From: <james.tamm@dot.gov> 

To: William Charmley/AA/USEPA/US@EPA, Ed Nam/AA/USEPA/US@EPA, Robin 

Moran/AA/USEPA/US@EPA 

Cc: <rebecca.yoon@dot.gov>, <Steve.Wood@dot.gov> 

Date: 03/17/2010 04:52 PM 

Subject

: 

FW: Joint EPA/NHTSA Preamble Chapter 2.G Safety Effects - DRAFT 

Bill and Ed,

  

Thanks to you and your team for today’s productive meeting.  Attached is the draft we sent to

OMB.  Per Chandana, this OMB will wait until tomorrow’s final version is received to distribute

to other agencies

  

Jim

  

From: Tamm, James (NHTSA) 

Sent: Wednesday, March 17, 2010 4:42 PM

To: 'Achanta, Chandana L.'

Cc: Yoon, Rebecca (NHTSA); Wood, Steve (NHTSA); Kratzke, Stephen (NHTSA); Vincent, Kevin 

(NHTSA); Eisner, Neil (OST); Simons, Jim (NHTSA); Blincoe, Larry (NHTSA); Kahane, Chuck 

(NHTSA); Thomson, Kathryn (OST); Medford, Ronald (NHTSA); Strickland, David (NHTSA)

Subject: Joint EPA/NHTSA Preamble Chapter 2.G Safety Effects - DRAFT 

 

Chandana,

Per the OMB request, NHTSA is forwarding the current draft of joint EPA/NHTSA Preamble

Chapter 2.G on the estimated safety effects of the final MY’s 2012‐2016 CAFE and GHG

standards.    

EPA and NHTSA are completing final review of the document, so some revisions may be

incorporated into the final document.    

EPA and NHTSA are working to provide the final versions of Chapter 2.G by close of business

tomorrow, Thursday 3/18.  

  

Jim

  

James Tamm 

Fuel Economy Division Chief 

Office of Rulemaking 

National Highway Traffic Safety Administration 

Rm W45-340 

1200 New Jersey Ave. SE 

Washington, DC 20590 

202-493-0515 

 

   

[attachment "Section II.G ‐‐ draft joint safety section for preamble ‐‐ 17 Mar 10.docx" deleted

by Tad Wysor/AA/USEPA/US]
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

G. What are the estimated safety effects of the final MYs 2012-2016 

CAFE and GHG standards? 

The primary goals of the final CAFE and GHG standards are to reduce fuel 

consumption and GHG emissions, but in addition to these intended effects, the agencies 

must consider the potential of the standards to affect vehicle safety, which the agencies 

have assessed in evaluating the appropriate levels at which to set the final standards. 

Safety trade-offs associated with fuel economy increases have occurred in the past, and 

the agencies must be mindful of the possibility of future ones. These past safety tradeoffs occurred because manufacturers chose, at the time, to build smaller and lighter 

vehicles in response to CAFE standards rather than adding more expensive fuel-saving 

technologies (and maintaining vehicle size and safety), and the smaller and lighter 

vehicles did not fare as well in crashes as larger and heavier vehicles. Historically, as 

shown in FARS data analyzed by NHTSA, the safest vehicles have been heavy and large, 

while the vehicles with the highest fatal-crash rates have been light and small, both 

because the crash rate is higher for small/light vehicles and because the fatality rate per 

crash is higher for small/light vehicle crashes. 

The primary mechanism in this rulemaking for mitigating the potential negative 

effects on safety is the application of footprint-based standards, which create a 

disincentive for manufacturers to produce smaller-footprint vehicles. Manufacturers have 

stated, however, that they will reduce vehicle weight as one of the cost-effective means of 

increasing fuel economy and reducing CO2 emissions, and the agencies have incorporated 

this expectation into our modeling analysis supporting today’s final standards. NHTSA’s 

previous analyses examining the relationship between vehicle mass and fatalities found 

fatality increases as vehicle weight and size were reduced, but these previous analyses did 

not differentiate between weight reductions and size (i.e., weight and footprint) 

reductions. 

The question of the effect of changes in vehicle mass on safety in the context of 

fuel economy is a complex question that poses serious analytic challenges and has been a 

contentious issue for many years, as discussed by a number of commenters to the NPRM. 

This contentiousness arises, at least in part, from the difficulty of isolating vehicle mass 

from other confounding factors (e.g., driver behavior, or vehicle factors such as engine 

size and wheelbase). In addition, several vehicle factors have been closely related 

historically, such as vehicle mass, wheelbase, and track width. The issue has been 

reviewed and analyzed in the literature for more than two decades. For the reader’s 

reference, much more information about safety in the CAFE context is available in 

Chapter IX of NHTSA’s FRIA. Chapter 7.6 of EPA’s final RIA also contained 

additional discussion on mass and safety. 

Over the past several years, as also discussed by a number of commenters to the 

NPRM, contention has arisen with regard to the applicability of analysis of historical 

crash data to future safety effects due to mass reduction. The agencies recognize that 

there are a host of factors that may make future mass reduction different than what is 

reflected in the historical data. For one, the footprint-based standards have been carefully 

1 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

developed by the agencies so that they do not encourage vehicle footprint reductions as a 

way of meeting the standards, but so that they do encourage application of fuel-saving 

technologies, including mass reduction. This in turn encourages manufacturers to find 

ways to separate mass reduction from footprint reduction, which will very likely result in 

a future relationship between mass and fatalities that is safer than the historical 

relationship. However, as manufacturers pursue these methods of mass reduction, the 

fleet moves further away from the historical trends, which the agencies recognize. 

NHTSA’s NPRM analysis of the safety effects of the proposed CAFE standards 

was based on NHTSA’s 2003 report concerning mass and size reduction in MYs 1991-

1999 vehicles, and evaluated a “worst-case scenario” in which the safety effects of the 

combined reductions of both mass and size for those vehicles were determined for the 

future passenger car and light truck fleets.1 In the NPRM analysis, mass and size could 

not be separated from one another, resulting in what NHTSA recognized was a larger 

safety disbenefit than was likely under the MYs 2012-2016 footprint-based CAFE 

standards. NHTSA emphasized, however, that actual fatalities would likely be less than 

these “worst-case” estimates, and possibly significantly less, based on the various factors 

discussed in the NPRM that could reduce the estimates, such as careful mass reduction 

through material substitution, etc. 

For the final rule, as discussed in the NPRM and in recognition of the importance 

of conducting analysis that better reflects, within the limits of our current knowledge, the 

potential safety effects of future mass reduction in response to the final CAFE and GHG 

standards that is highly unlikely to involve concurrent reductions in footprint, NHTSA 

has revised its analysis in consultation with EPA. Perhaps the most important change has 

been that NHTSA agreed with commenters that it was both possible and appropriate to 

separate the effect of mass reductions from the effect of footprint reductions. NHTSA 

thus performed a new statistical analysis, hereafter referred to as the 2010 Kahane 

analysis, of the MYs 1991-99 vehicle database from its 2003 report (now including rather 

than excluding 2-door cars in the passenger car fleet), assessing relationships between 

fatality risk, mass, and footprint for both passenger cars and LTVs (light trucks and 

vans).2

 As part of its results, the new report presents an “upper-estimate scenario,” a 

“lower-estimate scenario,” and an “unmodified regression scenario” representing 

potential safety effects of future mass reductions without corresponding vehicle size 

reductions, that assume, by virtue of being a cross-sectional analysis of historical data, 

that historical relationships between vehicle mass and fatalities are maintained. The 

“upper-estimate scenario” and “lower-estimate scenario” are based on NHTSA’s 

judgment as a vehicle safety agency, and are not meant to convey any more or less 

likelihood in the results, but more to convey a sense of bounding for potential safety 

effects. The upper-estimate scenario reflects potential safety effects given the report’s 

 

1

 The analysis excluded 2-door cars. 

2

 “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 1991-1999 and Other 

Passenger Cars and LTVs,” Charles J. Kahane, NCSA, NHTSA, March 2010, available at Docket No. 

NHTSA-2009-0059-[item no.]. We note that this report has not yet been externally peer-reviewed, but that 

NHTSA has begun the process for peer review in accordance with OMB guidance by the time of 

publication of this final rule. The results of the peer review and any subsequent revisions to the report will 

be made available in a public docket and on NHTSA’s website as they are completed. 

2 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

finding that that, using the one-step regression method of the 2003 Kahane report, the 

regression coefficients show that mass and footprint each accounted for about half the 

fatality increase associated with downsizing in a cross-sectional analysis of MYs 1991-

1999 cars. A similar effect was found for lighter LTVs. Applying the same regression 

method to heavier LTVs, however, the coefficients indicated a significant societal fatality 

reduction when mass, but not footprint, is reduced in the heavier LTVs.3 Fatalities are 

reduced primarily because mass reduction in the heavier LTVs will reduce risk to 

occupants of the other cars and lighter LTVs involved in collisions with these heavier 

LTVs.4

 Thus, even in the “upper-estimate scenario,” the potential fatality increases 

associated with mass reduction in the passenger cars would be to a large extent offset by 

the benefits of mass reduction in the heavier LTVs. 

The lower-estimate scenario, in turn, reflects NHTSA’s estimate of potential 

safety effects if future mass reduction is accomplished entirely by things like material 

substitution, smart design,5

 and component integration, among other things, that can 

reduce mass without perceptibly changing a vehicle’s shape, functionality, or safety 

performance, maintaining structural strength without compromising other aspects of 

safety. If future mass reduction follows this path, it could limit the added risk close to 

only the effects of mass per se (the ability to transfer momentum to other vehicles or 

objects in a collision), resulting in estimated effects in passenger cars that are 

substantially smaller than in the upper-estimate scenario based directly on the regression 

results. The lower-estimate scenario also covers both passenger cars and LTVs. 

Overall, based on the new analyses, NHTSA estimated that fatality effects could 

be markedly less than those estimated in the “worst-case scenario” presented in the 

NPRM. The agencies believe that the overall effect of mass reduction in cars and LTVs 

may be close to zero, and may possibly be beneficial in terms of the fleet as a whole if 

mass reduction is carefully applied in the future (as with careful material substitution and 

other methods of mass reduction that can reduce mass without perceptibly changing a 

car’s shape, functionality, or safety performance, and maintain its structural strength 

without making it excessively rigid). This is especially important if the mass reduction in 

the heavier LTVs is greater (in absolute terms) than in passenger cars, as discussed 

further below and in the 2010 Kahane report. 

 

3

 Conversely, the coefficients indicate a significant increase if footprint is reduced. 4

 We note that there may be some (currently non-quantifiable) welfare losses for purchasers of these 

heavier LTVs, the mass of which is reduced in response to these final standards. This is due to the fact that 

in certain crashes, as discussed below and in greater detail in Chapter IX of the NHTSA FRIA, more mass 

will always be helpful (although certainly in other crashes, the amount of mass reduction modeled by the 

agency will not be enough to have any significant effect on driver/occupant safety). However, we believe 

the effects of this will likely be minor. Consumer welfare impacts of the final rule are discussed in more 

detail in Chapter VIII of the NHTSA FRIA. 

5

 Manufacturers implement “smart design” through computer aided engineering (CAE) tools that can be 

used to better optimize load paths within structures by reducing stresses and bending moments applied to 

structures. This allows better optimization of the sectional thicknesses of structural components to reduce 

mass while maintaining or improving the function of the component. Smart designs also integrate separate 

parts in a manner that reduces mass by combining functions or the reduced use of separate fasteners. In 

addition, some “body on frame” vehicles are redesigned with a lighter “unibody” construction. 

