Subject: Preamble II.G Safety for Interagency Review From: [james.tamm@dot.gov]

 Sent: 11/12/2011 02:00 PM EST

 To: <Chandana_L._Achanta@omb.eop.gov>

 Cc: <rebecca.yoon@dot.gov>; William Charmley

 

Hi Chandana, 

 

Attached is draft revised text for the Preamble II.G Safety Considerations in Establishing CAFE/GHG 

Standards. The section includes revised text to address the November 10, 2011 interagency comment 

#3 ( begins in the last paragraph on page 30), and text that more fully describes the agencies’ 

consideration of safety in selecting mass reduction levels (begins in the last paragraph on page 28). 

 

For reference, below is November 10, 2011 Interagency Comment #3. 

Interagency Comment 3) Effect of rule on the distribution of the fleet footprint . Interagency 

commenters recommend that EPA and RIA include the following description in the Preamble discussion 

of safety impacts: 

“The standards in this proposal are intended to eliminate to comply by changing the size (i.e., footprint) 

of vehicles. This is a well-understood problem with prior “flat” standards, which the National Academy

of Sciences concluded [2002 report] led to adverse safety consequences, among other effects. 

Nevertheless the agencies acknowledge it is possible that changing the footprint could, 

under certain circumstances, be a relatively low-cost means of achieving compliance with the standards

proposed today. This could happen in one of two ways. Manufacturers could change the size of a vehicle

during the redesign cycle in order to increase overall fuel economy relative to the target for that 

particular vehicle size. (Footnote: Suppose, for example, a 50-square-foot vehicle would exceed its 

target by 2 mpg, but a redesigned new vehicle would cost the same but exceed the slightly larger target 

by 3 mpg.) Alternatively, instead of redesigning vehicles, manufacturers could lower the price and 

increase production of vehicles of sizes whose fuel economy is favorable (well above the target for that 

footprint) and/or increase the price and decrease production those whose fuel economy is unfavorable. 

For example, the agencies note that the standard is flat for vehicles smaller than roughly 41 square feet. 

Further downsizing in this category could be a relatively low-cost way of helping achieve overall 

compliance. To the extent manufacturers engage in either behavior, the net result would be vehicles 

larger or smaller on average than they otherwise would have been in the absence of this regulation. 

Changes to the average size, and/ or the distribution of vehicle sizes, could also have important 

implications for the overall safety of the new vehicle fleet. 

For various reasons, the analysis assumes this rule will not have either of the effects described above. In 

other words, it will have no effect on the sales-weighted distribution of vehicle footprints. In particular, 

the analysis assumes production of vehicles with footprints where the curve is flat (i.e., below 41 square 

feet) will not increase over their baseline levels (i.e., those that will exist in MY 2016) as a result of this 

rule. We seek comment on the appropriateness of the overall assumptions that this rule will have no 

effect on the overall sales-weighted distribution of vehicle footprints of on the sales of vehicles smaller 

than 41 sq ft. Please provide data and/or other evidence to support any claim general or specific that

the size distribution of the fleet will or will not change as a result of this rule.” 

 

 

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 

II.G Safety Considerations to OMB 11-12-11.docx 

 

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Page 1 of 32

G. Safety Considerations in Establishing CAFE/GHG Standards 

1. Why do the agencies consider safety?

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

consumption and GHG emissions from the on-road light-duty vehicle fleet, but in addition to 

these intended effects, the agencies also consider the potential of the standards to affect vehicle 

safety.1 As a safety agency, NHTSA has long considered the potential for adverse safety 

consequences when establishing CAFE standards,2

 and under the CAA, EPA considers factors 

related to public health and human welfare, and safety, in regulating emissions of air pollutants 

from mobile sources.3

 Safety trade-offs associated with fuel economy increases have occurred in 

the past (particularly before NHTSA CAFE standards were attribute-based), and the agencies 

must be mindful of the possibility of future ones. These past safety trade-offs may have occurred 

because manufacturers chose, at the time, to build smaller and lighter vehicles—partly 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 cars generally have been heavy and large, while the cars with the highest 

fatal-crash rates have been light and small. The question, then, is whether past is necessarily 

prologue when it comes to potential changes in vehicle size (both footprint and “overhang”) and 

mass in response to these proposed future CAFE and GHG standards. Manufacturers have stated 

that they will reduce vehicle mass as one of the cost-effective means of increasing fuel economy 

and reducing CO2 emissions in order to meet the proposed standards, and the agencies have 

incorporated this expectation into our modeling analysis supporting the proposed standards. 

Because the agencies discern a historical relationship between vehicle mass, size, and safety, it is 

reasonable to assume that these relationships will continue in the future. The question of 

whether vehicle design can mitigate the adverse effects of mass reduction is discussed below. 

Manufacturers are less likely than they were in the past to reduce vehicle footprint in 

order to reduce mass for increased fuel economy. 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. See section 

II. C.1, above. This is because, as footprint decreases, the corresponding fuel economy/GHG 

 

1

 In this rulemaking document, “vehicle safety” is defined as societal fatality rates per vehicle miles traveled (VMT), 

which include fatalities to occupants of all the vehicles involved in the collisions, plus any pedestrians. 

2

 This practice is recognized approvingly in case law. As the United States Court of Appeals for the D.C. Circuit 

stated in upholding NHTSA’s exercise of judgment in setting the 1987-1989 passenger car standards, “NHTSA has 

always examined the safety consequences of the CAFE standards in its overall consideration of relevant factors 

since its earliest rulemaking under the CAFE program.” Competitive Enterprise Institute v. NHTSA (“CEI I”), 901 

F.2d 107, 120 at n. 11 (D.C. Cir. 1990). 

3

 See NRDC v. EPA, 655 F. 2d 318, 332 n. 31 (D.C. Cir. 1981).(EPA may consider safety in developing standards 

under section 202 (a) and did so appropriately in the given instance).
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emission target becomes more stringent. We also believe that the shape of the footprint curves 

themselves is approximately “footprint-neutral,” that is, that it should neither encourage 

manufacturers to increase the footprint of their fleets, nor to decrease it. Upsizing footprint is 

also discouraged through the curve “cut-off” at larger footprints.4 However, the 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 of other areas of the vehicle outside the wheels. The crush space 

provided by those portions of a vehicle can make important contributions to managing crash 

energy. Additionally, simply because footprint-based standards create no incentive to downsize 

vehicles does not mean that manufacturers will not downsize if doing so makes it easier to meet 

the overall CAFE/GHG standard, as for example if the smaller vehicles are so much lighter that 

they exceed their targets by much greater amounts. On balance, however, we believe the target 

curves and the incentives they provide generally will not encourage down-sizing (or up-sizing) in 

terms of footprint reductions (or increases).5

Given that we expect manufacturers to reduce vehicle mass in response to the proposed 

standards, and do not expect manufacturers to reduce vehicle footprint in response to the 

proposed standards, the agencies must attempt to predict the safety effects, if any, of the 

proposed standards based on the best information currently available. This section explained 

why the agencies consider safety; the following section discusses how the agencies consider 

safety. 

2. How do the agencies consider safety? 

Assessing the effects of vehicle mass reduction and size on societal safety is a complex 

issue. One part of estimating potential safety effects involves trying to understand better the 

relationship between mass and vehicle design. The extent of mass reduction that manufacturers 

may be considering to meet more stringent fuel economy and GHG standards may raise different 

safety concerns from what the industry has previously faced. The principal difference between 

the heavier vehicles, especially truck-based LTVs, and the lighter vehicles, especially passenger 

cars, is that mass reduction has a different effect in collisions with another car or LTV. When 

 

4

 The agencies recognize that at the other end of the curve, manufacturers who make small cars and trucks below 41 

square feet (the small footprint cut-off point) have some incentive to downsize their vehicles to make it easier to 

meet the constant target. That cut-off may also create some incentive for manufacturers who do not currently offer 

models that size to do so in the future. However, at the same time, the agencies believe that there is a limit to the 

market for cars and trucks smaller than 41 square feet: most consumers likely have some minimum expectation 

about interior volume, for example, among other things. Additionally, vehicles in this segment are the lowest price 

point for the light-duty automotive market, with several models in the $10,000-$15,000 range. Manufacturers who 

find themselves incentivized by the cut-off will also find themselves adding technology to the lowest price segment 

vehicles, which could make it challenging to retain the price advantage. Because of these two reasons, the agencies 

believe that the incentive to increase the sales of vehicles smaller than 41 square feet due to this rulemaking, if any, 

is small. See Section II.C.1 above and Chapter 1 of the draft Joint TSD for more information on the agencies’ 

choice of “cut-off” points for the footprint-based target curves. 

5

 This statement makes no prediction of how consumer choices of vehicle size will change in the future, independent 

of this proposal. 
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two vehicles of unequal mass collide, the change in velocity (delta V) is higher in the lighter 

vehicle, similar to the mass ratio proportion. As a result of the higher change in velocity, the 

fatality risk may also increase. Removing more mass from the heavier vehicle than in the lighter 

vehicle by amounts that bring the mass ratio closer to 1.0 reduces the delta V in the lighter 

vehicle, possibly resulting in a net societal benefit. 

