Estimating the Lives Saved by Safety Belts and Air Bags 

Donna Glassbrenner, Ph. D. 

National Highway Traffic Safety Administration, 400 Seventh St SW, Washington DC 20590 

donna.glassbrenner@nhtsa.dot.gov 

0. Introduction 

Safety belts and air bags are among the most important safety devices in society today, together saving thousands of lives 

each year. The National Highway Traffic Safety Administration (NHTSA) estimates the number saved each year, as well as 

the number that would have been saved if more occupants had buckled up. These estimates are not determined by examining 

crashes on a case by case basis, but rather are estimated from the number of occupants who died, the restraints they used, and 

the effectiveness of the restraints in preventing 

fatality. 

Specifically, NHTSA estimates on an annual basis: 

• the passenger vehicle occupants over 4 years old 

saved by safety belts, 

• the passenger vehicle occupants over 12 years 

old saved by frontal air bags, and 

• the passenger vehicle occupants over 4 years old 

that would have been saved by safety belts if 

more of them had buckled up. 

Chart 1: Passenger Vehicle Occupants Saved by Safety Belts and 

Air Bags, and Lost from not Using Belts 

0 

2,000 

4,000 

6,000 

8,000 

10,000 

12,000 

1991 1993 1995 1997 1999 2001 

Occupants 5 and 

Older Saved by 

Safety Belts 

Occupants 13 

and Older Saved 

by Frontal Air 

Bags 

Occupants 5 and 

Older Who Died 

Because of Not 

Using Belts 

Source: National Center for Statistics and Analysis, 

NHTSA, FARS 1991-2001 

Page 132 

Note that these do not include the relatively small 

numbers of occupants of large trucks saved by belts, 

children under five saved by belts, children under 13 

saved by frontal air bags, and occupants saved by 

side air bags. Side air bags have not yet been rated 

for effectiveness, and so the lives that they save 

cannot currently be estimated. The current data do not support reliable air bag effectiveness ratings for children under 13. 

NHTSA recommends that these children not be in front of an air bag, unless no other seating position is available. 

Lives saved and savable are used in calculating the economic impact of belt use and nonuse. (Blincoe, Seay, et al, 2002) This 

paper presents improvements made the calculations of lives saved and savable. More detailed information can be found in 

(Glassbrenner, 2003). In the rest of this paper, “air bag” will mean “frontal air bag”, although we occasionally write “frontal 

air bag” for emphasis. “Lives saved by belts” will mean “passenger vehicle occupants over 4 years old saved by belts”, and 

“lives saved by bags” will mean “passenger vehicle occupants over 12 years old saved by (frontal) air bags”. 

1. Input to the Lives Saved Calculations: Effectiveness Ratings, Restraint Configurations, and Fatality Counts 

1.1 Effectiveness Ratings 

NHTSA rates the effectiveness of belts and bags in preventing fatality, finding for instance the following ratings for the 

driver’s seat of passenger cars: 

1. Three-point belts are 48% effective for occupants over 4 years old. 

2. Air bags are 14% effective for occupants over 12 years old. 

3. Air bags are 11% effective for belted occupants over 12 years old who are not saved by their belt. 

4. Three-point belts in conjunction with air bags are 53.72% effective for occupants over 12 years old. 

The first rating means that 48% of the drivers (over 4 years old) of passenger cars in crashes severe enough that they would 

die if they were unbuckled and any air bag in their position were removed, would live if they buckled up. The second means 

that 14% of the passenger car drivers over 12 who would die without a belt and bag would live with the bag. The third means 

that 11% of the passenger car drivers over 12 who would die with a belt but no bag, would live with a belt and bag. The 

fourth means that 53.72% of the drivers over 12 years old who would die without a belt and bag would live if they buckled 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

and had an airbag. This rating and can be derived from 1 and 3. (Report to Congress, 2001; Glassbrenner, 2003) Note that 

the belt and bag ratings are specified for different age ranges. Of course, there will be few drivers under 12 years old. 

Three–point belts are the manual lap/shoulder belts in today’s vehicles and the automatic lap/shoulder belts that appeared 

primarily in vehicles made by General Motors. Two-point belts consist of either a) an automatic shoulder belt combined with a 

manual lap belt, a configuration that appeared in some passenger cars in the 1980’s and into the 1990’s, or b) an automatic or 

manual shoulder belt together with a knee bolster under the dashboard, a less common configuration. 