3 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

The following sections will address how the agencies addressed potential safety 

effects in the NPRM for the proposed standards, how commenters responded, and the 

work that NHTSA has done since the NPRM to revise its estimates of potential safety 

effects for the final rule. The final section discusses some of the agencies’ plans for the 

future with respect to potential analysis and studies to further enhance our understanding 

of this important and complex issue. 

1. What did the agencies say in the NPRM with regard to potential 

safety effects? 

In the NPRM preceding these final standards, NHTSA’s safety assessment 

derived from the agency’s belief that some of these vehicle factors, namely vehicle mass 

and footprint, could not be accurately separated. NHTSA relied on the 2003 study by Dr. 

Charles Kahane, which estimates the effect of 100-pound reductions in MYs 1991-1999 

heavy light trucks and vans (LTVs), light LTVs, heavy passenger cars, and light 

passenger cars.6

 The study compares the fatality rates of LTVs and cars to quantify 

differences between vehicle types, given drivers of the same age/gender, etc. In that 

analysis, the effect of “weight reduction” is not limited to the effect of mass per se, but 

includes all the factors, such as length, width, structural strength, safety features, and size 

of the occupant compartment, that were naturally or historically confounded with mass in 

MYs 1991-1999 vehicles. The rationale was that adding length, width, or strength to a 

vehicle historically also made it heavier. 

NHTSA utilized the relationships between mass and safety from Kahane (2003), 

expressed as percentage increases in fatalities per 100-pound mass reduction, and 

examined the mass effects assumed in the NPRM modeling analysis. While previous 

CAFE rulemakings had limited mass reduction as a “technology option” to vehicles over 

5,000 pounds GVWR, both NHTSA’s and EPA’s modeling analyses in the NPRM 

included mass reduction of up to 5-10 percent of baseline curb weight, depending on 

vehicle subclass, in response to recently-submitted manufacturer product plans as well as 

public statements indicating that these levels were possible and likely. 5-10 percent 

represented a maximum bound; EPA’s modeling, for example, included average vehicle 

weight reductions of 4 percent between MYs 2011 and 2016, although the average pervehicle mass reduction was greater in absolute terms for light trucks than for passenger 

cars. NHTSA’s assumptions for mass reduction were also limited by lead time such that 

mass reductions of 1.5 percent were included for redesigns occurring prior to MY 2014, 

and mass reductions of 5-10 percent were only “achievable” in redesigns occurring in 

MY 2014 or later. NHTSA further assumed that mass reductions would be limited to 5 

percent for small vehicles (e.g., subcompact passenger cars), and that reductions of 10 

percent would only be applied to the larger vehicle types (e.g., large light trucks). 

 

6

 Kahane, Charles J., Ph.D., “Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 1991-

99 Passenger Cars and Light Trucks,” DOT HS 809 662, October 2003, Executive Summary. Available at

http://www.nhtsa.dot.gov/cars/rules/regrev/evaluate/809662.html (last accessed March 10, 2010). 

4 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

Based on these assumptions of how manufacturers might comply with the 

standards, NHTSA examined the effects of the identifiable safety trends over the lifetime 

of the vehicles produced in each model year. The effects were estimated on a year-byyear basis, assuming that certain known safety trends would result in a reduction in the 

target population of fatalities from which the mass effects are derived.7 Using this 

method, NHTSA found a 12.6 percent reduction in fatality levels between 2007 and 

2020. The estimates derived from applying Kahane’s 2003 percentages to a baseline of 

2007 fatalities were then multiplied by 0.874 to account for changes that the agency 

believed would take place in passenger car and light truck safety between the 2007 

baseline on-road fleet used for that particular analysis and year 2020. 

NHTSA and EPA both emphasized that the safety effect estimates in the NPRM 

needed to be understood in the context of the 2003 Kahane report, which is based upon a 

cross-sectional analysis of the actual on-road safety experience of 1991-1999 vehicles. 

For those vehicles, heavier usually also meant larger-footprint. Hence, the numbers in 

those analyses were used to predict the safety-related fatalities that could occur in the 

unlikely event that weight reduction for MYs 2012-2016 is accomplished entirely by 

reducing mass and reducing footprint. Any estimates derived from those analyses 

represented a “worst-case” estimate of safety effects, for several reasons. 

 First, manufacturers are far less likely to reduce mass by “downsizing” (making 

vehicles smaller overall) under the current attribute-based standards, because the 

standards are based on vehicle footprint. The selection of footprint as the attribute in 

setting CAFE and GHG standards helps to minimize the incentive to reduce a vehicle’s 

physical dimensions. This is because as footprint decreases, the corresponding fuel 

economy/GHG emission target becomes more stringent.8 The shape of the footprint 

curves themselves have also been designed to be approximately “footprint neutral” within 

the sloped portion of the functions – that is, to neither encourage manufacturers to 

 

7

 NHTSA explained that there are several identifiable safety trends that are already in place or expected to 

occur in the foreseeable future and that were not accounted for in the study. For example, two important 

new safety standards that have already been issued and will be phasing in during the rulemaking time 

frame. Federal Motor Vehicle Safety Standard No. 126 (49 CFR 571.126) will require electronic stability 

control in all new vehicles by MY 2012, and the upgrade to Federal Motor Vehicle Safety Standard No. 

214 (Side Impact Protection, 49 CFR 571.214) will likely result in all new vehicles being equipped with 

head-curtain air bags by MY 2014.7

 Additionally, the agency stated that it anticipates continued 

improvements in driver (and passenger) behavior, such as higher safety belt use rates. All of these will 

tend to reduce the absolute number of fatalities resulting from mass reductions. Thus, while the percentage 

increases in Kahane (2003) was applied, the reduced base resulted in smaller absolute increases than those 

that were predicted in the 2003 report.

8

 We note, however, that vehicle footprint is not synonymous with vehicle size. Since the footprint is only 

that portion of the vehicle between the front and rear axles, footprint-based standards do not discourage 

downsizing the portions of a vehicle in front of the front axle and to the rear of the rear axle, or to other 

portions of the vehicle outside the wheels. The crush space provided by those portions of a vehicle can 

make important contributions to managing crash energy. NHTSA noted in the NPRM that at least one 

manufacturer has confidentially indicated plans to reduce overhang as a way of reducing mass on some 

vehicles during the rulemaking time frame. Additionally, simply because footprint-based standards create 

no incentive to downsize vehicles, does not mean that manufacturers may not choose to do so if doing so 

makes it easier to meet the overall standard (as, for example, if the smaller vehicles are so much lighter that 

they exceed their targets by much greater amounts). 

5 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

increase the footprint of their fleets, nor to decrease it. For further discussion on these 

aspects of the standards, please see Section II.C above and Chapter 2 of the joint TSD. 

However, as discussed in [consumer welfare discussion section], the agencies 

acknowledge some uncertainty regarding how consumer purchases will change in 

response to the vehicles designed to meet the MYs 2012-2016 standards. This could 

potentially affect the mix of vehicles sold in the future, including the mass and footprint 

distribution. 

As a result, the agencies found it likely that a significant portion of the mass 

reduction in the MY 2012-2016 vehicles would be accomplished by strategies, such as 

material substitution, smart design, reduced powertrain requirements, and mass 

compounding, that have a lesser safety effect than the prevalent 1980s strategy of simply 

making the vehicles smaller. The agencies noted that to the extent that future mass 

reductions could be achieved by these methods—without any accompanying reduction in 

the size or structural strength of the vehicle—then the fatality increases associated with 

the mass reductions anticipated by the model as a result of the proposed standards could 

be significantly smaller than those in the worst-case scenario. 

However, even though the agencies recognized that these methods of mass 

reduction could be technologically feasible in the rulemaking time frame, and included 

them as such in our modeling analyses, the agencies diverged as to how potential safety 

effects accompanying such methods of mass reduction could be evaluated, particularly in 

relation to the worst-case scenario presented by NHTSA. NHTSA stated that it could not 

predict how much smaller those increases would be for any given mixture of mass 

reduction methods, since the data on the safety effects of mass reduction alone (without 

size reduction) was not available due to the low numbers of vehicles in the current onroad fleet that have utilized these technologies extensively. Further, to the extent that 

mass reductions were accomplished through use of light, high-strength materials, 

NHTSA emphasized that there would be significant additional costs that would need to 

be determined and accounted for than were reflected in the agency’s proposal. 

 Additionally, NHTSA emphasized that while it thought material substitution and 

other methods of mass reduction could considerably lessen the potential safety effects 

compared to the historical trend, NHTSA also stated that it did not believe the effects in 

passenger cars would be smaller than zero. EPA disagreed with this, and stated in the 

NPRM that the safety effects could very well be smaller than zero. Even though 

footprint-based standards discourage downsizing as a way of “balancing out” sales of 

larger/heavier vehicles, they do not discourage manufacturers from reducing crush space 

in overhang areas or from reducing structural support as a way of taking out mass.9

 

Moreover, NHTSA’s analysis had also found that lighter cars have a higher involvement 

rate in fatal crashes, even after controlling for the driver’s age, gender, urbanization, and 

region of the country. Being unable to explain this clear trend in the crash data, NHTSA 

stated that it must assume that mass reduction is likely to be associated with higher fatalcrash rates, no matter how the weight reduction is achieved.

 

9

 However, we recognize that FMVSS and NCAP ratings may limit the manufacturer’s ability to reduce 

crush space or structural support. 

6 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

NHTSA also noted in the NPRM that several studies by Dynamic Research, Inc. 

(DRI) had been repeatedly cited to the agency in support of the proposition that reducing 

vehicle mass while maintaining track width and wheelbase would lead to significant 

safety benefits. In its 2005 studies, one of which was published and peer-reviewed 

through the Society of Automotive Engineers as a technical paper, DRI attempted to 

assess the independent effects of vehicle weight and size (in terms of wheelbase and track 

width) on safety, and presented results indicating that reducing vehicle weight tends to 

reduce fatalities, but that reducing vehicle wheelbase and track width tends to increase 

fatalities. DRI’s analysis was based on FARS data for MYs 1985-1998 passenger cars 

and 1985-1997 light trucks, similar to the MYs 1991-1999 car and truck data used in the 

2003 Kahane report. However, DRI included 2-door passenger cars, while the 2003 

Kahane report excluded those vehicles out of concern that their inclusion could bias the 

results of the regression analysis, because a significant proportion of MYs 1991-1999 2-

door cars were sports and “muscle” cars, which have particularly high fatal crash rates for 

their relatively short wheelbases compared to the rest of the fleet. While in the NPRM 

NHTSA rejected the results of the DRI studies based in part on this concern, the agencies 

note that upon further consideration, NHTSA has agreed for this final rule that the 

inclusion of 2-door cars in regression analysis of historical data is appropriate, and indeed 

has no overly-biasing effects. 

The 2005 DRI studies also differed from the 2003 Kahane report in terms of their 

estimates of the effect of vehicle weight on rollover fatalities. The 2003 Kahane report 

analyzed a single variable, curb weight, as a surrogate for both vehicle size and weight, 

and found that curb weight reductions would increase rollover fatalities. The DRI study, 

in contrast, attempted to analyze curb weight, wheelbase, and track width separately, and 

found that curb weight reduction would decrease rollover fatalities, while wheelbase 

reduction and track width reduction would increase them. DRI suggested that heavier 

vehicles may have higher rollover fatalities for two reasons: first, because taller vehicles 

tend to be heavier, so the correlation between vehicle height and weight and vehicle 

center-of-gravity height may make heavier vehicles more rollover-prone; and second, 

because heavier vehicles may have been less rollover-crashworthy due to FMVSS No. 