Another complexity is that if a vehicle is made lighter, adjustments must be made to the 

vehicle’s structure such that it will be able to manage the energy in a crash while limiting 

intrusion into the occupant compartment after adopting materials that may be stiffer. To maintain 

an acceptable occupant compartment deceleration, the effective front end stiffness has to be 

managed such that the crash pulse does not increase as stiffer yet lighter materials are utilized. If

the energy is not well managed, the occupants may have to “ride down” a more severe crash 

pulse, putting more burdens on the restraint systems to protect the occupants. There may be 

technological and physical limitations to how much the restraint system may mitigate these 

effects. 

The agencies must attempt to estimate now, based on the best information currently 

available to us, how the assumed levels of mass reduction without additional changes (i.e. 

footprint, performance, functionality) might affect the safety of vehicles, and how lighter 

vehicles might affect the safety of drivers and passengers in the entire on-road fleet, as we are 

analyzing potential future CAFE and GHG standards. The agencies seek to ensure that the 

standards are designed to encourage manufacturers to pursue a path toward compliance that is 

both cost-effective and safe. 

To estimate the possible safety effects of the MY 2017-2025 standards, then, the agencies 

have undertaken research that approaches this question from several angles. First, we are using a 

statistical approach to study the effect of vehicle mass reduction on safety historically, as 

discussed in greater detail in section C below. Statistical analysis is performed using the most 

recent historical crash data available, and is considered as the agencies’ best estimate of potential 

mass-safety effects. The agencies recognize that negative safety effects estimated based on the 

historical relationships could potentially be tempered with safety technology advances in the 

future, and may not represent the current or future fleet. Second, we are using an engineering 

approach to investigate what amount of mass reduction is affordable and feasible while 

maintaining vehicle safety and other major functionalities such as NVH and acceleration 

performance. Third, we are also studying the new challenges these lighter vehicles might bring 

to vehicle safety and potential countermeasures available to manage those challenges effectively. 

The sections below discuss more specifically the state of the research on the mass-safety 

relationship, and how the agencies integrate that research into our assessment of the potential 

safety effects of the MY 2017-2025 CAFE and GHG standards. 
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3. What is the current state of the research on statistical analysis of historical crash 

data? 

a. Background 

Researchers have been using statistical analysis to examine the relationship of vehicle 

mass and safety in historical crash data for many years, and continue to refine their techniques 

over time. In the MY 2012-2016 final rule, the agencies stated that we would conduct further 

study and research into the interaction of mass, size and safety to assist future rulemakings, and 

start to work collaboratively by developing an interagency working group between NHTSA, 

EPA, DOE, and CARB to evaluate all aspects of mass, size and safety. The team would seek to 

coordinate government supported studies and independent research, to the greatest extent 

possible, to help ensure the work is complementary to previous and ongoing research and to 

guide further research in this area. 

The agencies also identified three specific areas to direct research in preparation for 

future CAFE/GHG rulemaking in regards to statistical analysis of historical data. 

First, NHTSA would contract with an independent institution to review the statistical 

methods that NHTSA and DRI have 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 will include a consideration of

potential near multicollinearity in the historical data and how best to address it in a regression 

analysis. The 2010 NHTSA report was also peer reviewed by two other experts in the safety 

field - Charles Farmer (Insurance Institute for Highway Safety) and Anders Lie (Swedish 

Transport Administration).6

 

Second, NHTSA and EPA, in consultation with DOE, would update the MYs 1991–1999 

database on which the safety analyses in the NPRM and final rule are based with newer vehicle 

data, and create a common database that could be made publicly available to help address 

concerns that differences in data were leading to different results in statistical analyses by 

different researchers. 

And 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, the 

agencies sought to identify vehicles that are using material substitution and smart design, and to 

try to assess if there is sufficient crash data involving those vehicles for statistical analysis. If

sufficient data exists, statistical analysis would be conducted to compare the relationship among 

 

6

 All three of the peer reviews are in docket, NHTSA-2010-0152. You can access the docket at 

http://www.regulations.gov/#!home by typing ‘NHTSA-2010-0152’ where it says “enter keyword or ID” and then 

clicking on “Search.”
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mass, size and safety of these smart design vehicles to vehicles of similar size and mass with 

more traditional designs. 

Significant progress has been made on these tasks since the MY 2012-2016 final rule, as 

follows: The independent review of recent and updated statistical analyses of the relationship 

between vehicle mass, size, and crash fatality rates has been completed. NHTSA contracted with 

the University of Michigan Transportation Research Institute (UMTRI) to conduct this review, 

and the UMTRI team led by Paul Green evaluated over 20 papers, including studies done by 

NHTSA’s Charles Kahane, Tom Wenzel of the US Department of Energy’s Lawrence Berkeley 

National Laboratory, Dynamic Research, Inc., and others. UMTRI’s basic findings will be 

discussed below. Some commenters in recent CAFE rulemakings, including some vehicle 

manufacturers, suggested that the designs and materials of more recent model year vehicles may 

have weakened the historical statistical relationships between mass, size, and safety. The 

agencies agree that the statistical analysis would be improved by using an updated database that 

reflects more recent safety technologies, vehicle designs and materials, and reflects changes in 

the overall vehicle fleet. The agencies also believe, as UMTRI also found, that different 

statistical analyses may have had different results because they each used slightly different 

datasets for their analyses. In order to try to mitigate this problem and to support the current 

rulemaking, NHTSA has created a common, updated database for statistical analysis that consists 

of crash data of model years 2000-2007 vehicles in calendar years 2002-2008, as compared to 

the database used in prior NHTSA analyses which was based on model years 1991–1999 

vehicles in calendar years 1995-2000. The new database is the most up-to-date possible, given 

the processing lead time for crash data and the need for enough crash cases to permit statistically 

meaningful analyses. NHTSA has made the new databases available to the public,7

 enabling 

other researchers to analyze the same data and hopefully minimizing discrepancies in the results 

that would have been due to inconsistencies across databases.8

 The agencies recognize, 

however, that the updated database may not represent the future fleet, because vehicles have 

continued and will continue to change. 

The agencies are aware that several studies have been initiated using NHTSA’s 2011 

newly established safety database. In addition to a new Kahane study, which is discussed in 

section II.G.4, other on-going studies include two by Wenzel at Lawrence Berkeley National 

Laboratory (LBNL) under contract with the U.S. DOE, and one by Dynamic Research, Inc. 

(DRI) contracted by the International Council on Clean Transportation (ICCT). These studies 

may take somewhat different approaches to examine the statistical relationship between fatality 

risk, vehicle mass and size. In addition to a detailed assessment of the NHTSA 2011 report, 

Wenzel is expected to consider the effect of mass and footprint reduction on casualty risk per 

crash, using data from thirteen states. Casualty risk includes both fatalities and serious or 

 

7

 The new databases are available at http://www.nhtsa.gov/fuel-economy (look for “Download Crash Databases for 

Statistical Analysis of Relationships Between Vehicles’ Fatality Risk, Mass, and Footprint.” 

8

 75 Fed. Reg. 25324 (May 7, 2010); the discussion of planned statistical analyses is on pp. 25395-25396. 
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incapacitating injuries. DRI is expected to use a two-stage approach to separate the effect of 

mass reduction on two components of fatality risk, crash avoidance and crashworthiness. The 

LBNL assessment of the NHTSA 2011 report is available in the docket for this NPRM.9 The 

casualty risk effect study was not available in time to inform this NPRM. The completed final 

peer reviewed-report on both assessments will be available prior to the final rule. DRI has also 

indicated that it expects its study to be publicly available prior to the final rule. The agencies will 

consider these studies and any others that become available, and the results may influence the 

safety analysis for the final rule. 

Other researchers are free to download the database from NHTSA’s website, and we 

expect to see additional papers in the coming months and as comments to the rulemaking that 

may also inform our consideration of these issues for the final rule. Kahane’s updated study for 

2011 is currently undergoing peer-review, and is available in the docket for this rulemaking for 

review by commenters. 

Finally, EPA and NHTSA with DOT’s Volpe Center, part of the Research and Innovative 

Technology Administration (RITA), attempted to investigate the implications of “Smart Design,” 

by identifying and describing the types of “Smart Design” and methods for using “Smart 

Design” to result in vehicle mass reduction, selecting analytical pairs of vehicles, and using the 

appropriate crash database to analyze vehicle crash data. The analysis identified several onevehicle and two-vehicle crash datasets with the potential to shed light on the issue, but the 

available data for specific crash scenarios was insufficient to produce consistent results that 

could be used to support conclusions regarding historical performance of “smart designs.” 

Undertaking these tasks has helped the agencies come closer to resolving some of the 

ongoing debates in statistical analysis research of historical crash data. We intend to apply these 

conclusions going forward, and we believe that the public discussion of the issues will be 

facilitated by the research conducted. The following sections discuss the findings from these 

studies and others in greater detail, to present a more nuanced picture of the current state of the 

statistical research. 

b. NHTSA workshop on vehicle mass, size and safety 

On February 25, 2011, NHTSA hosted a workshop on mass reduction, vehicle size, and 

fleet safety at the Headquarters of the U.S. Department of Transportation in Washington, DC.10

 

The purpose of the workshop was to provide the agencies with a broad understanding of current 

research in the field and provide stakeholders and the public with an opportunity to weigh in on 

 

9

 –Wenzel, T.P. (2011b). Assessment of NHTSA’s Report “Relationships between Fatality Risk, Mass, and 

Footprint in Model Year 2000-2007 Passenger Cars and LTVs”, available at... 10 A video recording, transcript, and the presentations from the NHTSA workshop on mass reduction, vehicle size 

and fleet safety is available at http://www.nhtsa.gov/fuel-economy (look for “NHTSA Workshop on Vehicle MassSize-Safety on Feb. 25”) 
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this issue. NHTSA also created a public docket to receive comments from interested parties that 

were unable to attend. 