Defining ratings this way follows standard practice for two devices, A and B, that act in conjunction. In general, defining 

potential fatalities as those who would die with neither A nor B, the effectiveness of A is the percent of potential fatalities 

who would live if they used A but not B, the effectiveness of B is the percent who would live if they used B but not A, and 

the residual effectiveness of B is the percent who would live if they used A and B but A did not save them. 

We will call the effectiveness in 2 the effectiveness of bags and that in 3 the residual effectiveness of bags, since it is the 

remaining effectiveness after belts are applied. These are the only two ratings of air bags. Belts are rated for their 

performance as they are used, so the rates reflect both proper and improper use (such as not buckling the manual lap belt if 

the shoulder belt is automatic). Also note that the air bags are rated for the life-saving capability of their presence, not their 

deployment. However the deployment ratings would be the same as those for presence. 

Vehicle technology is constantly improving and the types of potentially fatal crashes may change over time. For instance the 

increasing popularity of SUVs has been accompanied by an increase in rollover crashes. Consequently NHTSA periodically 

updates its ratings to reflect changing conditions. The most recent belt effectiveness ratings are in (Kahane, 2000) and 

(Morgan, 1999), and the most recent bag ratings in (Report to Congress, 2001). Kahane and Morgan were computed using 

data from 1986 – 1999 and 1988-1997 respectively, and the Report to Congress used 1986-2000. 

1.2 Restraint Configurations and Fatality Counts 

We define a restraint configuration to be a 6-tuple consisting of the 

coordinates in Table 1. Note that this includes information on the 

vehicle and occupant as well as the restraint. The coordinate “air 

bag?” is to take the value “yes” if the occupant is the driver or right 

front passenger and there is an air bag in their seating position, and 

“no” otherwise. The effectiveness of belts and bags in (Kahane, 

2000), (Morgan, 1999), and (Report to Congress, 2001) are specified 

in terms of the restraint configurations. 

NHTSA compiles a census of all motor vehicle fatalities in the U.S. from police reports, hospital records and other state 

documents called the Fatality Analysis Reporting System (FARS). Fatal crashes are defined as police-reported crashes 

involving a motor vehicle in transport on a public road, street, or highway in which at least one person, called a fatality, died 

within 30 days of the crash. From FARS, we produce counts of occupant fatalities in passenger vehicles, called the fatality 

counts, who are ages 5 or older and had access to a belt, for the various restraint configurations. 

Table 1: Restraint Configurations 

Coordinate Values 

vehicle type passenger car; light truck or van 

seating 

position 

driver, right front passenger, front 

center, rear outboard, rear center 

belt type 3-point, 2-point, lap 

belt used? yes, no 

air bag? yes, no 

age 5-12, 13 or older 

For instance, the fatality counts for drivers of passenger cars 

with 3-point belts in 2001 are given in Table 2. We will refer to 

this as Example 1. Note that belted children under 13 have the 

same effectiveness rating, whether or not they had an air bag. 

1.3 Notation 

Table 3 defines the notation used in the remainder of this paper. 

For instance, the second restraint configuration in Table 2 is 

i=(passenger car, driver, 3-point belt, no, yes, 13 or older), Fi= 

3,354, belt(i)=0, bag(i)=1, ei(belt)= 48%, ei(used) = e(bag) = 

14%, ei(system)=53.72%, and ei(belt | bag)=46.19%, which is 

the residual effectiveness of these belts. 

Table 2: 2001 Fatality Counts Ratings for Drivers of 

Passenger Cars with 3-Point Belts 

Age 

5-

12? 

Belt 

Used? 

Air Bag 

in Seating 

Position? 

Effectiveness 

of Restraint 

Used 

Fatality 

Count 

No Yes Yes 53.72% 3,555 

No No Yes 14% 3,354 

No Yes No 48% 1,989 

No No No 0% 2,873 

Yes Yes NA 48% 0 

Yes No NA 0% 2 

Source: National Center for Statistics and Analysis, 

NHTSA, FARS, 2001 

Page 133 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. The Total Lives that Were 

Saved by a Belt or a Bag 

It is a relatively straightforward 

matter to estimate the lives saved 

by both restraints combined. If x 

people die using a safety device 

that has an effectiveness e (i.e. that 

reduces fatalities in settings in 

which people would otherwise die 

by e×100%), then one can infer 

that a total of x/(1-e) used the 

device in a setting in which they 

would otherwise die (the potential 

fatalities), ex/(1-e) of which were 

saved by the device. 