216’s constant (as opposed to proportional) requirements for MYs 1995-1999 vehicles 

weighing more than 3,333 lbs unloaded. 

Overall, DRI’s 2005 studies found a reduction in fatalities for cars (580 in the first 

study, and 836 in the second study) and for trucks (219 in the first study, 682 in the 

second study) for a 100 pound reduction in curb weight without accompanying wheelbase 

or track width reductions. In the NPRM, NHTSA disagreed with the results of the DRI 

studies, out of concern that DRI’s inclusion of 2-door cars in its analysis biased the 

results, and because NHTSA was unable to reproduce DRI’s results despite repeated 

attempts. NHTSA stated that it agreed intuitively with DRI’s conclusion that vehicle 

mass reductions without accompanying size reductions (as through substitution of a 

heavier material for a lighter one) would be less harmful than downsizing, but without 

supporting real-world data and unable to verify DRI’s results, NHTSA stated that it could 

not conclude that mass reductions would result in safety benefits. EPA, in contrast, 

7 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

believed that DRI’s results contained some merit, in particular because of the study 

separated the effects of mass and size and EPA stated that applying them using the curb 

weight reductions in EPA’s modeling analysis would show an overall reduction of 

fatalities for the proposed standards. 

On balance, both agencies recognized that mass reduction could be an important 

tool for achieving higher levels of fuel economy and reducing CO2 emissions, and 

emphasized that NHTSA’s fatality estimates represented a worst-case scenario for the 

potential effects of the proposed standards, and that actual fatalities will be less than these 

estimates, possibly significantly less, based on the various factors discussed in the NPRM 

that could reduce the estimates. The agencies sought comment on the safety analysis and 

discussions presented in the NPRM. 

2. What public comments did the agencies receive on the safety analysis 

and discussions in the NPRM?

 Several dozen commenters addressed the safety issue. Comments in response to 

the safety effects analysis and discussion in the NPRM tended to follow several general 

themes, as follows: 

• NHTSA’s safety effects estimates are inaccurate because they do not account for 

the fact that: 

o While NHTSA’s study only considers vehicles from MYs 1991-1999, 

more recently-built vehicles are safer than those, and future vehicles will 

be safer still; 

o Lighter vehicles are safer than heavier cars in terms of crash-avoidance, 

because they handle and brake better; 

o Fatalities are linked more to other factors than mass; 

o The structure of the standards reduces/contributes to potential safety 

effects from mass reduction; 

o NHTSA could mitigate additional safety effects from mass reduction, if 

there are any, by simply regulating safety more; 

o Casualty risks range widely for vehicles of the same weight or footprint, 

which skews regression analysis and makes computer simulation a better 

predictor of the safety effects of mass reduction; 

• DRI’s analysis shows that lighter vehicles will save lives, and NHTSA reaches 

the opposite conclusion without disproving DRI’s analysis; 

o Reasons that NHTSA and DRI may have reached different conclusions: 

 NHTSA’s study should distinguish between reductions in size and 

reductions in weight like DRI’s; 

 NHTSA’s study should include two-door cars; 

 NHTSA’s study should have used different assumptions; 

 NHTSA’s study should include confidence intervals; 

• NHTSA should include a “best-case” estimate in its study; 

• NHTSA should not include a “worst-case” estimate in its study; 

8 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

The agencies recognize that the issue of the potential safety effects of mass 

reduction, which was one of the many factors considered in the balancing that led to the 

agencies’ conclusion as to appropriate stringency levels for the MYs 2012-2016 

standards, is of great interest to the public and could possibly be a more significant factor 

in regulators’ and manufacturers’ decisions with regard to future standards beyond MY 

2016. The agencies are committed to analyzing this issue thoroughly and holistically 

going forward, based on the best available science, in order to further their closely related 

missions of safety, energy conservation, and environmental protection. We respond to 

the issues and concerns raised by commenters in turn below. 

NHTSA’s estimates are inaccurate because NHTSA’s study only considers vehicles 

from MYs 1991-1999, but more recently-built vehicles are safer than those, and future 

vehicles will be safer still

 A number of commenters (CAS, Adcock, NACAA, NJ DEP, NY DEC, UCS, and 

Wenzel) argued that the 2003 Kahane report, on which the “worst-case scenario” in the 

NPRM was based, is outdated because it considers the relationship between vehicle 

weight and safety in MYs 1991-1999 passenger cars. These commenters generally stated 

that data from MYs 1991-1999 vehicles provide an inaccurate basis for assessing the 

relationship between vehicle weight and safety in current or future vehicles, because the 

fleets of vehicles now and in the future are increasingly different from that 1990s fleet 

(more crossovers, fewer trucks, lighter trucks, etc.), with different vehicle shapes and 

characteristics, different materials, and more safety features. Several of these 

commenters argued that NHTSA should conduct an updated analysis for the final rule 

using more recent data – Wenzel, for example, stated that an updated regression analysis 

that accounted for the recent introduction of crossover SUVs would likely find reduced 

casualty risk, similar to DRI’s previous finding using fatality data. CEI, in contrast, 

argued that the “safety trade-off” would not be eliminated by new technologies and 

attribute-based standards, because additional weight inherently makes a vehicle safer to 

its own occupants, citing the 2003 Kahane report, while AISI argued that Desapriya had 

found that passenger car drivers and occupants are two times more likely to be injured 

than drivers and occupants in larger pickup trucks and SUVs. 

 Several commenters (Adcock, CARB, Daimler, NESCAUM, NRDC, Public 

Citizen, UCS, Wenzel) suggested that NHTSA’s analysis was based on overly 

pessimistic assumptions about how manufacturers would choose to reduce mass in their 

vehicles, because manufacturers have a strong incentive in the market to build vehicles 

safely. Many of these commenters stated that several manufacturers have already 

committed publicly to fairly ambitious mass reduction goals in the mid-term, but several 

stated further that NHTSA should not assume that manufacturers will reduce the same 

amount of mass in all vehicles, because it is likely that they will concentrate mass 

reduction in the heaviest vehicles, which will improve compatibility and decrease 

aggressivity in the heaviest vehicles. Daimler emphasized that all vehicles will have to 

comply with the Federal Motor Vehicle Safety Standards, and will likely be designed to 

test well in NHTSA’s NCAP tests. 

9 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

Other commenters (Aluminum Association, CARB, CAS, ICCT, MEMA, NRDC, 

U.S. Steel) also emphasized the need for NHTSA to account for the safety benefits to be 

expected in the future from use of advanced materials for lightweighting purposes and 

other engineering advances. The Aluminum Association stated that advanced vehicle 

design and construction techniques using aluminum can improve energy management and 

minimize adverse safety effects of their use,10 but that NHTSA’s safety analysis could 

not account for those benefits if it were based on MYs 1991-1999 vehicles. CAS, ICCT, 

and U.S. Steel discussed similar benefits for more recent and future vehicles built with 

high strength steel (HSS), although U.S. Steel cautioned that given the stringency of the 

proposed standards, manufacturers would likely be encouraged to build smaller and 

lighter vehicles in order to achieve compliance, which fare worse in head-on collisions 

than larger, heavier vehicles. AISI, in contrast to U.S. Steel, stated that in its research 

with the Auto/Steel Partnership and in programs supported by DOE, it had found that the 

use of new Advanced HSS steel grades could enable mass of critical crash structures, 

such as front rails and bumper systems, to be reduced by 25 percent without degrading 

performance in standard NHTSA frontal or IIHS offset instrumented crash tests 

compared to their “heavier counterparts.” 

Agencies’ response:

NHTSA, in consultation with EPA and DOE, plans to begin updating the MYs 

1991-1999 database on which NHTSA’s safety analyses in the NPRM and final rule are 

based in the next several months in order to analyze the differences in safety effects 

against vehicles built in more recent model years. As this task will take at least a year to 

complete, beginning it immediately after the NPRM would not have enabled the agency 

to complete it and then conduct a new analysis during the period between the NPRM and 

the final rule. 

For purposes of this final rule, however, we believe that using the same MYs 

1991-1999 database as that used in the 2003 Kahane study provides a reasonable basis for 

attempting to estimate safety effects due to reductions in mass. While commenters often 

stated that updating the database would help to reveal the effect of recently-introduced 

lightweight vehicles with extensive material substitution, there have in fact not yet been a 

significant number of vehicles with substantial mass reduction/material substitution to 

analyze, and they must also show up in the crash databases for NHTSA to be able to add 

them to its analysis. Based on NHTSA’s research, specifically, on three statistical 

analyses over a 12-year period (1991-2003) covering a range of 22 model years (1978-

1999), NHTSA believes that the relationships between mass, size, and safety change 

slowly over time, although we recognize that they may change somewhat more rapidly in 

 

10 The Aluminum Association (NHTSA-2009-0059-0067.3) stated that its research on vehicle safety 

compatibility between an SUV and a mid-sized car, done jointly with DRI, shows that reducing the weight 

of a heavier SUV by 20% (a realistic value for an aluminum-intensive vehicle) could reduce the combined 

injury rate for both vehicles by 28% in moderately severe crashes. The commenter stated that it would 

keep NHTSA apprised of its results as its research progressed. Based on the information presented, 

NHTSA believes that this research appears to agree with NHTSA’s latest analysis, which finds that a 

reduction in weight for the heaviest vehicles may improve overall fleet safety.

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the future.11 As the on-road fleet gains increasing numbers of vehicles with increasing 

amounts of different methods of mass reduction applied to them, we may begin to discern 

changes in the crash databases due to the presence of these vehicles, but any such 

changes are likely to be slow and evolutionary, particularly in the context of MYs 2000-

2009 vehicles. The agencies do expect that further analysis of historical data files will 

provide the most robust basis practicable for estimating the potential safety effects that 

might occur with future reductions in vehicle mass. However, we recognize that 

estimates derived from analysis of historical data, like estimates from any other type of 

analysis (including simulation-based analysis, which cannot feasibly cover all relevant 

scenarios), will be uncertain in terms of predicting actual future outcomes with respect to 

a vehicle fleet, driving population, and operating environment that does not yet exist. 

 

The agencies also recognize that more recent vehicles have more safety features 

than 1990s vehicles, which are likely to make them safer overall. To account for this, 

NHTSA did adjust the results of both its NPRM and final rule analysis to include known 

safety improvements, like ESC and increases in seat belt use, that have occurred since 

MYs 1991-1999.12 However, simply because newer vehicles have more safety 

countermeasures, does not mean that the weight/safety relationship necessarily changes. 

More likely, it would change the target population (the number of fatalities) to which one 

would apply the weight/safety relationship. Thus, we still believe that some mass 

reduction techniques for both passenger cars and light trucks can make them less safe, in 

certain crashes as discussed in NHTSA’s FRIA, than if mass had not been reduced.13

As for NHTSA’s assumptions about mass reduction, in its analysis, NHTSA 

generally assumed that lighter vehicles could be reduced in weight by 5 percent while 

heavier light trucks could be reduced in weight by 10 percent. NHTSA recognizes that 

manufacturers might choose a different mass reduction scheme than this, and that its 

quantification of the estimated effect on safety would be different if they did. We 

emphasize that our estimates are based on the assumptions we have employed and are 

intended to help the agency consider the potential effect of the final standards on vehicle 

safety. Thus, based on the 2010 Kahane analysis, reductions in weight for the heavier 

light trucks would have positive overall safety effects,14 while mass reductions for 

passenger cars and smaller light trucks would have negative overall safety effects. 