The speakers included Charles Kahane of NHTSA, Tom Wenzel of Lawrence Berkeley 

National Laboratory, R. Michael Van Auken of Dynamic Research Inc. (DRI), Jeya Padmanaban 

of JP Research, Inc., Adrian Lund of the Insurance Institute for Highway Safety, Paul Green of 

the University of Michigan Transportation Research Institute (UMTRI), Stephen Summers of 

NHTSA, Gregg Peterson of Lotus Engineering, Koichi Kamiji of Honda, John German of the 

International Council on Clean Transportation (ICCT), Scott Schmidt of the Alliance of 

Automobile Manufacturers, Guy Nusholtz of Chrysler, and Frank Field of the Massachusetts 

Institute of Technology. 

The wide participation in the workshop allowed the agencies to hear from a broad range 

of experts and stakeholders. The contributions were particularly relevant to the agencies’ 

analysis of the effects of weight reduction for this proposed rule. The presentations were divided 

into two sessions that addressed the two expansive sets of issues – statistical evidence of the 

roles of mass and size on safety, and engineering realities – structural crashworthiness, occupant 

injury and advanced vehicle design. 

The first session focused on previous and ongoing statistical studies of crash data that 

attempt to identify the relative effects of vehicle mass and size on fleet safety. There was 

consensus that there is a complicated relationship with many confounding influences in the data. 

Wenzel summarized a recent study he conducted comparing four types of risk (fatality or 

casualty risk, per vehicle registration-years or per crash) using police-reported crash data from 

five states.11 He showed that the trends in risk for various classes of vehicles (e.g., non-sports 

car passenger cars, vans, SUVs, crossover SUVs, pickups) were similar regardless of what risk 

was being measured (fatality or casualty) or what exposure metric was used (e.g., registration 

years, police-reported crashes, etc.). In general, most trends showed a lower risk for drivers of 

larger, heavier vehicles. 

Although Wenzel’s analysis was focused on differences in the four types of risk on the 

relative risk by vehicle type, he cautioned that, when analyzing casualty risk per crash, analysts 

should control for driver age and gender, crash location (urban vs. rural), and the state in which 

the crash occurred (to account for crash reporting biases). 

Several participants pointed out that analyses must also control for individual 

technologies with significant safety effects (e.g., Electronic Stability Control, airbags).It was not 

always conclusive whether a specialty vehicle group (e.g., sports cars, two-door cars, early 

crossover SUVs) were outliers that confound the trend or unique datasets that isolate specific 

 

11 Wenzel, T.P. (2011a). Analysis of Casualty Risk per Police-Reported Crash for Model Year 2000 to 2004 

Vehicles, using Crash Data from Five States, March 2011, LBNL-4897E, available at: 

http://eetd.lbl.gov/EA/teepa/pub.html#Vehicle 
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vehicle characteristics. Unfortunately, specialty vehicle groups are usually adopted by specific 

driver groups, often with outlying vehicle usage or driver behavior patterns. Green, who 

conducted an independent review of the previous statistical analyses, suggested that evaluating 

residuals will give an indication of whether or not a data subset can be legitimately removed 

without inappropriately affecting the analytical results. 

It was recognized that the physics of a two-vehicle crash require that the lighter vehicle 

experience a greater change in velocity, which often leads to disproportionately more injury risk. 

Lund noted persistent historical trends that, in any time period, occupants of the smallest and 

lightest vehicles had, on average, fatality rates approximately twice those of occupants of the 

largest and heaviest vehicles but predicted “the sky will not fall” as the fleet downsizes, we will 

not see an increase in absolute injury risk because smaller cars will become increasingly 

protective of their occupants. Padmanaban also noted in her research of the historical trends that 

mass ratio and vehicle stiffness are significant predictors with mass ratio consistently the 

dominant parameter when correlating harm. Reducing the mass of any vehicle may have 

competing societal effects as it increases the injury risk in the lightened vehicle and decreases 

them in the partner vehicle 

The separation of key parameters was also discussed as a challenge to the analyses, as 

vehicle size has historically been highly correlated with vehicle mass. Presenters had varying 

approaches for dealing with the potential multicollinearity between these two variables. Van 

Auken of DRI stated that there was latitude in the value of Variance Inflation Factor (VIF, a 

measure of multicollinearity) that would call results into question, and suggested that the large 

value of VIF for curb weight might imply “perhaps the effect of weight is too small in 

comparison to other factors.” Green, of UMTRI, stated that highly correlated variables may not 

be appropriate for use in a predictive model and that “match[ing] on footprint” (i.e., conducting 

multiple analyses for data subsets with similar footprint values) may be the most effective way to 

resolve the issue. 

There was no consensus on the overall effect of the maneuverability of smaller, lighter 

vehicles. German noted that lighter vehicles should have improved handling and braking 

characteristics and “may be more likely to avoid collisions”. Lund presented crash involvement 

data that implied that, among vehicles of similar function and use rates, crash risk does not go 

down for more “nimble” vehicles. Several presenters noted the difficulties of projecting past 

data into the future as new technologies will be used that were not available when the data were 

collected. The advances in technology through the decades have dramatically improved safety 

for all weight and size classes. A video of IIHS’s 50th anniversary crash test of a 1959 Chevrolet 

Bel Air and 2009 Chevrolet Malibu graphically demonstrated that stark differences in design and 

technology that can possibly mask the discrete mass effects, while videos of compatibility crash 

tests between smaller, lighter vehicles and contemporary larger, heavier vehicles graphically 

showed the significance of vehicle mass and size. 
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Kahane presented results from his 2010 report12 that found that a scenario which took 

some mass out of heavier vehicles but little or no mass out of the lightest vehicles did not impact 

safety in absolute terms. Kahane noted that if the analyses were able to consider the mass of 

both vehicles in a two-vehicle crash, the results may be more indicative of future crashes. There 

is apparent consistency with other presentations (e.g., Padmanaban, Nusholtz) that reducing the 

overall ranges of masses and mass ratios seems to reduce overall societal harm. That is, the effect 

of mass reduction exclusively does not appear to be a “zero sum game” in which any increase in 

harm to occupants of the lightened vehicle is precisely offset by a decrease in harm to the 

occupants of the partner vehicle. If the mass of the heavier vehicle is reduced by a larger 

percentage, the changes in velocity from the collision are more nearly equal and the injuries 

suffered in the lighter vehicle are likely to be reduced more than the injuries in the heavier 

vehicle are increased. Alternatively, a fixed mass reduction (say, 100 lbs) in all vehicles could 

increase societal harm whereas a fixed percentage mass reduction is more likely to be neutral. 

Padmanaban described a series of studies conducted in recent years. She included 

numerous vehicle parameters including bumper height and several measures of vehicle size and 

stiffness and also commented on previous analyses that using weight and wheelbase together in a 

logistic model distorts the estimates, resulting in inflated variance with wrong signs and 

magnitudes in the results. Her results consistently showed that vehicle mass ratio was a more 

important parameter than those describing vehicle geometry or stiffness. Her ultimate 

conclusion was that removing mass (e.g., 100 lbs.) from all passenger cars would cause an 

overall increase in fatalities in truck-to-car crashes while removing the same amount from light 

trucks would cause an overall decrease in fatalities. 

c. Report by Green et al., UMTRI – “Independent Review: Statistical Analyses of 

Relationship Between Vehicle Curb Weight, Track Width, Wheelbase and 

Fatality Rates,” April 2011. 

As explained above, NHTSA contracted with the University of Michigan Transportation 

Research Institute (UMTRI) to conduct an independent review13 of a set of statistical analyses of 

relationships between vehicle curb weight, the footprint variables (track width, wheelbase) and 

fatality rates from vehicle crashes. The purpose of this review was to examine analysis methods, 

data sources, and assumptions of the statistical studies, with the objective of identifying the 

reasons for any differences in results. Another objective was to examine the suitability of the 

various methods for estimating the fatality risks of future vehicles. 

 

12 Kahane, C. J. (2010). “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 1991-1999 and 

Other Passenger Cars and LTVs,” Final Regulatory Impact Analysis: Corporate Average Fuel Economy for MY 

2012-MY 2016 Passenger Cars and Light Trucks. Washington, DC: National Highway Traffic Safety 

Administration, pp. 464-542, available at http://www.nhtsa.gov/staticfiles/rulemaking/pdf/cafe/CAFE_2012-

2016_FRIA_04012010.pdf. 

13 The review is independent in the sense that it was conducted by an outside third party without any interest in the 

reported outcome. 
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 UMTRI reviewed a set of papers, reports, and manuscripts provided by NHTSA 

(listed in Appendix A of UMTRI’s report, which is available in the docket to this rulemaking) 

that examined the statistical relationships between fatality or casualty rates and vehicle properties 

such as curb weight, track width, wheelbase and other variables. 

 It is difficult to summarize a study of that length and complexity for purposes of this 

discussion, but fundamentally, the UMTRI team concluded the following: 

• Differences in data may have complicated comparisons of earlier analyses, but if the 

methodology is robust, and the methods were applied in a similar way, small changes in 

data should not lead to different conclusions. The main conclusions and findings should 

be reproducible. The data base created by Kahane appears to be an impressive collection 

of files from appropriate sources and the best ones available for answering the research 

questions considered in this study. 