Table 3: Notation 

Notation Definition 

e(bag) the effectiveness of air bags, i.e. 14% 

e(bag | belt) the residual effectiveness of air bags, i.e. 11% 

R the set of all restraint configurations 

In the remaining definitions, i denotes a restraint configuration. 

Fi the fatality count for i 

belt(i) 1 if a belt is used in i, and 0 otherwise 

bag(i) 1 if a bag is present and the occupant is over 12 in i, 0 otherwise 

ei(belt) the effectiveness of the belt in I 

ei(system) ei(belt) if bag(i)=0, otherwise the effectiveness of the belt-bag 

system in i 

ei(used) the effectiveness of the restraint (belt, bag, or belt-bag) used in i 

ei(belt | bag) 

ei (system) - e(bag) when bag(i)=1, otherwise undefined 

1 - e(bag) 

Applying this to Example 1 gives 

that 6,508 drivers in passenger 

cars equipped with 3-point belts 

were saved in 2001. (See Table 

4.) Formulaically, 

e (used) F i i ∑ people were 

1- e (used) i

i∈R 

saved. Of course Example 1 

only comprises drivers of 

passenger cars with three-point 

belts while the national 

calculation uses all passenger 

vehicles and all seating positions. 

Table 4: Lives Saved in 2001 Among Drivers of Passenger Cars with 3-Point Belts 

Age 5-

12? 

Belt 

Used 

? 

Vehicle Has 

Driver’s Side 

Air Bag? 

Effectiveness of 

Restraint 

Used 

Fatalities Potential 

Fatalities 

Lives 

Saved 

No Yes Yes 53.72% 3,555 7,682 4,127 

No No Yes 14% 3,354 3,900 546 

No Yes No 48% 1,989 3,824 1,836 

No No No 0% 2,873 2,873 0 

Yes Yes NA 48% 0 0 0 

Yes No NA 0% 2 2 0 

Totals* 11,774 18,282 6,508 

*Row entries may not sum to totals due to rounding. 

Source: National Center for Statistics and Analysis, NHTSA, FARS, 2001 

3. Attributing the Lives Saved to Belts and Bags 

To quantify the benefits of belts and bags separately, NHTSA wishes to partition the total lives saved from the previous 

section into those that were saved by belts and those that were saved by bags. Unfortunately, there is no clear way to 

determine such a partition. One can determine the maximum number that could have been saved by either restraint, but this 

only places limits on the attribution, without determining it. (Logically there should also be occupants for whom both 

restraints were necessary for their survival, but there is no clear way to calculate this number.) 

We illustrate on Example 1. Obviously the 1,836 drivers that were saved but did not have an air bag must have been saved 

by the belt. Had any children under 13 been saved, they would have also been saved by the belt, since we do not have a bag 

effectiveness rating in this age range. Similarly the 546 people who were unbelted were saved by the air bag. The only issue 

is how to attribute the 4,127 that were saved while being protected by both restraints. 

Since their belts are 48% effective, belts could only have saved at most 3,687 of the 4,127 occupants (0.48 ×

3,555/(1-0.5372) = 3,687), leaving 439 saved by bags. Similarly, bags could have saved at most 1,075 of these occupants, 

leaving 3,051 for belts. We call these two attribution methods the belt-maximizing and bag-maximizing methods, 

respectively. Note that we do not mean to suggest by these names that the methods were contrived to favor belts or bags. 

There are simply the bounds set by logic. 

NHTSA considered these attributions and a middle ground that attributes the 4,127 lives saved using both restraints to belts 

and bags in proportion to the effectiveness of belts and bags separately. This method, which we call restraint-neutral, 

attributes 3,195 of the 4,127 occupants to belts (i.e. 4,127× .48/(.48+.14)= 3,195), and the remaining 932 to bags. Combining 

these calculations with the 1,836 occupants who were saved using only a belt and the 546 saved using only a bag yields the 

three attributions for Example 1 in Table 5. Table 6 gives the formulas for the three attribution methods. 