 

11 NHTSA notes the CAS’ comments regarding changes in the vehicle fleets since the introduction of 

CAFE standards in the late 1970s, but believes they apply more to the differences between late 1970s 

through 1980s vehicles and 2010s vehicles than to the differences between 1990s and 2010s vehicles. 

NHTSA believes that the CAS comments regarding the phase-out of 1970s vehicles and their replacement 

with safer, better fuel-economy-achieving 1980s vehicles paint with rather too large a brush to be relevant 

to the main discussion of whether the 2003 Kahane report database can reasonably be used to estimate 

safety effects of mass reduction for the MYs 2012-2016 fleet. 

12 [ref FRIA how we did this adjustment] 13 If one has a vehicle (vehicle A), and both reduces the vehicle’s mass and adds new safety equipment to it, 

thus creating a variant (vehicle A1), the variant might conceivably have a level of overall safety for its 

occupants is equal to that of the original vehicle (vehicle A). However, vehicle A1 might not be as safe as 

second variant (vehicle A2) of vehicle A, one that is produced by adding to vehicle A the same new safety 

equipment added to the first variant, but this time without any mass reduction. 

14 This is due to the beneficial effect on the occupants of vehicles struck by the downweighted larger 

vehicles. 

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NHTSA’s estimates are inaccurate because they do not account for the fact that 

lighter vehicles are safer than heavier cars in terms of crash-avoidance, 

because they handle and brake better 

ICCT stated that lighter vehicles are better able to avoid crashes because they 

“handle and brake slightly better,” arguing that size-based standards encourage lighterweight car-based SUVs with “significantly better handling and crash protection” than 

1996-1999 mid-size SUVs, which will reduce both fatalities and fuel consumption. ICCT 

stated that NHTSA did not include these safety benefits in its analysis. DRI also stated 

that its 2005 report found that crash avoidance improves with reduction in curb weight 

and/or with increases in wheelbase and track, because “Crash avoidance can depend, 

amongst other factors, on the vehicle directional control and rollover characteristics.” 

DRI argued that, therefore, “These results indicate that vehicle weight reduction tends to 

decrease fatalities, but vehicle wheelbase and track reduction tends to increase fatalities.” 

Agencies’ response:

In fact, NHTSA’s regression analysis of crash fatalities per million registration 

years measures the effects of crash avoidance, if there are any, as well as 

crashworthiness. Given that the historical empirical data for passenger cars show a trend 

of higher crash rates for lighter cars, it is unclear whether lighter cars have, in the net, 

superior crash avoidance, although the agencies recognize that they may have advantages 

in certain individual situations. EPA presents a discussion of improved accident 

avoidance as vehicle mass is reduced in Chapter 7.6 of its final RIA. The important point 

to emphasize is that it depends on the situation – it would oversimplify drastically to 

point to one situation in which extra mass helps or hurts and then extrapolate effects for 

crash avoidance across the board based on only that. 

For example, the relationship of vehicle mass to rollover and directional stability 

is more complex than commenters imply. For rollover, it is true that if heavy pickups 

were always more top-heavy than lighter pickups of the same footprint, their higher 

center of gravity could make them more rollover-prone, yet some mass can be placed so 

as to lower a vehicle’s center of gravity and make it less rollover-prone. For mass 

reduction to be beneficial in rollover crashes, then, it must take center of gravity height 

into account along with other factors such as passenger compartment design and 

structure, suspension, the presence of various safety equipment, and so forth. 

Similarly, for directional stability, it is true that having more mass increases the 

“understeer gradient” of cars – i.e., it reinforces their tendency to proceed in a straight 

line and slows their response to steering input, which would be harmful where prompt 

steering response is essential, such as in a double-lane-change maneuver to avoid an 

obstacle. Yet more mass and a higher understeer gradient could help when it is better to 

remain on a straight path, such as on a straight road with icy patches where wheel slip 

might impair directional stability. Thus, while less vehicle mass can sometimes improve 

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crash avoidance capability, there can also be situations when more vehicle mass can help 

in other kinds of crash avoidance. 

Further, NHTSA’s research suggests that additional vehicle mass may be even 

more helpful, as will be discussed in Section 6, when the average driver’s response to a 

vehicle’s maneuverability is taken into account. Lighter cars have historically (1976-

2009) had higher collision-involvement rates than heavier cars – even in multi-vehicle 

crashes where directional and rollover stability is not particularly an issue.15 Based on 

our analyses using nationally-collected FARS and GES data, drivers of lighter cars are 

more likely to be the culpable party in a 2-vehicle collision, even after controlling for 

footprint, the driver’s age, gender, urbanization, and region of the country. 

Thus, based on this data, it appears that lighter cars may not be driven as well as 

heavier cars, although it is unknown why this is so. If poor drivers intrinsically chose 

light cars (self-selection), it might be evidenced by an increase in antisocial driving 

behavior (such as DWI, drug involvement, speeding, or driving without a license) as car 

weight decreases, after controlling for driver age and gender – in addition to the increases 

in merely culpable driver behavior (such as failure to yield the right of way). But 

analyses in NHTSA’s 2003 report did not show an increase in antisocial driver behavior 

in the lighter cars paralleling their increase in culpable involvements. 

NHTSA also hypothesizes that certain aspects of lightness and/or smallness in a 

car may give a driver a perception of greater maneuverability that ultimately results in 

driving with less of a “safety margin,” e.g., encouraging them to weave in traffic. That 

may appear paradoxical at first glance, as maneuverability is, in the abstract, a safety 

plus. Yet the situation is not unlike powerful engines that could theoretically enable a 

driver to escape some hazards, but in reality have long been associated with high crash 

and fatality rates.16

NHTSA’s estimates are inaccurate because fatalities are linked more to other 

factors than mass 

Tom Wenzel stated that the safety record of recent model year crossover SUVs 

indicates that weight reduction in this class of vehicles (small to mid-size SUVs) resulted 

in a reduction in fatality risk. Wenzel argued that NHTSA should acknowledge that other 

vehicle attributes may be as important, if not more important, than vehicle weight or 

footprint in terms of occupant safety, such as unibody construction as compared to 

 

15 See, e.g., NHTSA (2000). Traffic Safety Facts 1999. Report No. DOT HS 809 100. Washington, DC: 

National Highway Traffic Safety Administration, p. 71; Najm, W.G., Sen, B., Smith, J.D., and Campbell, 

B.N. (2003). Analysis of Light Vehicle Crashes and Pre-Crash Scenarios Based on the 2000 General 

Estimates System, Report No. DOT HS 809 573. Washington, DC: National Highway Traffic Safety 

Administration, p. 48. 

16 Robertson, L.S. (1991), “How to Save Fuel and Reduce Injuries in Automobiles,” The Journal of 

Trauma, Vol. 31, pp. 107-109; Kahane, C.J. (1994). Correlation of NCAP Performance with Fatality Risk 

in Actual Head-On Collisions, NHTSA Technical Report No. DOT HS 808 061. Washington, DC: 

National Highway Traffic Safety Administration, http://www-nrd.nhtsa.dot.gov/Pubs/808061.PDF, pp. 4-

7. 

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ladder-frame, lower bumpers, and less rigid frontal structures, all of which make 

crossover SUVs more compatible with cars than truck-based SUVs. 

Marc Ross commented that fatalities are linked more strongly to intrusion than to 

mass, and stated that research by safety experts in Japan and Europe suggests the main 

cause of serious injuries and deaths is intrusion due to the failure of load-bearing 

elements to properly protect occupants in a severe crash. Ross argued that the results 

from this project have “overturned the original views about compatibility,” which thought 

that mass and the mass ratio were the dominant factors. Since footprint-based standards 

will encourage the reduction of vehicle weight through materials substitution while 

maintaining size, Ross stated, they will help to reduce intrusion and consequently 

fatalities, as the lower weight reduces crash forces while maintaining size preserves crush 

space. Ross argued that this factor was not considered by NHTSA in its discussion of 

safety. ICCT agreed with Ross’ comments on this issue. 

In previous comments on NHTSA rulemakings and in several studies, Wenzel and 

Ross have argued generally that vehicle design and “quality” is a much more important 

determinant of vehicle safety than mass. In comments on the NPRM, CARB, NRDC, 

Sierra Club, and UCS echoed this theme. 

ICCT commented as well that fatality rates in the EU are much lower than rates in 

the U.S., even though the vehicles in the EU fleet tend to be smaller and lighter than 

those in the U.S. fleet. Thus, ICCT argued, “This strongly supports the idea that vehicle 

and highway design are far more important factors than size or weight in vehicle safety.” 

ICCT added that “It also suggests that the rise in SUVs in the U.S. has not helped reduce 

fatalities.” CAS also commented that Germany’s vehicle fleet is both smaller and lighter 

than the American fleet, and has lower fatality rates. 

Agencies’ response:

NHTSA and EPA agree that there are many features that affect safety. While 

crossover SUVs have lower fatality rates than truck-based SUVs, there are no analyses 

that attribute the improved safety to mass alone, and not to other factors such as the lower 

center of gravity or the unibody construction of these vehicles. While a number of 

improvements in safety can be made, they do not negate the potential that another 100 

lbs. could make a passenger car or crossover vehicle safer for its occupants, because of 

the effects of mass per se as discussed in NHTSA’s FRIA, albeit similar mass reductions 

could make heavier LTVs safer to other vehicles without necessarily harming their own 

drivers and occupants. Moreover, in the 2004 response to docket comments, NHTSA 

explained that the significant relationship between mass and fatality risk persisted even 

after controlling for vehicle price or nameplate, suggesting that vehicle “quality” as cited 

by Wenzel and Ross is not necessarily more important than vehicle mass. 

As for reductions in intrusions due to material substitution, the agencies agree 

generally that the use of new and innovative materials may have the potential to reduce 

crash fatalities, but such vehicles have not been introduced in large numbers into the 

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vehicle fleet. The agencies will continue to monitor the situation, but ultimately the 

effects of different methods of mass reduction on overall safety in the real world (not just 

in simulations) will need to be analyzed when vehicles with these types of mass reduction 

are on the road in sufficient quantities to provide statistically significant results. For 

example, a vehicle that is designed to be much stiffer to reduce intrusion is likely to have 

a more severe crash pulse and thus impose greater forces on the occupants during a crash, 

and might not necessarily be good for elderly and child occupant safety in certain types 

of crashes. Such trade-offs make it difficult to estimate overall results accurately without 

real world data. The agencies will continue to evaluate and analyze such real world data 

as it becomes available, and will keep the public informed as to our progress. 

ICCT’s comment illustrates the fact that different vehicle fleets in different 

countries can face different challenges. EPA and NHTSA do not believe that the fact that 

the EU vehicle fleet is generally lighter than the U.S. fleet is the exclusive reason, or even 

the primary factor, for the EU’s lower fatality rates. Significant differences exist in belt 

usage, drunk driving, rural/urban roads, driving culture, etc. 

The structure of the standards reduces/contributes to potential safety risks from 

mass reduction 

Since switching in 2006 to setting attribute-based light truck CAFE standards, 

NHTSA has emphasized that one of the benefits of a footprint-based standard is that it 

discourages manufacturers from building smaller, less safe vehicles to achieve CAFE 

compliance by “balancing out” their larger vehicles, and thus avoids a negative safety 

consequence of increasing CAFE stringency.17 Some commenters on the NPRM 

(Daimler, IIHS, NADA, NRDC, Sierra Club et al.) agreed that footprint-based standards 

would protect against downsizing and help to mitigate safety risks, while others stated 

that there would still be safety risks even with footprint-based standards – CEI, for 

example, argued that mass reduction inherently creates safety risks, while IIHS and 

Porsche expressed concern about footprint-based standards encouraging manufacturers to 

manipulate wheelbase, which could reduce crush space and worsen vehicle handling. 