• In statistical analysis simpler models generally lead to improved inference, assuming the 

data and model assumptions are appropriate. In that regard, the disaggregate logistic 

regression model used by NHTSA in the 2003 report14 seems to be the most appropriate 

model, and valid for the analysis in the context that it was used: finding general 

associations between fatality risk and mass – and the general directions of the reported 

associations are correct. 

• The two-stage logistic regression model in combination with the two-step aggregate 

regression used by DRI seems to be more complicated than is necessary based on the data 

being analyzed, and summing regression coefficients from two separate models to arrive 

at conclusions about the effects of reductions in weight or size on fatality risk seems to 

add unneeded complexity to the problem. 

• One of the biggest issues regarding this work is the historical correlation between curb 

weight, wheelbase, and track width. Including three variables that are highly correlated 

in the same model can have adverse effects on the fit of the model, especially with 

respect to the parameter estimates, as discussed by Kahane. UMTRI makes no 

conclusions about multicollinearity, other than to say that inferences made in the 

presence of multicollinearity should be judged with great caution. At the NHTSA 

workshop on size, safety and mass, Paul Green suggested that a matched analysis, in 

which regressions are run on the relationship between mass reduction and risk separately 

for vehicles of similar footprint, could be undertaken to investigate the effect of 

multicollinearity between vehicle mass and size. Kahane has combined wheelbase and 

track width into one variable (footprint) to compare with curb weight. NHTSA believes 

that the 2011 Kahane analysis has done all it can to lessen concerns about 

multicollinearity, but a concern still exists. In considering other studies provided by 

NHTSA for evaluation by the UMTRI team: 

o Papers by Wenzel, and Wenzel and Ross, addressing associations between fatality 

risk per vehicle registration-year, weight, and size by vehicle model contribute to 

understanding some of the relationships between risk, weight, and size. However, 

least squares linear regression models, without modification, are not exposure-

 
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based risk models and inference drawn from these models tends to be weak since 

they do not account for additional differences in vehicles, drivers, or crash 

conditions that could explain the variance in risk by vehicle model. 

o A 2009 J.P. Research paper focused on the difficulties associated with separating 

out the contributions of weight and size variables when analyzing fatality risk 

properly recognized the problem arising from multicollinearity and included a 

clear explanation of why fatality risk is expected to increase with increasing mass 

ratio. UMTRI concluded that the increases in fatality risk associated with a 100-

pound reduction in weight allowing footprint to vary with weight as estimated by 

Kahane and JP Research, are broadly more convincing than the 6.7 percent 

reduction in fatality risk associated with mass reduction while holding footprint 

constant, as reported by DRI. 

o A paper by Nusholtz et al. focused on the question of whether vehicle size can 

reasonably be the dominant vehicle factor for fatality risk, and finding that 

changing the mean mass of the vehicle population (leaving variability unchanged) 

has a stronger influence on fatality risk than corresponding (feasible) changes in 

mean vehicle dimensions, concluded unequivocally that reducing vehicle mass 

while maintaining constant vehicle dimensions will increase fatality risk. UMTRI 

concluded that if one accepts the methodology, this conclusion is robust against 

realistic changes that may be made in the force vs. deflection characteristics of the 

impacting vehicles. 

o Two papers by Robertson, one a commentary paper and the other a peer-reviewed 

journal article, were reviewed. The commentary paper did not fit separate models 

according to crash type, and included passenger cars, vans, and SUVs in the same 

model. UMTRI concluded that some of the claims in the commentary paper 

appear to be overstated, and intermediate results and more documentation would 

help the reader determine if these claims are valid. The second paper focused 

largely on the effects of electronic stability control (ESC), but generally followed 

on from the first paper except that curb weight is not fit and fuel economy is used 

as a surrogate. 

 

The UMTRI study provided a number of useful suggestions that Kahane considered in 

updating his 2011 analysis, and that have been incorporated into the safety effects estimates for 

the current rulemaking. 

d. Report by Dr. Charles Kahane, NHTSA – “Relationships Between Fatality Risk, 

Mass, and Footprint in Model Year 2000-2007 Passenger Cars and LTVs,” 2011 

The relationship between a vehicle’s mass, size, and fatality risk is complex, and it varies 

in different types of crashes. NHTSA, along with others, has been examining this relationship 

for over a decade. The safety chapter of NHTSA’s April 2010 final regulatory impact analysis 

(FRIA) of CAFE standards for MY 2012-2016 passenger cars and light trucks included a 

statistical analysis of relationships between fatality risk, mass, and footprint in MY 1991-1999 

passenger cars and LTVs (light trucks and vans), based on calendar year (CY) 1995-2000 crash 
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and vehicle-registration data.15 The 2010 analysis used the same data as the 2003 analysis, but 

included vehicle mass and footprint in the same regression model. 

The principal findings of NHTSA’s 2010 analysis were that mass reduction in lighter 

cars, even while holding footprint constant, would significantly increase societal fatality risk, 

whereas mass reduction in the heavier LTVs would significantly reduce net societal fatality risk, 

because it would reduce the fatality risk of occupants in lighter vehicles which collide with the 

heavier LTVs. NHTSA concluded that, as a result, any reasonable combination of mass 

reductions while holding footprint constant in MY 2012-2016 vehicles – concentrated, at least to 

some extent, in the heavier LTVs and limited in the lighter cars – would likely be approximately 

safety-neutral; it would not significantly increase fatalities and might well decrease them. 

NHTSA’s 2010 report partially agreed and partially disagreed with analyses published 

during 2003-2005 by Dynamic Research, Inc. (DRI). NHTSA and DRI both found a significant 

protective effect for footprint, and that reducing mass and footprint together (downsizing) on 

smaller vehicles was harmful. DRI’s analyses estimated a significant overall reduction in 

fatalities from mass reduction in all light-duty vehicles if wheelbase and track width were 

maintained, whereas NHTSA’s report showed overall fatality reductions only in the heavier 

LTVs, and benefits only in some types of crashes for other vehicle types. Much of NHTSA’s 

2010 report, as well as recent work by DRI, involved sensitivity tests on the databases and 

models, which generated a range of estimates somewhere between the initial DRI and NHTSA 

results.16 

Immediately after issuing the final rule for MYs 2012-2016 CAFE and GHG standards in 

May 2010, NHTSA and EPA began work on the next joint rulemaking to develop CAFE and 

GHG standards for MY 2017 to 2025 and beyond. The preamble to the 2012-2016 final rule 

stated that NHTSA, working closely with EPA and the Department of Energy (DOE), would 

perform a new statistical analysis of the relationships between fatality rates, mass and footprint, 

updating the crash and exposure databases to the latest available model years, refining the 

 

15 Kahane, C. J. (2010). “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 1991-1999 and 

Other Passenger Cars and LTVs,” Final Regulatory Impact Analysis: Corporate Average Fuel Economy for MY 

2012-MY 2016 Passenger Cars and Light Trucks. Washington, DC: National Highway Traffic Safety 

Administration, pp. 464-542, available at http://www.nhtsa.gov/staticfiles/rulemaking/pdf/cafe/CAFE_2012-

2016_FRIA_04012010.pdf. 

16 Van Auken, R. M., and Zellner, J. W. (2003). A Further Assessment of the Effects of Vehicle Weight and Size 

Parameters on Fatality Risk in Model Year 1985-98 Passenger Cars and 1986-97 Light Trucks. Report No. DRITR-03-01. Torrance, CA: Dynamic Research, Inc.; Van Auken, R. M., and Zellner, J. W. (2005a). An Assessment of 

the Effects of Vehicle Weight and Size on Fatality Risk in 1985 to 1998 Model Year Passenger Cars and 1985 to 

1997 Model Year Light Trucks and Vans. Paper No. 2005-01-1354. Warrendale, PA: Society of Automotive 

Engineers; Van Auken, R. M., and Zellner, J. W. (2005b). Supplemental Results on the Independent Effects of Curb 

Weight, Wheelbase, and Track on Fatality Risk in 1985-1998 Model Year Passenger Cars and 1986-97 Model Year 

LTVs. Report No. DRI-TR-05-01. Torrance, CA: Dynamic Research, Inc.; Van Auken, R.M., and Zellner, J. W. 

(2011). “Updated Analysis of the Effects of Passenger Vehicle Size and Weight on Safety,” NHTSA Workshop on 

Vehicle Mass-Size-Safety, Washington, February 25, 2011, 

http://www.nhtsa.gov/staticfiles/rulemaking/pdf/MSS/MSSworkshop_VanAuken.pdf 
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methodology in response to peer reviews of the 2010 report and taking into account changes in 

vehicle technologies. The previous databases of MY 1991-1999 vehicles in CY 1995-2000 

crashes has become outdated as new safety technologies, vehicle designs and materials were 

introduced. The new databases comprising MY 2000-2007 vehicles in CY 2002-2008 crashes 

with the most up-to-date possible, given the processing lead time for crash data and the need for 

enough crash cases to permit statistically meaningful analyses. NHTSA has made the new 

databases available to the public17, enabling other researchers to analyze the same data and 

hopefully minimizing discrepancies in the results due to inconsistencies across the data used.18

One way to estimate these effects is via statistical analyses of societal fatality rates per 

vehicle miles traveled (VMT), by vehicles’ mass and footprint, for the current on-road vehicle 

fleet. The basic analytical method used for the 2011 NHTSA report is the same as in NHTSA’s 

2010 report: cross-sectional analyses of the effect of mass and footprint reductions on the 

societal fatality rate per billion vehicle miles of travel (VMT), while controlling for driver age 

and gender, vehicle type, vehicle safety features, crash times and locations, and other factors. 