Page 134 
 

 

 

 

 

 

 

 

 

 

 

Each method has merits. The belt-maximizing method is used in 

classical benefits analysis. Classically, the safety device that is 

instituted first (in this case, belts) is attributed the maximal benefits 

possible, and subsequent safety devices (in this case, bags) are attributed 

only the residual benefits. Also one can view air bags as restraints that 

supplement belts, and from this view, bags should only be attributed the

residual benefits. In comparison, the restraint-neutral attribution does 

not give preference to either restraint. Finally, since bags activate 

automatically, belts can be viewed as restraints that supplement air bags. 

From this point of view, belts should only receive the residual savings, 

supporting the bag-maximizing attribution. 

Table 5: Attributions Considered for the Lives 

Saved in 2001 Among Drivers of Passenger 

Cars with 3-Point Belts 

Attribution Method Lives Saved 

Belts Bags 

Belt-maximizing 

Restraint-neutral

5,523 985 

 5,030 1,478 

Bag-maximizing 4,887 1,621 

Source: National Center for Statistics and 

Analysis, NHTSA, FARS, 2001 

Since each method, and in fact 

any method that falls within 

the limits set by the beltmaximizing and bagmaximizing attributions, is 

scientifically valid, choosing 

among them is a policy 

decision. NHTSA chose to 

use the belt-maximizing 

attribution, which is the 

method it has used in the past. 

The formulas for this 

attribution in Table 6 are 

improvements over the 

formulas previously used that 

better reflect the contributions 

of air bags. 

4. The Lives that Would 

Have Been Saved if Belt Use 

Had Been Higher 

NHTSA estimates the number 

Table 6: Formulas for the Attribution Methods 

Method Restraint Lives Saved Belt-maximizing 

Belt 

e (belt) F i i ∑ 1 - e (used) belt(i)=1 i 

Bag 

e(bag | belt) e(bag) ∑Fi + ∑Fi

1- e(bag | belt) 1- e(bag) belt(i) 1, = belt(i) 0, =

bag(i) 1 = bag(i) 1 =

Restraint-neutral 

Belt 

e (belt) F i i e (belt) e (system) F i i i ∑ 1- e (belt) 

+ ∑ e (belt) + e(bag) 1- e (system) belt(i) 1, = i belt(i) 1, = i i 

bag(i) 0 = bag(i) 1 =

Bag 

e(bag) e(bag) e (system) F i i

1 - e(bag) e (belt) ∑Fi + ∑ + e(bag) 1- e (system) belt(i) 0, = belt(i) 1, = i i 

bag(i) 1 = bag(i) 1 =

Bagmaximizing

Belt 

e (belt | bag) F e (belt) F i i i i ∑ 1- e (belt | bag) 1- e (belt)

+ ∑

belt(i) 1, = i belt(i) 1, = i 

bag(i) 1 = bag(i) 0 =

Bag 

Fi

e(bag) ∑ 1- e (used) bag(i)=1 i 

of lives that would have been 

saved in a given year if front seat daytime belt use had been at various higher rates, such as one percentage point higher, 

90%, or 100%. “Front seat daytime” is used because our best measurements of belt use arise from an observational survey 

that employs this restriction for practical reasons. (Glassbrenner, 2002) However, the resulting estimates are frequently 

referred to as the lives saved if belt use had been, e.g. 90% or 100%, instead of front seat daytime use being 90% or 100%, as 

will we. 

The calculations utilize a belt use model developed by NHTSA. The most recent version of this model is UPF(x) = 

0.47249 x

2 + 0.43751 x, where x denotes belt use in the front seat during daytime and UPF(x) denotes the belt use among 

potential fatalities when daytime front seat use is x. (Wang and Blincoe, 2003) We will refer to belt use among potential 

fatalities as UPF. 

As with the total lives saved, it is a relatively straightforward matter to estimate the number of lives that would have been 

saved if belt use had been higher. We illustrate with Example 1, calculating the additional lives that would have been saved 

if (daytime front seat) belt use had been 90% in 2001. The UPF model from (Wang and Blincoe, 2002) estimates that UPF 

would have been 21 percentage points higher in 2001, when front seat occupants buckled up during 73% of their daylight 

driving time (i.e. 0.2053 = UPF(0.9) – UPF(0.73)). That is, the model predicts that 21% more of the potential fatalities would 

have buckled up. Applying this to the 18,282 potential fatalities gives 3,753 additional potential fatalities that would have 

buckled up. We would expect 2,160 of them to be over 12 and have an air bag, since 3,900 of the 6,776 original unbelted 

potential fatalities fit this description (i.e. 2,160=3,753 × 3,900 / 6,776). 