U.S. Steel and AISI both commented that the “aggressive schedule” for the proposed 

increases in stringency could encourage manufacturers to build smaller, lighter vehicles 

in order to comply. 

Some commenters also focused on the shape and stringency of the target curves 

and their potential effect on vehicle safety. IIHS agreed with the agencies’ tentative 

decision to cut off the target curves at the small-footprint end. Although the agencies’ 

did so in relation to concerns about the feasibility of attribute-based standards as applied 

to manufacturers emphasizing small vehicles, IIHS’s comments suggest IIHS is not 

concerned with the standards’ resultant neutrality with respect to incremental reductions 

 

17 We note that commenters were divided on whether they believed there was a clear correlation between 

vehicle size/weight and safety (CEI, Congress of Racial Equality, Heritage Foundation, IIHS, Spurgeon, 

University of PA Environmental Law Project) or whether they believed that the correlation was less clear, 

for example because they believed that vehicle design was more important than vehicle mass (CARB, 

Public Citizen). 

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in the size of the smallest vehicles. Regarding the safety effect of the curves requiring 

less stringent targets for larger vehicles, while IIHS stated that increasing footprint is 

good for safety, CAS, Wenzel, and the UCSB students stated that decreasing footprint 

may be better for safety in terms of risk to occupants of other vehicles. Daimler, Wenzel, 

and the University of PA Environmental Law Project commented generally that more 

similar passenger car and light truck targets at identical footprints (as Wenzel put it, a 

single target curve) would improve fleet compatibility and thus, safety, by encouraging 

manufacturers to build more passenger cars instead of light trucks. 

Agencies’ response:

 The agencies continue to believe that footprint-based standards help to mitigate 

potential safety risks from downsizing if the target curves maintain sufficient slope, 

because, based on NHTSA’s analysis, larger-footprint vehicles are safer than smallerfootprint vehicles.18 Nevertheless, we recognize that footprint-based standards are not a 

panacea – NHTSA’s analysis continues to show that there was a historical relationship 

between lower vehicle mass and increased safety risk in passenger cars even if footprint 

is maintained, and there are ways that manufacturers may increase footprint that either 

improve or reduce vehicle safety, as indicated by IIHS and Porsche. 

 With regard to whether the agencies should set separate curves or a single one, 

NHTSA also notes in Section II.C that EPCA requires NHTSA to establish standards 

separately for passenger cars and light trucks, and thus concludes that the standards for 

each fleet should be based on the characteristics of vehicles in each fleet. In other words, 

the passenger car curve should be based on the characteristics of passenger cars, and the 

light truck curve should be based on the characteristics of light trucks—thus to the extent 

that those characteristics are different, an artificially-forced convergence would not 

accurately reflect those differences. However, such convergence could be appropriate 

depending on future trends in the light vehicle market, specifically further reduction in 

the differences between passenger car and light truck characteristics. While that trend 

was more apparent when car-like 2WD SUVs were classified as light trucks, it seems 

likely to diminish for the model year vehicles subject to these rules as the truck fleet will 

be more purely “truck-like” than has been the case in recent years. 

NHTSA’s estimates are inaccurate because NHTSA could mitigate additional 

safety risks from mass reduction, if there are any, by simply regulating safety 

more 

 Since NHTSA began considering the potential safety risks from mass reduction in 

response to increased CAFE standards, some commenters have suggested that NHTSA 

could mitigate those safety risks, if any, by simply regulating more.19 In response to the 

safety analysis presented in the NPRM, several commenters stated that NHTSA should 

develop additional safety regulations to require vehicles to be designed more safely, 

whether to improve compatibility (Adcock, NY DEC, Public Citizen, UCS), to require 

 

18 See the FRIA. 

19 See, e.g., [cites]; MY 2011 CAFE final rule, 74 Fed. Reg. at 14403-05 (Mar. 30, 2009). 

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seat belt use (CAS, UCS), to improve rollover and roof crush resistance (UCS), or to 

improve crashworthiness generally by strengthening NCAP and the star rating system 

(Adcock). Wenzel commented further that “Improvements in safety regulations will have 

a greater effect on occupant safety than FE standards that are structured to maintain, but 

may actually increase, vehicle size.” 

Agencies’ response:

 NHTSA appreciates the commenters’ suggestions and notes that the agency is 

continually striving to improve motor vehicle safety consistent with its mission. As noted 

above, improving safety in other areas affects the target population that the 

mass/footprint relationship could affect, but it does not necessarily change the 

relationship. 

 The 2010 Kahane analysis discussed in this final rule evaluates the relative safety 

risk when vehicles are made lighter than they might otherwise be absent the final MYs 

2012-2016 standards. It does consider the effect of known safety regulations as they are 

projected to affect the target population. 

 

Casualty risks range widely for vehicles of the same weight or footprint, which 

skews regression analysis and makes computer simulation a better predictor of 

the safety effects of mass reduction 

Wenzel commented that he had found, in his most recent work,20 after accounting 

for drivers and crash location, that there is a wide range in casualty risk for vehicles with 

the same weight or footprint. Wenzel stated that for drivers, casualty risk does generally 

decrease as weight or footprint increases, especially for passenger cars, but the degree of 

variation in the data for vehicles (particularly light trucks) at a given weight or footprint 

makes it difficult to say that a decrease in weight or footprint will necessarily result in 

increased casualty risk. In terms of risk imposed on the drivers of other vehicles, Wenzel 

stated that risk increases as light truck weight or footprint increases. 

Wenzel further stated that because a regression analysis can only consider the 

average trend in the relationship between vehicle weight/size and risk, it must “ignore” 

vehicles that do not follow that trend. Wenzel therefore recommended that the agency 

employ computer crash simulations for analyzing the effect of vehicle weight reduction 

on safety, because they can “pinpoint the effect of specific vehicle designs on safety,” 

and can model future vehicles which do not yet exist and are not bound to analyzing 

historical data. Wenzel cited, as an example, a DRI simulation study commissioned by 

the Aluminum Association (Kebschull 2004), which used a computer model to simulate 

the effect of changing SUV mass or footprint (without changing other attributes of the 

vehicle) on crash outcomes, and showed a 15 percent net decrease in injuries, while 

increasing wheelbase by 4.5 inches while maintaining weight showed a 26 percent net 

decrease in serious injuries. 

 

20 [cite draft report?]

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Agencies’ response: 

The agencies have reviewed Mr. Wenzel’s draft report for DOE to which he 

referred in his comments, but based on NHTSA’s work do not find such a wide range of 

safety risk for vehicles with the same weight, although we agree there is a range of risk 

for a given footprint. Wenzel found that for drivers, casualty risk does generally decrease 

as weight or footprint increases, especially for passenger cars, and that in terms of risk 

imposed on the drivers of other vehicles, risk increases as light truck weight or footprint 

increases, but concluded that the variation in the data precluded the possibility of drawing 

any conclusions. In the 2010 Kahane study presented in the FRIA, NHTSA undertook a 

similar analysis in which it correlated weight to fatality risk for vehicles of essentially the 

same footprint.21 The “decile analysis,” provided as a check on the trend/direction of 

NHTSA’s regression analysis, shows that fatality risk generally increases and rarely 

decreases for lighter relative to heavier cars of the same footprint. Thus, while Mr. 

Wenzel was reluctant to draw a conclusion, NHTSA believes that both our research and 

Mr. Wenzel’s appear to point to the same conclusion. We agree that there is a wide range 

in casualty risk among cars of the same footprint, but we find that that casualty risk is 

correlated with weight. The correlation shows that heavier cars have lower overall 

fatality rates than lighter cars of very similar footprint. 

The agencies agree that simulation can be beneficial in certain circumstances. 

NHTSA cautions, however, that it is difficult for a simulation analysis to capture the full 

range of variations in crash situations in the way that a statistical regression analysis 

does. Vehicle crash dynamics are complex, and small changes in initial crash conditions 

(such as impact angle or closing speed) can have large effects on injury outcome. This 

condition is a consequence of variations in the deformation mode of individual 

components (e.g., buckling, bending, crushing, material failure, etc.) and how those 

variations affect the creation and destruction of load paths between the impacting object 

and the occupant compartment during the crash event. It is therefore difficult to predict 

and assess structural interactions using computational methods when one does not have a 

detailed, as-built geometric and material model. Even when a complete model is 

available, prudent engineering assessments require extensive physical testing to verify 

crash behavior and safety. Despite all this, the agencies recognize that detailed crash 

simulations can be useful in estimating the relative structural effects of design changes 

over a limited range of crash conditions, and will continue to evaluate the appropriate use 

of this tool in the future. 

Simplified crash simulations can also be valuable tools, but only when employed 

as part of a comprehensive analytical program. They are especially valuable in 

evaluating the relative effect and associated confidence intervals of feasible design 

alternatives. For example, the method employed by Nusholtz et al.

22 could be used by a 

 

21 Sections 2.4 and 3.3 of new report. 

22 Nusholtz, G.S., G. Rabbiolo, and Y. Shi, “Estimation of the Effects of Vehicle Size and Mass on CrashInjury Outcome Through Parameterized Probability Manifolds,” Society of Automotive Engineers (2003), 

Document No. 2003-01-0905. Available at http://www.sae.org/technical/papers/2003-01-0905 (last 

accessed Feb. 15, 2010). 

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vehicle designer to estimate the benefit of incremental changes in mass or wheelbase as 

well as the tradeoffs that might be made between them once that designer has settled on a 

preliminary design. A key difference between the research by Nusholtz and the research 

by Kebschull that Mr. Wenzel cited23 is in their suggested applications. The former is 

useful in evaluating proposed alternatives early in the design process – Nusholtz 

specifically warns that the model provides only “general insights into the overall risk ... 

and cannot be used to obtain specific response characteristics.” Mr. Wenzel implies the 

latter can “isolate the effect of specific design changes, such as weight reduction” and 

thus quantify the fleet-wide effect of substantial vehicle redesigns. Yet while Kebschull 

reports injury reductions to three significant digits, there is no validation that vehicle 

structures of the proposed weight and stiffness are even feasible with current technology. 

Thus, while the agencies agree that computer simulations can be useful tools, we also 

recognize the value of statistical regression analysis for determining fleet-wide effects, 

because it inherently incorporates all real-world factors in historical safety assessments.

DRI’s analysis shows that lighter vehicles will save lives, and NHTSA reaches the 

opposite conclusion without disproving DRI’s analysis

 The difference between NHTSA’s results and DRI’s results for the relationship 

between vehicle mass and on vehicle safety has been at the crux of this issue for several 

years. While NHTSA offered some theories in the NPRM as to why DRI might have 

found a safety benefit for mass reduction, NHTSA’s work since then has enabled it to 

identify what we believe is the most likely reason for DRI’s findings. The potential near 

multicollinearity of the variables of curb weight, track width, and wheelbase creates some 

degree of risk that any regression models with those variables could inaccurately calibrate 

their effects. However, based on its own experience with statistical analysis, NHTSA 

believes that the specific two-step regression model used by DRI increases this risk, 

because it weakens relationships between curb weight and dependent variables by 

splitting the effect of curb weight across the two regression steps. 