Separate logistic regression models are run for three types of vehicles and nine types of crashes. 

Societal fatality rates include occupants of all vehicles in the crash, as well as non-occupants, 

such as pedestrians and cyclists. NHTSA’s 2011 Report19 analyzes MY 2000-2007 cars and 

LTVs in CY 2002-2008 crashes. Fatality rates were derived from FARS data, 13 State crash 

files, and registration and mileage data from R.L. Polk. 

The most noticeable change in MY 2000-2007 vehicles from MY 1991-1999 has been 

the increase in crossover utility vehicles (CUV), which are SUVs of unibody construction, often 

but not always built upon a platform shared with passenger cars. CUVs have blurred the 

distinction between cars and trucks. The new analysis treats CUVs and minivans as a separate 

vehicle class, because they differ in some respects from pickup-truck-based LTVs and in other 

respects from passenger cars. In the 2010 report, the many different types of LTVs were 

combined into a single analysis and NHTSA believes that this may have made the analyses too 

complex and might have contributed to some of the uncertainty in the results. 

The new database has accurate VMT estimates, derived from a file of odometer readings 

by make, model, and model year recently developed by R.L. Polk and purchased by NHTSA.20 

For the 2011 report, the relative distribution of crash types has been changed to reflect the 

projected distribution of crashes during the period from 2017 to 2025, based on the estimated 

effectiveness of electronic stability control (ESC) in reduction the number of fatalities in rollover 

crashes and crashes with a stationary object. The annual target population of fatalities or the 

 

17 http://www.nhtsa.gov/fuel-economy

18 75 Fed. Reg. 25324 (May 7, 2010); the discussion of planned statistical analyses is on pp. 25395-25396. 

19 Kahane, C. J. (2011). “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 2000-2007 

Passenger Cars and LTVs,”, July 2011. The report is available in the NHTSA docket, NHTSA-2010-0152. You can 

access the docket at http://www.regulations.gov/#!home by typing ‘NHTSA-2010-0152’ where it says “enter 

keyword or ID” and then clicking on “Search.” 

20 In the 1991-1999 data base, VMT was estimated only by vehicle class, based on NASS CDS data. 
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annual fatality distribution baseline21 was not decreased in the period between 2017 and 2025 for 

the safety statistics analysis, but is taken into account later in the Volpe model analysis, since all

vehicles in the future will be equipped with ESC22. 

For the 2011 report, vehicles are now grouped into five classes rather than four: 

passenger cars (including both 2-door and 4-door cars) are split in half by median weight; CUVs 

and minivans; and truck-based LTVs, which are also split in half by median weight of the model 

year 2000-2007 vehicles. Table Error! No text of specified style in document.-1 presents the 

estimated percent increase in U.S. societal fatality risk per ten billion VMT for each 100-pound 

reduction in vehicle mass, while holding footprint constant, for each of the five classes of 

vehicles. 

Table Error! No text of specified style in document.-1 Results of 2011 NHTSA report 

Fatality Increase (%) per 100-Pound Mass Reduction While Holding Footprint Constant 

MY 2000-2007 

CY 2002-2008 

Fatality Increase (%) Per 100-Pound Mass 

Reduction While Holding Footprint Constant 

Point Estimate 95% Confidence Bounds 

Cars < 3,106 pounds 1.44 + .29 to +2.59

Cars > 3,106 pounds .47 - .58 to +1.52

CUVs and minivans - .46 -1.75 to + .83

Truck-based LTVs < 4,594 pounds .52 - .43 to +1.46

Truck-based LTVs > 4,594 pounds - .39 -1.06 to + .27

Only the 1.44 percent risk increase in the lighter cars is statistically significant. There are 

non-significant increases in the heavier cars and the lighter truck-based LTVs, and nonsignificant societal benefits for mass reduction in CUVs, minivans, and the heavier truck-based 

LTVs. Based on these results, potential combinations of mass reductions that maintain footprint 

and are proportionately somewhat higher for the heavier vehicles may be safety-neutral or better 

as point estimates and, in any case, unlikely to significantly increase fatalities. The primarily 

non-significant results are not due to a paucity of data, but because the societal effect of mass 

reduction while maintaining footprint, if any, is small. 

MY 2000-2007 vehicles of all types are heavier and larger than their MY 1991-1999 

counterparts. The average mass of passenger cars increased by 5 percent from 2000 to 2007 and 

the average mass of pickup trucks increased by 19 percent. Other types of vehicles became 

heavier, on the average, by intermediate amounts. There are several reasons for these increases: 

 

21 MY 2004 -2007 vehicles with fatal crashes occurred in CY 2004-2008 are selected as the annual fatality 

distribution baseline in the Kahane analysis. 

22 In the Volpe model, NHTSA assumed that the safety trend would result in 12.6 percent reduction between 2007 

and 2020 due to the combination of ESC, new safety standard, and behavior changes anticipated. 
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during this time frame, some of the lighter make-models were discontinued; many models were 

redesigned to be heavier and larger; and consumers more often selected stretched versions such 

as crew cabs in their new-vehicle purchases. 

It is interesting to compare the new results to NHTSA’s 2010 analysis of MY 1991-1999 

vehicles in CY 1995-2000, especially the new point estimate to the “actual regression result 

scenario” in the 2010 report: 

Table Error! No text of specified style in document.-2 2010 Report: MY 1991-1999, 

CY 1995-2000 Fatality Increase (%) per 100-Pound Mass Reduction While Holding 

Footprint Constant 

Actual Regression 

Result Scenario 

Upper-Estimate 

Scenario 

Lower-Estimate 

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 

Table Error! No text of specified style in document.-3 Fatality Increase (%) per 100-

Pound Mass Reduction While Holding Footprint Constant 

NHTSA (2010) NHTSA (2011) 

Lighter cars 2.21% 1.43% 

Heavier cars 0.89% 0.48% 

Lighter LTVs 0.17%* 0.52% 

Heavier LTVs -1.90%* -0.40% 

CUV/ minivan -0.47% 

*Includes CUV/minivan 

The new results are directionally the same as in 2010: fatality increase in the lighter cars, 

safety benefit in the heavier LTVs, but the effects may have become weaker at both ends. (The 

agencies do not consider this conclusion to be definitive because of the relatively wide 

confidence bounds of the estimates.) The fatality increase in the lighter cars tapered off from 

2.21 percent to 1.44 percent while the societal benefit of mass reduction in the heaviest LTVs 

diminished from 1.90 percent to 0.39 percent and is no longer statistically significant. 

The agencies believe that the changes may be due to a combination of both changes in the 

characteristics of newer vehicles and revisions to the analysis. NHTSA believes, above all, that 

several light, small car models with poor safety performance were discontinued by 2000 or 

during 2000-2007. Also, the tendency of light, small vehicles to be driven poorly is not as strong 

as it used to be – perhaps in part because safety improvements in lighter and smaller vehicles 
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have made some good drivers more willing to buy them. Both agencies believe that at the other 

end of the weight/size spectrum, blocker beams and other voluntary compatibility improvements 

in LTVs, as well as compatibility-related self-protection improvements to cars, have made the 

heavier LTVs less aggressive in collisions with lighter vehicles (although the effect of mass 

disparity remains). This report’s analysis of CUVs and minivans as a separate class of vehicles 

may have relieved some inaccuracies in the 2010 regression results for LTVs. Interestingly, the 

new actual-regression results are quite close to the previous report’s “lower-estimate scenario,” 

which was an attempt to adjust for supposed inaccuracies in some regressions and for a 

seemingly excessive trend toward higher crash rates in smaller and lighter cars. 

The principal difference between the heavier vehicles, especially truck-based LTVs, and 

the lighter vehicles, especially passenger cars, is that mass reduction has a different effect in 

collisions with another car or LTV. When two vehicles of unequal mass collide, the delta V is 

higher in the lighter vehicle, in the same proportion as the mass ratio. As a result, the fatality 

risk is also higher. Removing some mass from the heavy vehicle reduces delta V in the lighter 

vehicle, where fatality risk is high, resulting in a large benefit, offset by a small penalty because 

delta V increases in the heavy vehicle, where fatality risk is low – adding up to a net societal 

benefit. Removing some mass from the lighter vehicle results in a large penalty offset by a 

small benefit – adding up to net harm. These considerations drive the overall result: fatality 

increase in the lighter cars, reduction in the heavier LTVs, and little effect in the intermediate 

groups. However, in some types of crashes, especially first event rollovers and impacts with 

fixed objects, mass reduction is usually not harmful and often beneficial, because the lighter 

vehicles respond more quickly to braking and steering and are often more stable because their 

center of gravity is lower. Offsetting that benefit is the continuing historical tendency of lighter 

and smaller vehicles to be driven less well – although it continues to be unknown why that is so, 

and to what extent, if any, the lightness or smallness of the vehicle contributes to people driving 

it less safely. 