Page 135 
 

 

 

 

 

 

 

 

 

 

 

 

 

Table 7 computes that a 

total of 1,925 of the 3,753 

newly belted would be 

saved. However not all of 

the newly belted died when 

they were unbelted. One 

would expect that 302 of 

them were saved by their 

air bags when they were 

unbelted since 302 = 0.14 

× 2,160. Subtracting these 

from the 1,925 that would 

have been saved if belted, 

we find that 1,623 additional occupants would have been saved if belt use had been 90%. 

Table 7: The Newly Buckled Potential Fatalities Among Drivers of Passenger Cars with 3-

Point Belts if Belt Use Had Been 90% in 2001 

Age 5-

12? 

No 

No 

Yes 

Belt 

Used? 

Yes 

Yes 

Yes 

Airbag 

Present? 

Effectiveness Potential 

Fatalities 

Lives 

Saved 

Lives Previously 

Saved by Bags 

Net Lives 

Saved 

Yes 

No 

54% 

48% 

2,160 

1,592 

1,160 

764 

302 

0 

858 

764 

NA 48% 2 1 0 1 

Totals* 3,753 1,925 302 1,623 

*Row entries do not necessarily sum to totals due to rounding. 

Source: National Center for Statistics and Analysis, NHTSA, FARS, 2001 

That is, the additional lives that would have been saved in a given time period (e.g. a given year) had belt use been xhypoth is 

 

 

i i i (UPF(xhypoth ) − UPF(xcurrent ))∑

F e(system| unbelted) − e(bag) ∑F ∑F . ,

1- ei (used)  1- ei (used) 1- e (used) 

belt(i)=0 ii∈R bag(i)=1,  

 belt(i)=0 

where xcurrent is the belt use in the time period considered, and e(system | unbelted) is average value of ei(system) among the 

e (system) F F

unbelted potential fatalities, which is ∑

i i ∑

i and was 53% in 2001. If one imagines 

belt(i)=0 1- ei

(used) belt(i)=0 1- ei (used) 

choosing a random subset from the potential fatalities equal to the number you wish to additionally buckle, the formula takes 

the number saved if you buckle all of them and subtracts the number that were already saved by their bag. Note that we are 

not computing the additional lives saved by estimating the total lives saved in the natural way from the UPF model and 

subtracting the current lives saved. This would have resulted in a discontinuity, since the UPF model does not precisely 

predict the use rate that actually occurred among the potential fatalities when inputted the actual daytime front seat use. 

The additional lives saved were previously calculated using a variety of formulas for various hypothesized use rates, which 

produced inconsistent estimates. For instance, the estimated lives saved at 100% use had been precisely this, which resulted 

in a large gap between the estimate at 99% use, which hypothesized 99% daytime front seat use, and the 100% estimate, 

which hypothesized 100% use. In addition to producing consistent estimates, the new calculation also uses an updated belt 

use model from (Wang and Blincoe, 2002). This model is better than that previously used, which predicted daytime front 

seat use among potential fatalities. 

4. Attributing to the Total Lives Savable at Higher Belt Use to Belts and Bags 

Table 8 applies the three attribution methods to the 1,925 people that would have been saved among the newly belted in 

Example 1 if belt use had been 90%. Recall that 302 of these occupants were saved by their air bags when they were 

unbelted. The bag-maximizing attribution continues to attribute all 302 to bags, while the belt-maximizing and restraintneutral attributions revise the attribution for 

some of them to belts. Namely under the beltmaximizing attribution, 178 unbelted occupants 

saved by bags would have been saved by belts if 

they had buckled up, while the restraint neutral 

method would have reattributed 40 occupants. 

Table 9 contains the formulas for these 

attribution methods. Note that all methods 

attribute the additional lives saved (e.g. the 

1,623 people in Example 1) to belts. 

Table 8: Attributing the Lives Saved Among the Newly Buckled 

Potentially Fatal Drivers of Passenger Cars with 3-Point Belts if Belt 

Use Had Been 90% in 2001 

Age 5-

12? 

Belt 

Used? 