 

 The comments below are in response to NHTSA’s theories in the NPRM about 

the source of the differences between NHTSA’s and DRI’s results. The majority of them 

are answered more fully in the 2010 Kahane report included in NHTSA’s FRIA, but we 

respond to them in this document as well for purposes of completeness. 

 

23 Mr. Wenzel cites the report by Kebschull et al [2004, DRI-TR-04-04-02] as an example of what he 

regards as the effective use of computer crash simulation. NHTSA does not concur that this analysis 

represents a viable analytical method for evaluating the fleet-wide tradeoffs between vehicle mass and 

societal safety. The simulation method employed was not a full finite element representation of each major 

structural component in the vehicles in question. Instead, an Articulated Total Body (ATB) representation 

was constructed for each of two representative vehicles. In the ATB model, large structural subsystems 

were represented by a single ellipsoid. Consolidated load-deflection properties of these subsystems and the 

joints that tie them together were “calibrated” for an ATB vehicle model by requiring that it reproduce the 

acceleration pulse of a physical NHTSA crash test. NHTSA notes that vehicle simulation models that are 

calibrated to a single crash test configuration (e.g., a longitudinal NCAP test into a rigid wall) are often illequipped to analyze alternative crash scenarios (e.g., vehicle-to-vehicle crashes at arbitrary angles and 

lateral offsets).

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NHTSA and DRI may have reached different conclusions because NHTSA’s 

study does not distinguish between reductions in size and reductions in weight 

like DRI’s 

Several commenters (CARB, CBD, EDF, ICCT, NRDC, and UCS) stated that 

DRI had been able to separate the effect of size and weight in its analysis, and in so doing 

proved that there was a safety benefit to reducing weight without reducing size. The 

commenters suggested that if NHTSA properly distinguished between reductions in size 

and reductions in weight, it would find the same result as DRI. 

Agencies’ response:

 In the 2010 Kahane analysis presented in the FRIA, NHTSA did attempt to 

separate the effects of vehicle size and weight by performing regression analyses with 

footprint (or alternatively track width and wheelbase) and curb weight as separate 

independent variables. For passenger cars, NHTSA found that the regressions attribute 

the fatality increase due to downsizing about equally to mass and footprint – that is, the 

effect of reducing mass alone is about half the effect of reducing mass and reducing 

footprint. Unlike DRI’s results, NHTSA’s regressions for passenger cars and for lighter 

LTVs did not find a safety benefit to reducing weight without reducing size; while 

NHTSA did find a safety benefit for reducing weight in the heaviest LTVs, the 

magnitude of the benefit as compared to DRI’s was significantly smaller. NHTSA 

believes that these differences in results may be an artifact of DRI’s two-step regression 

model, as explained above. 

NHTSA and DRI may have reached different conclusions because NHTSA’s 

study does not include two-door cars like DRI’s 

One of NHTSA’s primary theories in the NPRM as to why NHTSA and DRI’s 

results differed related to DRI’s inclusion in its analysis of 2-door cars. NHTSA had 

excluded those vehicles from its analysis on the grounds that 2-door cars had a 

disproportionate crash rate (perhaps due to their inclusion of muscle and sports cars) 

which appeared likely to skew the regression. Several commenters argued that NHTSA 

should have included 2-door cars in its analysis. DRI and James Adcock stated that 2-

door cars should not be excluded because they represent a significant portion of the lightduty fleet, while CARB and ICCT stated that because DRI found safety benefits whether 

2-door cars were included or not, NHTSA should include 2-door cars in its analysis. 

Wenzel also commented that NHTSA should include 2-door cars in subsequent analyses, 

stating that while his analysis of MY 2000-2004 crash data from 5 states indicates that, in 

general, 4-door cars tend to have lower fatality risk than 2-door cars, the risk is even 

lower when he accounts for driver age/gender and crash location. Wenzel suggested that 

the increased fatality risk in the 2-door car population seemed primarily attributable to 

the sports cars, and that that was not sufficient grounds to exclude all 2-door cars from 

NHTSA’s analysis. 

Agencies’ response: 

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DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

The agencies agree that 2-door cars can be included in the analysis, and NHTSA 

retracts previous statements that DRI’s inclusion of them was incorrect. In its 2010 

analysis, NHTSA finds that it makes little difference to the results whether 2-door cars 

are included, partially included, or excluded from the analysis. Thus, analyses of 2-door 

and 4-door cars combined, as well as other combinations, have been included in the 

analysis. That said, no combination of 2-door and 4-door cars resulted in NHTSA’s 

finding a safety benefit for passenger cars due to mass reduction. 

NHTSA and DRI may have reached different conclusions due to different 

assumptions

DRI commented that the differences found between its study and NHTSA’s may 

be due to the different assumptions about the linearity of the curb weight effect and 

control variable for driver age, vehicle age, road conditions, and other factors. NHTSA’s 

analysis was based on a two-piece linear model for curb weight with two different weight 

groups (less than 2,950 lbs., and greater than or equal to 2.950 lbs). The DRI analysis 

assumed a linear model for curb weight with a single weight group. Additionally, DRI 

stated that NHTSA’s use of eight control variables (rather than three control variables 

like DRI used) for driver age introduces additional degrees of freedom into the 

regressions, which it suggested may be correlated with the curb weight, wheelbase, and 

track width, and/or other control variables. DRI suggested that this may also affect the 

results and cause or contribute to the differences in outcomes between NHTSA and DRI. 

Agencies’ response: 

NHTSA’s FRIA documents that NHTSA analyzed its database using both a single 

parameter for weight (a linear model) and two parameters for weight (a two-piece linear 

model). In both cases, the logistic regression responded identically, allocating the same 

way between weight, wheelbase, track width, or footprint.24 Thus, NHTSA does not 

believe that the differences between its results and DRI’s results are due to whether the 

studies used a single weight group or two weight groups. 

The FRIA also documents that NHTSA examined NHTSA’s use of eight control 

variables for driver age (ages 14-30, 30-50, 50-70, 70+ for males and females separately , 

versus DRI’s use of three control variables for age (FEMALE = 1 for females, 0 for 

males, YOUNGDRV = 35-AGE for drivers under 35, 0 for all others, OLDMAN = 

AGE-50 for males over 50, 0 for all others; OLDWOMAN = AGE-45 for females over 

45, 0 for all others) to see if that affected the results. NHTSA ran its analysis using the 

eight control variables and again using three control variables for age, and obtained 

similar results each time.25 Thus, NHTSA does not believe that the differences between 

its results and DRI’s results are due to the number of control variables used for driver 

age. 

 

24 Sections 2.2 and 2.3 of new report 

25 Id.

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DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

NHTSA’s and DRI’s conclusions may be similar if confidence intervals are 

taken into account 

DRI commented that NHTSA has not reported confidence intervals, while DRI 

has reported them in its studies. Thus, DRI argued, it is not possible to determine 

whether the confidence intervals overlap and whether the differences between NHTSA’s 

and DRI’s analyses are statistically significant. 

Agencies’ response: 

NHTSA has included confidence intervals for the main results of the 2010 

Kahane analysis, as shown in Chapter IX of the FRIA. For passenger cars, the NHTSA 

results are a statistically significant increase in fatalities with a 100 pound reduction 

while maintaining track width and wheelbase (or footprint); the DRI results are a 

statistically significant decrease in fatalities with a 100 pound reduction while 

maintaining track width and wheelbase. The DRI results are thus outside the confidence 

bounds of the NHTSA results and do not overlap.

NHTSA should include a “best-case” estimate in its study 

Several commenters (Center for Auto Safety, NRDC, Public Citizen, Sierra Club 

et al., and Wenzel) urged NHTSA to include a “best-case” estimate in the final rule, 

showing scenarios in which lives were saved rather than lost. Public Citizen stated that 

there would be safety benefits to reducing the weight of the heaviest vehicles while 

leaving the weight of the lighter vehicles unchanged, and that increasing the number of 

smaller vehicles would provide safety benefits to pedestrians, bicyclists, and 

motorcyclists. Sierra Club et al. stated that new materials, smart design, and lighter, 

more advanced engines can all improve fuel economy while maintaining or increasing 

vehicle safety. Both Center for Auto Safety and Sierra Club argued that the agency 

should have presented a “best-case” scenario to balance out the “worst-case” scenario 

presented in the NPRM, especially if NHTSA itself believed that the worst-case scenario 

was not inevitable. NRDC requested that NHTSA present both a “best-case” and a “most 

likely” scenario. Wenzel simply stated that NHTSA did not present a “best-case” 

scenario, despite DRI’s finding in 2005 that fatalities would be reduced if track width 

was held constant. 

Agencies’ response:

NHTSA has included an “upper estimate” and a “lower estimate” in the new 2010 

Kahane analysis. The lower estimate assumes that mass reduction will be accomplished 

entirely by material substitution or other techniques that do not perceptibly change a 

vehicle’s shape, structural strength, or ride quality. The lower estimate examines specific 

crash modes and is meant to reflect the increase in fatalities for the specific crash modes 

in which a reduction in mass per se in the case vehicle would result in a reduction in 

safety: namely, collisions with larger vehicles not covered by the regulations (e.g., trucks 

with a GVWR over 10,000 lbs), collisions with partially-movable objects (e.g., some 

22 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

trees, poles, parked cars, etc.), and collisions of cars or light LTVs with heavier LTVs – 

as well as the specific crash modes where a reduction in mass per se in the case vehicle 

would benefit safety: namely, collisions of heavy LTVs with cars or lighter LTVs. 

NHTSA believes that this is the effect of mass per se, i.e., the effects of reduced mass 

will generally persist in these crashes regardless of how the mass is reduced. The lower 

estimate attempts to quantify that scenario, although any such estimate is hypothetical 

and subject to considerable uncertainty. NHTSA believes that a “most likely” scenario 

cannot be determined with any certainty, and would depend entirely upon agency 

assumptions about how manufacturers intend to reduce mass in their vehicles. While we 

can speculate upon the potential effects of different methods of mass reduction, we 

cannot predict with any certainty what manufacturers will ultimately do. 

NHTSA should not include a “worst-case” estimate in its study 

NRDC, Public Citizen and Sierra Club et al. commented that NHTSA should 

remove the “worst-case scenario” estimate from the rulemaking, generally because it was 

based on an analysis that evaluated historical vehicles, and future vehicles would be 

sufficiently different to render the “worst-case scenario” inapplicable. 

Agencies’ response: 

NHTSA stated in the NPRM that the “worst-case scenario” addressed the effect of 

a kind of downsizing (i.e., mass reduction accompanied by footprint reduction) that was 

not likely to be a consequence of attribute-based CAFE standards, and that the agency 

would refine its analysis of such a scenario for the final rule. NHTSA has not used the 

“worst-case scenario” in the final rule. Instead, we present three scenarios: the first is an 

estimate based directly on the regression coefficients of weight reduction while 

maintaining footprint in the statistical analyses of historical data. As discussed above, 

presenting this scenario is possible because NHTSA was able to separate the effects of 

weight and footprint reduction in the new analysis. However, even the new analysis of 

LTVs produced some coefficients that NHTSA did not consider entirely plausible. 

NHTSA also presents an “upper estimate” in which those coefficients for the LTVs were 

adjusted based on additional analyses and expert opinion as a safety agency and a “lower 

estimate,” which estimates the effect if mass reduction is accomplished entirely by 

safety-conscious technologies such as material substitution. 

3. How has NHTSA refined its analysis for purposes of estimating the 

potential safety effects of this final rule? 