The estimates of the model are formulated for each 100-pound reduction in mass; in other 

words, if risk increases by 1 percent for 100 pounds reduction in mass, it would increase by 2 

percent for a 200-pound reduction, and 3 percent for a 300-pound reduction (more exactly, 2.01 

percent and 3.03 percent, because the effects work like compound interest). Confidence bounds 

around the point estimates will grow wider by the same proportions. 

The regression results are best suited to predict the effect of a small change in mass, 

leaving all other factors, including footprint, the same. With each additional change from the 

current environment, the model may become somewhat less accurate and it is difficult to assess 

the sensitivity to additional mass reduction greater than 100 pounds. The agencies recognize that 

the light-duty vehicle fleet in the 2017-2025 timeframe will be different than the 2000-2007 fleet 

analyzed for this study. Nevertheless, one consideration provides some basis for confidence. 

This is NHTSA’s fourth evaluation of the effects of mass reduction and/or downsizing, 

comprising databases ranging from MY 1985 to 2007. The results of the four studies are not 
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identical, but they have been consistent up to a point. During this time period, many makes and 

models have increased substantially in mass, sometimes as much as 30-40 percent.23 If the 

statistical analysis has, over the past years, been able to accommodate mass increases of this 

magnitude, perhaps it will also succeed in modeling the effects of mass reductions on the order 

of 10-20 percent, if they occur in the future. 

e. Report by Tom Wenzel, LBNL, “An Assessment of NHTSA’s Report 

‘Relationships Between Fatality Risk, Mass, and Footprint in Model Year 2000-

2007 Passenger Cars and LTVs’”, 2011 

DOE contracted with Tom Wenzel of Lawrence Berkeley National Laboratory to conduct 

an assessment of NHTSA’s updated 2011 study of the effect of mass and footprint reductions on 

US fatality risk per vehicle miles traveled, and to provide an analysis of the effect of mass and 

footprint reduction on casualty risk per police-reported crash, using independent data from 

thirteen states. The assessment has been completed and reviewed by NHTSA and EPA staff, and 

a draft final version is included in the docket of today’s rulemaking; the separate analysis of 

crash data from thirteen states will be completed and included in the docket shortly. Both reports 

will be peer reviewed by outside experts. 

The LBNL report replicates Kahane’s analysis for NHTSA, using the same data and 

methods, and in many cases using the same SAS programs. The Wenzel report finds that 

although mass reduction in lighter (less than 3,106 lbs) cars leads to a statistically significant 

1.44% increase in fatality risk per vehicle miles travelled (VMT), the increase is small. He tests 

this result for sensitivity to changes in specifications of the regression models and what data are 

used. In addition Wenzel shows that there is a wide range in fatality rates by vehicle model for 

models that have the same mass, even after accounting for differences in drivers’ age and gender, 

safety features installed, and crash times and locations. This section summarizes the results of 

the Wenzel assessment of the most recent NHTSA analysis. 

The LBNL report highlights the effect of the other driver, vehicle, and crash control 

variables, in addition to the effect of mass and footprint reduction, on risk. Some of the other 

variables NHTSA included in its regression models have much larger effects on fatality risk than 

mass or footprint reduction. For example, the models indicate that a 100-lb increase in the mass 

of a lighter car results in a 1.44% reduction in fatality risk; this is the largest estimated effect of 

changes in vehicle mass, and the only one that is statistically significant. For comparison this 

reduction in fatality risk could also be achieved by a 13% increase in 4-door sedans equipped 

with ESC. 

 

23 For example, one of the most popular models of small 4-door sedans increased in curb weight from 1,939 pounds 

in MY 1985 to 2,766 pounds in MY 2007, a 43 percent increase. A high-sales mid-size sedan grew from 2,385 to 

3,354 pounds (41%); a best-selling pickup truck from 3,390 to 4,742 pounds (40%) in the basic model with 2-door 

cab and rear-wheel drive; and a popular minivan from 2,940 to 3,862 pounds (31%). 
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The 1.44% increase in risk from reducing mass in the lighter cars was tested for 

sensitivity changes in the specification of, or the data used in, the regression models. For 

example, using the current distribution of crashes, rather than adjusting the distribution to that 

expected after full adoption of ESC, reduces the effect to 1.18%; excluding the calendar year 

variables from the model, which may be weakening the modeled benefits of vehicle safety 

technologies, reduces the effect to 1.39%; and including vehicle make in the model increases the 

effect to 1.81%. The results also are sensitive to the selection of data to include in the analysis: 

excluding bad drivers increases the effect to 2.03%, while excluding crashes involving alcohol or 

drugs increases the effect to 1.66%, and including sports, police, and all-wheel drive cars 

increases the effect to 1.64%. Finally, changing the definition of risk also affects the result for 

lighter cars: using the number of fatalities per induced exposure crash reduces the effect to -

0.24% (that is, a 0.24% reduction in risk), while using the number of fatal crashes (rather than 

total fatalities) per VMT increases the effect to 1.84%. These sensitivity tests, except one, 

changed the estimated coefficient by less than 1 percentage point, which is within its statistical 

confidence bounds of 0.29 to 2.59 percent and may be considered compatible with the baseline 

result. Using two or more variables that are strongly correlated in the same regression model 

(referred to as multicollinearity) can lead to inaccurate results. However, the correlation between 

vehicle mass and footprint may not be strong enough to cause serious concern. Experts suggest 

that a correlation of greater than 0.60 (or a variance inflation factor of 2.5) raises concern about 

multicollinearity.24 The correlation between vehicle mass and footprint ranges from over 0.80 

for four-door sedans, pickups, and SUVs, to about 0.65 for two-door cars and CUVs, to 0.26 for 

minivans; when pickups and SUVs are considered together, the correlation between mass and 

footprint is 0.65. Wenzel notes that the 2011 NHTSA report recognizes that the “near” 

multicollinearity between mass and footprint may not be strong enough to invalidate the results 

from a regression model that includes both variables. In addition, NHTSA included several 

analyses to address possible effects of the near-multicollinearity between mass and footprint. 

First, NHTSA ran a sensitivity model specification, where footprint is not held constant, 

but rather allowed to vary as mass varies (i.e. NHTSA ran a regression model which includes 

mass but not footprint). If the multicollinearity was so great that including both variables in the 

same model gave misleading results, removing footprint from the model could give mass 

coefficients five or more percentage points different than keeping it in the model. NHTSA’s 

sensitivity test indicates that when footprint is allowed to vary with mass, the effect of mass 

reduction on risk increases from 1.44% to 2.64% for lighter cars, and from a non-significant 

0.47% to a statistically-significant 1.94% for heaver cars (changes of less than two percentage 

points); however, the effect of mass reduction on light trucks is unchanged, and is still not 

statistically significant for CUVs/minivans. 

 

24 Light-Duty Vehicle Greenhouse Gas Emission Standards and Corporate Average Fuel Economy 

Standards; Final Rule, April 1, 2010, Section II.G.3., page 139. 
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Second, NHTSA conducted a stratification analysis of the effect of mass reduction on 

risk by dividing vehicles into deciles based on their footprint, and running a separate regression 

model for each vehicle and crash type, for each footprint decile (3 vehicle types times 9 crash 

types times 10 deciles equals 270 regressions). This analysis estimates the effect of mass 

reduction on risk separately for vehicles with similar footprint. The analysis indicates that mass 

reduction does not consistently increase risk across all footprint deciles for any combination of 

vehicle type and crash type. Mass reduction increases risk in a majority of footprint deciles for 

13 of the 27 crash and vehicle combinations, but few of these increases are statistically 

significant. On the other hand, mass reduction decreases risk in a majority of footprint deciles 

for 9 of the 27 crash and vehicle combinations; in some cases these risk reductions are large and 

statistically significant.25 If reducing vehicle mass while maintaining footprint inherently leads 

to an increase in risk, the coefficients on mass reduction should be more consistently positive, 

and with a larger R2

, across the 27 vehicle/crash combinations, than shown in the analysis. 

These findings are consistent with the conclusion of the basic regression analyses, namely, that 

the effect of mass reduction while holding footprint constant, if any, is small. 

One limitation of using logistic regression to estimate the effect of mass reduction on risk 

is that a standard statistic to measure the extent to which the variables in the model explain the 

range in risk, equivalent to the R2

 statistic in a linear regression model, does not exist. (SAS 

does generate a pseudo-R2 value for logistic regression models; in almost all of the NHTSA 

regression models this value is less than 0.10). For this reason LBNL conducted an analysis of 

risk versus mass by vehicle model. LBNL used the results of the NHTSA logistic regression 

model to predict the number of fatalities expected after accounting for all vehicle, driver, and 

crash variables included in the NHTSA regression model except for vehicle weight and footprint. 

LBNL then plotted expected fatality risk per VMT by vehicle model against the mass of each 

model, and analyzed the change in risk as mass increases, as well as how much of the change in 

risk was explained by all of the variables included in the model. 

