEFADW will indicate if rear window defoggers reduce relevant crashes when it is raining or

snowing. DEFMORN will indicate if rear window defoggers reduce relevant crashes during 

6:00 am and 10:00 am. DEFWIN will indicate if rear window defoggers reduce relevant 

crashes during the winter. 

Similar to the previous model, DEF must be included in the model because logistic regression 

requires the main effect to be included in the model when interaction terms are included. If 

our hypothesis is correct that rear window defoggers reduce relevant crashes under these three 

conditions, then DEFADW, DEFMORN, and DEFWIN coefficients should be negative. The 

regression model considers the effects to be linear and additive. If the effects are intrinsically 

not linear and not additive (e.g., if our second hypothesis is correct that the full benefit of 

defogger is achieved from any one adverse condition), then the model may spuriously 

calibrate some coefficients positive and others negative. 

24 
Table 12 shows the results and in fact, some of the coefficients are positive and some are 

negative. When raining or snowing, the coefficients are negative but not statistically 

significant. During the early morning, the coefficients are positive indicating an increase in 

relevant crashes. Michigan’s coefficient is statistically significant, but Florida’s is not. The 

coefficients for rear window defoggers during the winter are mixed. Michigan’s coefficient is 

negative and Florida’s is positive, but neither is significant. In any case, none of the 

coefficients is significant in the “right” direction, and we cannot conclude that rear window 

defoggers reduce relevant crashes when it is raining or snowing, during the early morning, or 

during the winter. 

 

Table 12 

DEFADW, DEFMORN, DEFWIN And DEF Coefficients And 

Percent Reduction By State 

DEFADW DEFMORN DEFWIN DEF 

State Coeff 

% 

Reduced Coeff 

% 

Reduced Coeff 

% 

Reduced Coeff 

% 

Reduced 

MI -0.00044 4% 0.00153 -17%* 0.000323 -3% 0.000122 -1%

FL -0.00094 9% 0.000668 -7% -0.00006 1% -0.00061 6%

* Significant 

HYPOTHESES – 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES AND THE 

EFFECTS ARE EQUAL AND ADDITIVE FOR THE THREE ADVERSE 

CONDITIONS and 

• REAR WINDOW DEFOGGERS REDUCE RELEVANT CRASHES ALL THE 

TIME 

The analysis was repeated to test that the more adverse the conditions, the more effective are 

the defoggers at reducing relevant crashes. This may be because the defoggers are more 

likely to be used or that the defoggers improve the vision out of the rear window even more 

when there are two or more adverse conditions than when there is only one. We believe that 

if vision through the rear window is obscured, then the defoggers will be used whether one or 

more adverse conditions are present. It is probably unlikely that rearward vision can be 

improved more when more than one adverse condition is present. Either the driver will be 

able to see through the rear window or not. Nevertheless, this analysis tested to see if

defoggers are effective when more adverse conditions are present. 

The model included only one interaction variable, similar to the model with DEF_USED. 

The only difference was that this model used an interaction variable that escalated the effect 

of defoggers when more than one adverse condition existed. DEFAD is the weighted 

variable. It expresses the interaction between percent of rear window defoggers (DEF) and 

adverse conditions: adverse weather (ADWEA), winter (WINTER), and early morning 

(MORN). DEFAD = DEF * (ADWEA + WINTER + MORN). DEFAD is large when 

(according to this questionable hypothesis) there is a large probability that the rear window 

defoggers are being used. For example, if a vehicle equipped with a rear window defogger 

25 
had a collision that occurred during January at 8:00 AM when it was snowing, then ADWEA 

=1, WINTER = 1, MORN = 1, DEF = 100 and DEFAD = 100 * (1 + 1 +1) = 300. DEFAD is 

less when the probability is reduced. For instance, if a vehicle with a rear window defogger 

had a collision that occurred in May during the afternoon when it is raining, then DEFAD = 

100. DEFAD is zero when there is little or no chance that rear window defoggers are being 

used. For example, if a vehicle with a rear window defogger had a crash that occurred on a 

bright, sunny afternoon in April, or a vehicle not equipped with a rear window defogger had a 

crash anytime, then DEFAD = 0. 

The model included ADWEA, WINTER, and MORN, but it did not include a variable similar 

to USED. A similar variable to USED would have been the sum of ADWEA, WINTER, and 

MORN, but is not needed in the model since it is a linear combination of ADWEA, WINTER, 

and MORN. 

Table 13 shows the results by State. All of the results in Table 13 are non-significant. Again, 

we conclude that these data fail to support any conclusion that rear window defoggers had an 

effect in either direction on changing lane and backing crashes. 

Table 13 

DEFAD And DEF Coefficients And 

Percent Reduction By State 

DEFAD DEF 

State Coeff 

Percent 

Reduction Coeff 

Percent 

Reduction 

Michigan 0.000364 -4% 0.000121 -1%

Florida -0.00001 0% 0.00162 -18%

CONCLUSIONS 

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 

26 
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. 

27 
28 
APPENDIX A Excel (PDF) work-sheet

PRTCH is the difference between percentage of rear defoggers for the last and first model 

year for that make-model. 

The State Columns show the number of rear impacts that were stopped, changing lanes, and 

backing in that State. 

Shaded cells were excluded from the analysis because reported percentages were much higher 

or lower than expected relative to previous or subsequent percentages. 

CG and MM2 are codes used in NHTSA evaluations for indicating the basic car group and 

specific make-model. 

29