During the past months, NHTSA has extensively reviewed the literature on 

vehicle mass, size, and fatality risk. NHTSA now agrees with DRI and other commenters 

that it is essential to analyze the effect of mass independently from the effects of size 

parameters such as wheelbase, track width, or footprint – and that the NPRM’s “worstcase” scenario based on downsizing (in which weight, wheelbase, and track width could 

all be changed) is not useful for that purpose. The agency should instead provide 

23 
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estimates that better reflect the more likely effect of the regulation - estimating the effect 

of mass reduction that maintains footprint. 

Yet it is more difficult to analyze multiple, independent parameters than a single 

parameter (e.g., curb weight), because there is a potential concern that the near 

multicollinearity of the parameters – the strong, natural and historical correlation of mass 

and size – can lead to inaccurate statistical estimates of their effects.26 NHTSA has 

performed new statistical analyses of its historical database of passenger cars, light 

trucks, and vans (LTVs) from its 2003 report (now including also 2-door cars), assessing 

relationships between fatality risk, mass, and footprint. They are described in Sections 

2.2 (cars) and 3.2 (LTVs) of the 2010 Kahane report presented in Chapter IX of the 

FRIA. While the potential concerns associated with near multicollinearity are inherent in 

regression analyses with multiple size/mass parameters, NHTSA believes that the 

analysis approach in the 2010 Kahane report, namely a single-step regression analysis, 

generally minimizes those hazards27 and models the trends in the historical data. The 

results differ substantially from DRI’s, based on a two-step regression analysis. Sections 

2.3 and 2.4 of the 2010 Kahane report attempt to account for the differences primarily by 

applying selected techniques from DRI’s analyses to NHTSA’s database. 

The statistical analyses – logistic regressions – of trends in MYs 1991-1999 

vehicles generate one set of estimates of the possible effects of reducing mass by 100 

pounds while maintaining footprint. While these effects might conceivably carry over to 

future mass reductions, there are two reasons that future safety effects of mass reduction 

could differ from projections from historical data: 

• The statistical analyses are “cross-sectional” analyses that estimate the increase in 

fatality rates for vehicles weighing n-100 pounds relative to vehicles weighing n 

pounds, across the spectrum of vehicles on the road, from the lightest to the 

heaviest. They do not directly compare the fatality rates for a specific make and 

model before and after a 100–pound reduction from that model. Instead, they use 

the differences across makes and models as a surrogate for the effects of actual 

reductions within a specific model; those cross-sectional differences could include 

trends that are statistically, but not causally related to mass. 

• The manner in which mass changed across MY 1991-1999 vehicles might not be 

consistent with future mass reductions, due to the availability of newer materials 

and design methods. 

 

26 Greene, W. H. (1993). Econometric Analysis, Second Edition. New York: Macmillan Publishing 

Company, pp. 266-268; Allison, P.D. (1999), Logistic Regression Using the SAS System. Cary, NC: SAS 

Institute Inc., pp. 48-51. The report shows variance inflation factor (VIF) scores in the 5-7 range for curb 

weight, wheelbase, and track width (or, alternatively, curb weight and footprint) in NHTSA’s database, 

exceeding the 2.5 level where near multicollinearity begins to become a concern in logistic regression 

analyses. 

27 NHTSA believes that the two-step regression is more hazardous because it weakens relationships 

between curb weight and dependent variables by splitting the effect of curb weight across the two 

regression steps, as discussed further below. 

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Therefore, Sections 2.5 and 3.4 of the 2010 Kahane report supplement those estimates 

with one or more scenarios in which some of the logistic regression coefficients are 

replaced by numbers based on additional analyses and NHTSA’s judgment of the likely 

effect of mass per se (the ability to transfer momentum to other vehicles or objects in a 

collision) and of what trends in the historical data could be avoided by current massreduction technologies such as materials substitution. The various scenarios may be 

viewed as a plausible range of point estimates for the effects of mass reduction while 

maintaining footprint, but they should not be construed as upper and lower bounds. 

Furthermore, being point estimates, they are themselves subject to uncertainties, such as, 

for example, the sampling errors associated with statistical analyses. 

 The principal findings and conclusions of the 2010 Kahane report are as follows: 

Passenger cars: This database with the one-step regression method of the 2003 Kahane 

report estimates an increase of 700-800 fatalities when curb weight is reduced by 100 

pounds and footprint is reduced by 0.65 square feet (the historic average footprint 

reduction per 100-pound mass reduction in cars). The regression attributes the fatality 

increase about equally to curb weight and to footprint. The results are approximately the 

same whether 2-door cars are fully included or partially included in the analysis or 

whether only 4-door cars are included (as in the 2003 report). Regressions by curb 

weight, track width and wheelbase produce findings quite similar to the regressions by 

curb weight and footprint, but the results with the single “size” variable, footprint, rather 

than the two variables, track width and wheelbase vary even less with the inclusion or 

exclusion of 2-door cars. 

In Section 2.3 of the new report, a two-step regression method that resembles 

(without exactly replicating) the approach by DRI, when applied to the same (NHTSA’s) 

crash and registration data, estimates a large benefit when mass is reduced, offset by even 

larger fatality increases when track width and wheelbase (or footprint) are reduced. 

NHTSA believes that the benefit estimated by this method is inaccurate, due to the 

potential concerns with the near multicollinearity of the parameters (curb weight, track 

width, and wheelbase)28 even though the analysis is theoretically unbiased29. Almost any 

analysis incorporating those parameters has a possibility of inaccurate coefficients due to 

near multicollinearity; however, based on our own experience with other regression 

analyses of crash data, NHTSA believes a DRI-type two-step method augments the 

possibility of estimating inaccurate coefficients for curb weight, because it weakens 

relationships between curb weight and dependent variables by splitting the effect of curb 

weight across the two regression steps. 

 

28 As evidenced by VIF scores in the 5-7 range, exceeding the 2.5 level where near multicollinearity begins 

to become a concern in logistic regression analyses. 29 Section 2.3 of the 2010 Kahane report attempts to explain why the two-step method, when applied to 

NHTSA’s 2003 database, produces results a lot like DRI’s, but it does not claim that DRI obtained its 

results from its own database for exactly those reasons. NHTSA did not analyze DRI’s database. The twostep method is “theoretically unbiased” in the sense that it seeks to estimate the same parameters as the 

one-step analysis. 

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In Section 2.4 of the new report, as a check on the results from the regression 

methods, NHTSA also performed what we refer to as “decile” analyses: simpler, tabular 

data analysis that compares fatality rates of cars of different mass but similar footprint. 

Decile analysis is not a precise tool because it does not control for confounding factors 

such as driver age/gender or the specific type of car, but it may be helpful in identifying 

the general directional trend in the data when footprint is held constant and curb weight 

varies. The decile analyses show that fatality risk in MY 1991-1999 cars generally 

increased and rarely decreased for lighter relative to heavier cars of the same footprint. 

They suggest that the historical, cross-sectional trend was generally in the lighter ↔ more 

fatalities direction and not in the opposite direction, as might be suggested by the 

regression coefficients from the method that resembles DRI’s approach. 

The regression coefficients from NHTSA’s one-step method suggest that mass 

and footprint each accounted for about half the fatality increase associated with 

downsizing in a cross-sectional analysis of 1991-1999 cars. They estimate the historical 

difference in societal fatality rates (i.e., including fatalities to occupants of all the vehicles 

involved in the collisions, plus any pedestrians) of cars of different curb weights but the 

same footprint. They may be considered an “upper-estimate scenario” of the effect of 

future mass reduction – if it were accomplished in a manner that resembled the historical 

cross-sectional trend – i.e., without any particular regard for safety (other than not to 

reduce footprint). 

However, NHTSA believes that future vehicle design is likely to take advantage 

of safety-conscious technologies such as materials substitution that can reduce mass 

without perceptibly changing a car’s shape or ride and maintain its structural strength. 

This could avoid much of the risk associated with lighter and smaller vehicles in the 

historical analyses, especially the historical trend toward higher crash-involvement rates 

for lighter and smaller vehicles.30 It could thereby shrink the added risk close to just the 

effects of mass per se (the ability to transfer momentum to other vehicles or objects in a 

collision). Section 2.5 of the 2010 Kahane report attempts to quantify a “lower-estimate 

scenario” for the potential effect of mass reduction achieved by safety-conscious 

technologies; the estimated effects are substantially smaller than in the upper-estimate 

scenario based directly on the regression results. 

We note, again, that the preceding paragraph is conditional. Nothing in the CAFE 

standard requires manufacturers to use material substitution or, more generally, take a 

safety-conscious approach to mass reduction.31 Federal Motor Vehicle Safety Standards 

include performance tests that verify historical improvements in structural strength and 

crashworthiness, but few FMVSS provide test information that sheds light about how a 

vehicle rides or otherwise helps explain the trend toward higher crash-involvement rates 

for lighter and smaller vehicles. It is possible that using material substitution and other 

 

30 IIHS Advisory No. 5, July 1988, http://www.iihs.org/research/advisories/iihs_advisory_5.pdf; IIHS 

News Release, February 24, 1998, http://www.iihs.org/news/1998/iihs_news_022498.pdf; Auto Insurance 

Loss Facts, September 2009, http://www.iihs.org/research/hldi/fact_sheets/CollisionLoss_0909.pdf. 31 Footprint-based standards do not specify how or where to remove mass while maintaining footprint, nor 

do they categorically forbid footprint reductions, even if they discourage them. 

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current mass reduction methods could avoid the historical trend in this area, but that 

remains to be studied as manufacturers introduce more of these vehicles into the on-road 

fleet in coming years. 

LTVs: The principal difference between LTVs and passenger cars is that mass reduction 

in the heavier LTVs is estimated to have significant societal benefits, in that it reduces the 

fatality risk for the occupants of cars and light LTVs that collide with the heavier LTVs. 

By contrast, footprint (size) reduction in LTVs has a harmful effect (for the LTVs’ own 

occupants), as in cars. The regression method of the 2003 Kahane report applied to the 

database of that report estimates a societal increase of 231 fatalities when curb weight is 

reduced by 100 pounds and footprint is reduced by 0.975 square feet (the historic average 

footprint reduction per 100-pound mass reduction in LTVs). But the regressions attribute 

an overall reduction of 266 fatalities to the 100-pound mass reduction and an increase of 

497 fatalities to the .975-square-foot footprint reduction. The regression results constitute 

one of the scenarios for the possible societal effects of future mass reduction in LTVs. 

However, NHTSA cautions that some of the regression coefficients, even by 

NHTSA’s preferred method, might not accurately model the historical trend in the data, 

possibly due to near multicollinearity of curb weight and footprint or because of the 

interaction of both of these variables with LTV type.32 Based on supplementary analyses 

and discussion in Sections 3.3 and 3.4, the new report defines an additional upperestimate scenario that NHTSA believes may more accurately reflect the historical trend in 

the data and a lower-estimate scenario that may come closer to the effects of mass per se. 

All three scenarios, however, attribute a societal fatality reduction to mass reduction in 

the heavier LTVs. 