The analysis indicates that, after accounting for all the variables, risk does decrease as 

mass increases; however, risk and mass are not strongly correlated, with the R2 ranging from 

0.33 for CUVs to less than 0.15 for all other vehicle types (as shown in Figure x). This means 

that, on average, risk decreases as mass increases, but the variation in risk among individual 

vehicle models is stronger than the trend in risk from light to heavy vehicles. For fullsize (i.e. 3⁄4-

and 1-ton) pickups, risk increases as mass increases, with an R2

 of 0.43, consistent with 

NHTSA’s basic regression results for the heavier LTVs (societal risk increases as mass 

increases). LBNL also examined the relationship between residual risk, that is the remaining 

unexplained risk after accounting for all vehicle, driver and crash variables, and mass, and found 

similarly poor correlations. This implies that the remaining factors not included in the regression 

 

25 And in 5 of the 27 crash and vehicle combinations, mass reduction increased risk in 5 deciles and decreased risk 

in 5 deciles. 
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model that account for the observed range in risk by vehicle model also are not correlated with 

mass. (LBNL found similar results when the analysis compared risk to vehicle footprint.) 

Figure Error! No text of specified style in document.-1 indicates that some vehicles on 

the road today have the same, or lower, fatality rates than models that weigh substantially more, 

and are substantially larger in terms of footprint. After accounting for differences in driver age 

and gender, safety features installed, and crash times and locations, there are numerous examples 

of different models with similar weight and footprint yet widely varying fatality rates. The 

variation of fatality rates among individual models may reflect differences in vehicle design, 

differences in the drivers who choose such vehicles (beyond what can be explained by 

demographic variables such as age and gender), and statistical variation of fatality rates based on 

limited data for individual models. Differences in vehicle design can, and already do, mitigate 

some safety penalties from reduced mass; this is consistent with NHTSA’s opinion that some of 

the changes in its regression results between the 2003 study and the 2011 study are due to the 

redesign or removal of certain smaller and lighter models of poor design. 

Figure Error! No text of specified style in document.-1 Predicted US fatality risk per 

VMT vs. curb weight by vehicle type, after accounting for all driver, crash, and vehicle 

variables except mass and footprint 

f. Based on this information, what do the agencies consider to be the current state of 

statistical research on vehicle mass and safety? 

The agencies believe that statistical analysis of historical crash data continues to be an 

informative and important tool in assessing the potential safety impacts of the proposed 

R2 = 0.12 

R2 = 0.04 

R2 = 0.00 

R2 = 0.43 

R2 = 0.05 

R2 = 0.33 

R2 = 0.09 

0 

40 

80 

120 

160 

200 

240 

280 

320 

360 

400 

2000 2400 2800 3200 3600 4000 4400 4800 5200 5600 6000 6400 6800 7200 7600 

Predicted US fatality risk (crash fdatalities per 1010 VMT) 

Curb weight (lbs) 

2-dr cars 

4-dr cars 

Small pickups 

Fullsize pickups 

SUVs 

CUVs 

Minivans 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review ***

Page 21 of 32

standards. The effect of mass reduction while maintaining footprint is a complicated topic and 

there are open questions whether future designs will reduce the historical correlation between 

weight and size. It is important to note that while the updated database represents more current 

vehicles with technologies more representative of vehicles on the road today, they still do not 

fully represent what vehicles will be on the road in the 2017-2025 timeframe. The vehicles 

manufactured in the 2000-2007 timeframe were not subject to footprint-based fuel economy 

standards. The agencies expect that the attribute-based standards will likely facilitate the design 

of vehicles such that manufacturers may reduce mass while maintaining footprint. Therefore, it is 

possible that the analysis for 2000-2007 vehicles may not be fully representative of the vehicles 

that will be on the road in 2017 and beyond. 

While we recognize that statistical analysis of historical crash data may not be the only 

way to think about the future relationship between vehicle mass and safety, we also recognize 

that other assessment methods are also subject to uncertainties, which makes statistical analysis 

of historical data an important starting point if employed mindfully and recognized for how it 

can be useful and what its limitations may be. 

NHTSA undertook the independent review of statistical studies and held the mass-safety 

workshop in February 2011 in order to help the agencies sort through the ongoing debates over 

what statistical analysis of historical data is actually telling us. Previously, the agencies have 

assumed that differences in results were due in part to inconsistent databases; by creating the 

updated common database and making it publicly available, we are hopeful that that aspect of the 

problem has been resolved, and moreover, the UMTRI review suggested that differences in data 

were probably less significant than the agencies may have thought. Statistical analyses of 

historical crash data should be examined for potential multicollinearity issues. The agencies will 

continue to monitor issues with multicollinearity in our analyses, and hope that outside 

researchers will do the same. And finally, based on the findings of the independent review, the 

agencies continue to be confident that Kahane’s analysis is one of the best for the purpose of 

analyzing potential safety effects of future CAFE and GHG standards. UMTRI concluded that 

Kahane’s approach is valid, and Kahane has continued and refined that approach for the current 

analysis. The NHTSA 2011 statistical fatality report finds directionally similar but less 

statistically significant relationships between vehicle mass, size, and footprint, as discussed 

above. Based on these findings, the agencies believe that in the future, fatalities due to mass 

reduction will be best reduced if mass reduction is concentrated in the heaviest vehicles. 

NHTSA considers part of the reason that more recent historical data shows a dampened effect in 

the relationship between mass reduction and safety is that all vehicles, including traditionally 

lighter ones, grew heavier during that timeframe (2000s). As lighter vehicles might become 

more prevalent in the fleet again over the next decade, it is possible that the trend could 

strengthen again. On the other hand, extensive use of new lightweight materials and optimized 

vehicle design may weaken the relationship. Future updated analyses will be necessary to 

determine how the effect of mass reduction on risk changes over time. 
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Page 22 of 32

Both agencies agree that there are several identifiable safety trends already in place or 

expected to occur in the foreseeable future that are not accounted for in the study, since they 

were not in effect at the time that the vehicles in question were manufactured. For example, 

there are two important new safety standards that have already been issued and will be phasing in 

after MY 2008. FMVSS No. 126 (49 CFR § 571.126) requires electronic stability control in all 

new vehicles by MY 2012, and the upgrade to FMVSS 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. Additionally, we anticipate continued improvements in driver (and passenger) 

behavior, such as higher safety belt use rates. All of these may tend to reduce the absolute 

number of fatalities. On the other hand, as crash avoidance technology improves, future 

statistical analysis of historical data may be complicated by a lower number of crashes. In 

summary, the agencies have relied on the coefficients in the Kahane 2011 study for estimating 

the potential safety effects of the proposed CAFE and GHG standards for MYs 2017-2025, based 

on our assumptions regarding the amount of mass reduction that could be used to meet the 

standards in a cost-effective way without adversely affecting safety. Section E below discusses 

the methodology used by the agencies in more detail; while the results of the safety effects 

analysis are less significant than the results in the MY 2012-2016 final rule, the agencies still 

believe that any statistically significant results warrant careful consideration of the assumptions 

about appropriate levels of mass reduction on which to base future CAFE and GHG standards, 

and have acted accordingly in developing the proposed standards. 

4. How do the agencies think technological solutions might affect the safety estimates 

indicated by the statistical analysis? 

As mass reduction becomes a more important technology option for manufacturers in 

meeting future CAFE and GHG standards, manufacturers will invest more and more resources in 

developing increasingly lightweight vehicle designs that meet their needs for manufacturability 

and the public’s need for vehicles that are also safe, useful, affordable, and enjoyable to drive. 

There are many different ways to reduce mass, as discussed in Chapter 3 of this TSD and in 

Sections II, III, and IV of the preamble, and a considerable amount of information is available 

today on lightweight vehicle designs currently in production and that may be able to be put into 

production in the rulemaking timeframe. Discussion of lightweight material designs from 

NHTSA’s workshop is presented below. 

Besides “lightweighting” technologies themselves, though, there are a number of 

considerations when attempting to evaluate how future technological developments might affect 

the safety estimates indicated by the statistical analysis. As discussed in the first part of this 

chapter, for example, careful changes in design and/or materials used might mitigate some of the 

potential decrease in safety from mass reduction – through improved distribution of crash pulse 

energy, etc. – but these techniques can sometimes cause other problems, such as increased crash 

forces on vehicle occupants that have to be mitigated, or greater aggressivity against other 
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Page 23 of 32

vehicles in crashes. Manufacturers may develop new and better restraints – air bags, seat belts, 

etc. – to protect occupants in lighter vehicles in crashes, but NHTSA’s current safety standards 

for restraint systems are designed based on the current fleet, not the yet-unknown future fleet. 

The agency will need to monitor trends in the crash data to see whether changes to the safety 

standards (or new safety standards) become necessary. Manufacturers are also increasingly 

investigating a variety of crash avoidance technologies – ABS, electronic stability control (ESC), 

lane departure warnings, vehicle-to-vehicle (V2V) communications – that, as they become more 

prevalent in the fleet, are expected to reduce the number of overall crashes, and fatal, crashes. 

Until these technologies are present in the fleet in greater numbers, however, it will be difficult 

to assess whether they can mitigate the observed relationship between vehicle mass and safety in 

the historical data. 

Along with the California Air Resources Board (CARB), the agencies have initiated 

several projects to estimate the maximum potential for advanced materials and improved designs 

to reduce mass in the MY 2017-2021 timeframe, while continuing to meeting safety regulations 

and maintaining functionality of vehicles. Another NHTSA-sponsored study will estimate the 

effects of these design changes on overall fleet safety. 