Airbag 

Present 

? 

Lives 

Saved 

BeltMaximizing 

Attribution 

RestraintNeutral 

Attribution 

BagMaximizing 

Attribution 

Belt Bag Belt Bag Belt Bag 

No 

No 

Yes 

Yes 

Yes 

Yes 

Yes 1,160 

No 764 

NA 1 

1,037 124 

764 0 

1 0 

898 262 

764 0 

1 0 

858 302 

764 0 

1 0 

Totals 1,925 1,802 124 1,663 262 1,623 302 

Source: National Center for Statistics and Analysis, NHTSA, FARS, 

2001 

Page 136 
 

 

 

 

 

 

 

 

 

 

 

Table 9: Reattribution Formulas When Belt Use is Hypothesized to Be xhypoth 

Attribution Newly Belted Occupants Previously Saved by Bags Who Are Reattributed to Belts 

Beltmaximizing 

(UPF(x ) UPF(x )) hypoth current 

e (used) FiF

i e(bag | belt) i 











e(bag) Fi 1 - e (used) 1 - e(bag | belt) 1- e (used) i R i belt(i) 1, belt(i) 0, i 

bag(i) 1 bag(i) 1 

=

=

− ∑ =

=

∑

×

−

−

∑∈











Restraintneutral











+

(UPF(x ) UPF(x )) hypoth current 

F

i e(bag | belt) F e (belt) e (belt) e(bag) - e (used) i i i i e(bag) F i Fi

1- e (used) 1- e(bag | belt) e (belt) e(bag) 1- e (used) i R i bag(i) 1 belt(i) 1, i i +

bag(i) 1 

=

=

− ∑ =

∑

×

−

−

∑∈











Bagmaximizing 0 

Revising attributions for the 302 previously saved by bags makes perfect sense from the belt-maximizing and restraintneutral points of view. Buckling the 302 previously unbelted occupants poses a new scenario, in which these occupants 

would now be restrained by both a bag and a belt. Either restraint might save them. The bag-maximizing perspective doesn’t 

revise the attribution for these 302 occupants because they were saved by bags when they were unbelted. Again each method 

is justifiable. NHTSA had previously used the bag-maximizing approach for this computation. The other attributions would 

obviously cause much confusion among many of the people who use the estimates, many of whom are lay readers. 

Consequently it was decided to continue using the bag maximizing attribution. 

5. Summary 

In addition to the correction of oversights, we made three major improvements to the calculations of lives saved and savable 

by belts and bags. Formulas were improved to better reflect the contributions of air bags. The various calculations of 

savable lives were made consistent. In particular, the lives that would have been saved at 100% use was changed to be 

consistent with other estimates. Finally, updated effectiveness ratings and a better UPF model were incorporated. 

The improvements have a substantial impact on the estimates. Under the new calculations, the lives saved by belts increases 

by about 10% and that for bags by 6%. The lives that would have been saved if belt use had been one percentage point 

higher drops by 10%, that for 90% belt use drops by 25%, and that for 100% drops by 19%. 

References 

1. Blincoe, L., Seay, A., Zaloshnja, E., Miller, T., Romano, E., Luchter, S., Spicer, R., The Economic Impact of Motor 

Vehicle Crashes, 2000, NHTSA Technical Report, DOT HS 809 446, May 2002 

2. Fifth / Sixth Report to Congress, Effectiveness of Occupant Protection Systems and Their Use, DOT HS 809 442, 

November 2001 

3. Glassbrenner, D., Safety Belt and Helmet Use in 2002 – Overall Results, NHTSA Technical Report, DOT HS 809 500, 

August 2002 

4. Glassbrenner, D., Improving the Calculations of the Lives Saved by Safety Belts and Air Bags, NHTSA Technical 

Report, DOT HS 809, August 2003 

5. Kahane, C., Fatality Reduction by Safety Belts for Front-Seat Occupants of Cars and Light Trucks, NHTSA Technical 

Report, DOT HS 809 199, December 2000 

6. Morgan, C., Effectiveness of Lap/Shoulder Belts in the Back Outboard Seating Positions, NHTSA Technical Report, 

DOT HS 808 945, June 1999 

7. Wang, J. and Blincoe, L., Belt Regression Model - 2003 Update, NHTSA Research Note, DOT HS 809, May 2003 

Page 137