Overall effects of mass reduction while maintaining footprint in cars and LTVs: The 

immediate purpose of the new report’s analyses of relationships between fatality risk, 

mass, and footprint is to develop the four parameters that the Volpe model needs in order 

to predict the safety effects, if any, of the modeled mass reductions in MYs 2012-2016 

cars and LTVs over the lifetime of those vehicles. The four numbers are the overall 

percentage increases or decreases, per 100-pound mass reduction while holding footprint 

constant, in crash fatalities involving: (1) cars < 2,950 pounds (which was the median 

curb weight of cars in MY 1991-1999), (2) cars ≥ 2,950 pounds, (3) LTVs < 3,870 

pounds (which was the median curb weight of LTVs in those model years), and (4) LTVs 

≥ 3,870 pounds. Here are the percentage effects for each of the three alternative 

scenarios, again, the “upper-estimate scenario” and the “lower-estimate scenario” being 

developed based on NHTSA’s expert opinion as a vehicle safety agency: 

 

32 For example, mid-size SUVs of the 1990s typically had high mass relative to their short wheelbase and 

footprint (and exceptionally high rates of fatal rollovers); minivans typically have low mass relative to their 

footprint (and low fatality rates); heavy-duty pickup trucks used extensively for work tend to have more 

mass, for the same footprint, as basic full-sized pickup trucks that are more often used for personal 

transportation. 

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Fatality Increase per 100-Pound Reduction (%) 

Actual Regression 

Result Scenario 

NHTSA Expert 

Opinion UpperEstimate 

Scenario33

NHTSA Expert 

Opinion LowerEstimate Scenario 

Cars < 2,950 

pounds 2.21 2.21 1.02

Cars > 2,950 

pounds 0.90 0.90 0.44

LTVs < 3,870 

pounds 0.17 0.55 0.41

LTVs > 3,870 

pounds -1.90 -0.62 -0.73

In all three scenarios, the estimated effects of a 100-pound mass reduction while 

maintaining footprint are an increase in fatalities in cars < 2,950 pounds, substantially 

smaller increases in cars ≥ 2,950 pounds and LTVs < 3,870 pounds, and a societal benefit 

for LTVs ≥ 3,870 pounds (because it reduces fatality risk to occupants of cars and lighter 

LTVs they collide with). These are the estimated effects of reducing each vehicle by 

exactly 100 pounds. However, the actual mass reduction will vary by make, model, and 

year. The aggregate effect on fatalities can only be estimated by attempting to forecast, 

as NHTSA has using inputs to the Volpe model, the mass reductions by make and model. 

It should be noted, however, that a 100-pound reduction would be 5 percent of the mass 

of a 2000-pound car but only 2 percent of a 5000-pound LTV. Thus, a forecast that mass 

will decrease by an equal or greater percentage in the heavier vehicles than in the lightest 

cars would be proportionately more influenced by the benefit for mass reduction in the 

heavy LTVs than by the fatality increases in the other groups; it is likely to result in an 

estimated net benefit under one or more of the scenarios. It should also be noted, again, 

that the three scenarios are point estimates and are subject to uncertainties, such as the 

sampling errors associated with the regression results. In the scenario based on actual 

regression results, the 1.96-sigma sampling errors in the above estimates are + 0.91 

percentage points for cars , 2,950 pounds and also for cars > 2,950 pounds, + 0.82 

percentage points for LTVs < 3,870 pounds, and + 1.18 percentage points for LTVs >

3,870 pounds. In other words, the fatality increase in the cars < 2,950 pounds and the 

societal fatality reduction attributed to mass reduction in the LTVs > 3,870 pounds are 

statistically significant. The sampling errors associated with the scenario based on actual 

regression results perhaps also indicate the general level of statistical noise in the other 

two scenarios. 

4. What are the estimated safety effects of this final rule? 

The table below shows the estimated safety effects of the modeled reduction in 

vehicle mass provided in the NPRM and in this final rule in order to meet the MYs 2012-

 

33 For passenger cars, the upper-estimate scenario is the actual-regression-result scenario. 

28 
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2016 standards, based on the analysis described briefly above and in much more detail in 

Chapter IX of the FRIA. These are combined results for passenger cars and light trucks. 

A positive number is an estimated increase in fatalities and a negative number (shown in 

parentheses) is an estimated reduction in fatalities over the lifetime of the model year 

vehicles compared to the MY 2011 baseline fleet. 

MY 

2012

MY 

2013

MY 

2014

MY 

2015

MY 

2016

NPRM “Worst Case” 34 54 194 313 493 

NHTSA Expert 

Opinion Final Rule 

Upper Estimate

9 14 26 24 22

NHTSA Expert 

Opinion Final Rule 

Lower Estimate

2 4 (17) (53) (80)

Actual Regression 

Result Scenario 0 2 (94) (206) (301) 

NHTSA emphasizes that the table above is based on the NHTSA’s assumptions 

about how manufacturers might choose to reduce the mass of their vehicles in response to 

the final rule, which are very similar to EPA’s assumptions. In general, as discussed 

above, the agencies assume that mass will be reduced by as much as 10 percent in the 

heaviest LTVs but only by as much as 5 percent in other vehicles and that substantial 

mass reductions will take place only in the year that models are redesigned. The actual 

mass reduction that is likely to occur in response to the standards will of course vary by 

make and model, depending on each manufacturer’s particular approach, with likely 

more opportunity for the largest LTVs that still use separate frame construction. 

The “upper estimate” presented above, as discussed in the FRIA, assumes only 

that manufacturers will reduce vehicle mass without reducing footprint. Thus, under such 

a scenario, safety effects could be somewhat adverse if, for example, manufacturers 

chose to reduce crush space associated with vehicle overhang as a way of reducing mass 

without changing footprint. The “lower estimate,” in turn, is based on the assumption 

that manufacturers will reduce vehicle mass solely through methods like material 

substitution, which (under these assumptions) fully maintain not only footprint but also 

all structural integrity, and other aspects of vehicle safety. Under these scenarios, safety 

effects could be worse if mass reduction was not undertaken thoughtfully to maintain 

existing safety levels, but could also be better if it was undertaken with a thorough and 

extensive vehicle redesign to maximize both mass reduction and safety. 

 And finally, while NHTSA does not believe that the “worst-case” scenario 

presented in the NPRM is likely to occur during the MYs 2012-2016 timeframe, we 

cannot guarantee that manufacturers will never choose to reduce vehicle footprint, 

particularly if market forces lead to increased sales of small vehicles in response to sharp 

increases in the price of petroleum, though this case would not be in direct response to 

the CAFE/GHG standards. Thus, we cannot completely reject the worst-case scenario for 

29 
DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

all vehicles, although we can and do recognize that the footprint-based standards will 

significantly limit the likelihood of its occurrence within the context of this rulemaking. 

In summary, the agencies recognize the balancing inherent in achieving higher 

levels of fuel economy and lower levels of CO2 emissions through reduction of vehicle 

mass. Based on the 2010 Kahane analysis that is able to separate the effects of mass 

reductions and footprint reductions, and to account better for the possibility that mass 

reduction will be accomplished entirely through methods that preserves structural 

strength and vehicle safety, the agencies now believe that the likely deleterious safety 

effects of the MYs 2012-2016 standards may be much lower than originally estimated. 

They may be close to zero, or possibly beneficial if mass reduction is carefully 

undertaken in the future and if the mass reduction in the heavier LTVs is greater (in 

absolute terms) than in passenger cars. In light of these findings, we believe that the 

balancing is reasonable. 

5. How do the agencies plan to address this issue going forward?

NHTSA and EPA believe that it is important for the agencies to conduct further 

study and research into the interaction of mass, size and safety to assist future 

rulemakings. The agencies intend to begin working collaboratively and to explore with 

DOE, CARB, and perhaps other stakeholders an interagency/ intergovernmental working 

group to evaluate all aspects of mass, size and safety. It would also be the goal of this 

team to coordinate government supported studies and independent research, to the extent 

possible, to help ensure the work is complementary to previous and ongoing research and 

to guide further research in this area. DOE’s EERE office has long funded extensive 

research into component advanced vehicle materials and vehicle mass reduction. Other 

agencies may have additional expertise that will be helpful in establishing a coordinated 

work plan. The agencies are interested in looking at the weight-safety relationship in a 

more holistic (complete vehicle) way, and thanks to this CAFE rulemaking NHTSA has 

begun to bring together parts of itself— crashworthiness, and crash avoidance rulemaking 

offices and the agency’s Research & Development office—in an interdisciplinary way to 

better leverage the expertise of the agency. Extending this effort to other agencies will 

help to ensure that all aspects of the weight-safety relationship are considered completely 

and carefully with our future research. The agencies also intend to carefully consider 

comments received in response to the NPRM in developing plans for future studies and 

research and to solicit input from stakeholders. 

The agencies also plan to watch for safety effects as the U.S. light-duty vehicle 

fleet evolves in response both to the CAFE/GHG standards and to consumer preferences 

over the next several years. Additionally, as new and advanced materials and component 

smart designs are developed and commercialized, and as manufacturers implement them 

in more vehicles, it will be useful for the agencies to learn more about them and to try to 

track these vehicles in the fleet to understand the relationship between vehicle design and 

injury/fatality data. Specifically, the agencies intend to follow up with study and research 

of the following: 

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DRAFT – DELIBERATIVE – PRIVILEGED AND CONFIDENTIAL 

First, NHTSA is in the process of contracting with an independent institution to 

review the statistical methods that NHTSA has used to analyze historical data related to 

mass, size and safety, and to provide recommendation on whether the existing methods or 

other methods should be used for future statistical analysis of historical data. This study 

is being initiated because, in response to the NPRM, NHTSA received a number of 

comments related to the methodology NHTSA used for the NPRM to determine the 

relationship between mass and safety, as discussed in detail above. 

Second, NHTSA and EPA, in consultation with DOE, intend to begin updating 

the MYs 1991-1999 database on which the safety analyses in the NPRM and final rule 

are based with newer vehicle data in the next several months. This task will take at least 

a year to complete. This study is being initiated in response to the NPRM comments 

related to the use of data from MYs 1991-1999 in the NHTSA analysis, as discussed in 

detail above. 

Third, in order to assess if the design of recent model year vehicles that 

incorporate various mass reduction methods affect the relationships among vehicle mass, 

size and safety, NHTSA and EPA intend to conduct collaborative statistical analysis, 

beginning in the next several months. The agencies intend to work with DOE to identify 

vehicles that are using material substitution and smart design. After these vehicles are 

identified, the agencies intend to assess if there are sufficient data for statistical analysis. 

If there are sufficient data, statistical analysis would be conducted to compare the 

relationship among mass, size and safety of these smart design vehicles to vehicles of 

similar size and mass with more traditional designs. This study is being initiated because, 

in response to the NPRM, NHTSA received comments related to the use of data from 

MYs 1991-1999 in the NHTSA analysis that did not include new designs that might 

change the relationship among mass, size and safety, as discussed in detail above. 

NHTSA may initiate a two-year study of the safety of the fleet through an 

analysis of the trends in structural stiffness and whether any trends identified impact 

occupant injury response in crashes. Vehicle manufacturers may employ stiffer light 

weight materials to limit occupant compartment intrusion while controlling for mass that 

may expose the occupants to higher accelerations resulting in a greater chance of injury 

in real-world crashes. This study would provide information that would increase the 

understanding of the effects on safety of newer vehicle designs. 

In addition, NHTSA and EPA, possibly in collaboration with DOE, may conduct 

a longer-term computer modeling-based design and analysis study to help determine the 

maximum potential for mass reduction in the MYs 2017-2021 timeframe, through direct 

material substitution and smart design while meeting safety regulations and guidelines, 

and maintaining vehicle size and functionality. This study may build upon prior research 

completed on vehicle mass reduction. This study would further explore comprehensive 

vehicle effects, including dissimilar material joining technologies, manufacturer 

feasibility of both supplier and OEM, tooling costs, and crash simulation and perhaps 

eventual crash testing. 

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