A. NHTSA has awarded a contract to Electricore, with EDAG and George Washington 

University (GWU) as subcontractors, to study the maximum feasible amount of mass 

reduction for a mid-size car – specifically, a Honda Accord. The study tore down a MY 

2011 Honda Accord, studied each component and sub-system, and then redesigned each 

component and sub-system trying to maximize the amount of mass reduction with 

technologies that are considered feasible for 200,000 units per year production volume 

during the time frame of this rulemaking. Electricore and its sub-contractors are 

consulting industry leaders and experts for each component and sub-system when 

deciding which technologies are feasible. Electricore and its sub-contractors are also 

building detailed CAD/CAE/powertrain models to validate vehicle safety, stiffness, NVH, 

durability, drivability and powertrain performance. For OEM-supplied parts, a detailed 

cost model is being built based on a Technical Cost Modeling (TCM) approach developed 

by the Massachusetts Institute of Technology (MIT) Materials Systems Laboratory’s 

research26 to estimate the costs to OEMs for manufacturing parts. The cost will be broken 

down into each of the operations involved in the manufacturing; for example, for a sheet 

metal part, production costs will be estimated from the blanking of the steel coil to the 

final operation to fabricate the component. Total costs are then categorized into fixed 

cost, such as tooling, equipment, and facilities; and variable costs such as labor, material, 

energy, and maintenance. These costs will be assessed through an interactive process 

between the product designer, manufacturing engineers, and cost analysts. For OEMpurchased parts, the cost will be estimated by consultation with experienced cost analysts 

and Tier 1 system suppliers. This study will help to inform the agencies about the feasible 

 

26 Frank Field, Randolph Kirchain and Richard Roth, Process cost modeling: Strategic engineering and economic 

evaluation of materials technologies, JOM Journal of the Minerals, Metals and Materials Society, Volume 59, 

Number 10, 21-32. Available at http://msl.mit.edu/pubs/docs/Field_KirchainCM_StratEvalMatls.pdf (last accessed 

Aug. 22, 2011). 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review ***

Page 24 of 32

amount of mass reduction and the cost associated with it. NHTSA intends to have this 

study completed and peer reviewed before July 2012, in time for it to play an integral role 

in informing the final rule. 

B. EPA has awarded a similar contract to FEV, with EDAG and Monroe & Associates, Inc. 

as subcontractors, to study the maximum feasible amount of mass reduction for a mid-size 

CUV (cross over vehicle) specifically, a Toyota Venza. The study tears down a MY 2010 

vehicle, studies each component and sub-system, and then redesigns each component and 

sub-system trying to maximize the amount of mass reduction with technologies that are 

considered feasible for high volume production for a 2017 MY vehicle. FEV in 

coordination with EDAG is building detailed CAD/CAE/powertrain models to validate 

vehicle safety, stiffness, NVH, durability, drivability and powertrain performance to 

assess the safety of this new design. This study builds upon the low development (20% 

mass reduction) design in the 2010 Lotus Engineering study “An Assessment of Mass 

Reduction Opportunities for a 2017-2020 Model Year Vehicle Program”. This study 

builds upon the low development (20% mass reduction) design in the 2010 Lotus 

Engineering study “An Assessment of Mass Reduction Opportunities for a 2017-2020 

Model Year Vehicle Program”. This study will undergo a peer review. EPA intends to 

have this study completed and peer reviewed before July 2012, in time for it to play an 

integral role in informing the final rule. 

C. California Air Resources Board (CARB) has awarded a contract to Lotus Engineering, to 

study the maximum feasible amount of mass reduction for a mid-size CUV (cross over 

vehicle) specifically, a Toyota Venza. The study will concentrate on the Body-in-White 

and closures in the high development design (40% mass reduction) in the Lotus 

Engineering study cited above. The study will provide an updated design with crash 

simulation, detailed costing and manufacturing feasibility of these two systems for a 

MY2020 high volume production vehicle. This study will undergo a peer review. EPA 

intends to have this study completed and peer reviewed before July 2012, in time for it to 

play an integral role in informing the final rule. 

D. NHTSA has contracted with George Washington University (GWU) to build a fleet 

simulation model to study the impact and relationship of light-weight vehicle design and 

injuries and fatalities. This study will also include an evaluation of potential 

countermeasures to reduce any safety concerns associated with lightweight vehicles. 

NHTSA will include three light-weighted vehicle designs in this study: the one from 

Electricore/EDAG/GWU mentioned above, one from Lotus Engineering funded by 

California Air Resource Board for the second phase of the study, evaluating mass 

reduction levels around 35 percent of total vehicle mass, and two funded by EPA and the 

International Council on Clean Transportation (ICCT). This study will help to inform the 

agencies about the possible safety implications for light-weight vehicle designs and the 

appropriate counter-measures,27 if applicable, for these designs, as well as the feasible 

amounts of mass reduction. All of these analyses are expected to be finished and peerreviewed before July 2012, in time to inform the final rule. 

 

27 Countermeasures could potentially involve improved front end structure, knee bags, seat ramps, buckle

pretensioners, and others. 
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a. NHTSA workshop on vehicle mass, size and safety 

As stated above, in section C.2, on February 25, 2011, NHTSA hosted a workshop on 

mass reduction, vehicle size, and fleet safety at the Headquarters of the US Department of 

Transportation in Washington, DC. The purpose of the workshop was to provide the agencies 

with a broad understanding of current research in the field and provide stakeholders and the 

public with an opportunity to weigh in on this issue. The agencies also created a public docket 

to receive comments from interested parties that were unable to attend. The presentations were 

divided into two sessions that addressed the two expansive sets of issues. The first session 

explored statistical evidence of the roles of mass and size on safety, and is summarized in section 

C.2. The second session explored the engineering realities of structural crashworthiness, 

occupant injury and advanced vehicle design, and is summarized here. The speakers in the 

second session included Stephen Summers of NHTSA, Gregg Peterson of Lotus Engineering, 

Koichi Kamiji of Honda, John German of the International Council on Clean Transportation 

(ICCT), Scott Schmidt of the Alliance of Automobile Manufacturers, Guy Nusholtz of Chrysler, 

and Frank Field of the Massachusetts Institute of Technology. 

The second session explored what degree of weight reduction and occupant protection are 

feasible from technical, economic, and manufacturing perspectives. Field emphasized that 

technical feasibility alone does not constitute feasibility in the context of vehicle mass reduction. 

Sufficient material production capacity and viable manufacturing processes are essential to 

economic feasibility. Both Kamiji and German noted that both good materials and good designs 

will be necessary to reduce fatalities. For example, German cited the examples of hexagonally 

structured aluminum columns, such as used in the Honda Insight, that can improve crash 

absorption at lower mass, and of high-strength steel components that can both reduce weight and 

improve safety. Kamiji made the point that widespread mass reduction will reduce the kinetic 

energy of all crashes which should produce some beneficial effect. 

Summers described NHTSA’s plans for a model to estimate fleetwide safety effects 

based on an array of vehicle-to-vehicle computational crash simulations of current and 

anticipated vehicle designs. In particular, three computational models of lightweight vehicles are 

under development. They are based on current vehicles that have been modified to substantially 

reduce mass. The most ambitious was the “high development” derivative of a Toyota Venza 

developed by Lotus Engineering and discussed by Mr. Peterson. Its structure currently contains 

about 75% aluminum, 12% magnesium, 8% steel, and 5% advanced composites. Peterson 

expressed confidence that the design had the potential to meet federal safety standards. Nusholtz 

emphasized that computational crash simulations involving more advanced materials were less 

reliable than those involving traditional metals such as aluminum and steel. 

Nusholtz presented a revised data-based fleet safety model in which important vehicle 

parameters were modeled based on trends from current NCAP crash tests. For example, crash 

pulses and potential intrusion for a particular size vehicle were based on existing distributions. 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review ***

Page 26 of 32

Average occupant deceleration was used to estimate injury risk. Through a range of simulations 

of modified vehicle fleets, he was able to estimate the net effects of various design strategies for 

lighter weight vehicles, such as various scaling approaches for vehicle stiffness or intrusion. The 

approaches were selected based on engineering requirements for modified vehicles. Transition 

from the current fleet was considered. He concluded that protocols resulting in safer transitions 

(e.g., removing more mass from heavier vehicles with appropriate stiffness scaling according to a 

3/2 power law) were not generally consistent with those that provide the greatest reduction in 

GHG production. 

German discussed several important points on the future of mass reduction. Similar to 

Kahane’s discussion of the difficulties of isolating the impact of weight reduction, German stated 

that other important variables, such as vehicle design and compatibility factors, must be held 

constant in order for size or weight impacts to be quantified in statistical analyses. He presented 

results that, compared to driver, driving influences, and vehicle design influences, the safety 

impacts of size and weight are small and difficult to quantify. He noted that several scenarios, 

such as rollovers, greatly favored the occupants of smaller and lighter cars once a crash occurred. 

He pointed out that if size and design are maintained, lower weight should translate into a lower 

total crash force. He thought that advanced material designs have the potential to “decouple” the 

historical correlation between vehicle size and weight, and felt that effective design and driver 

attributes may start to dominate size and weight issues in future vehicle models. 

Other presenters noted industry’s perspective of the effect of incentivizing weight 

reduction. Field highlighted the complexity of institutional changes that may be necessitated by 

weight reduction, including redesign of material and component supply chains and 

manufacturing infrastructure. Schmidt described an industry perspective on the complicated 

decisions that must be made in the face of regulatory change, such as evaluating goals, gains, and 

timing. 

Field and Schmidt noted that the introduction of technical innovations is generally an 

innate development process involving both tactical and strategic considerations that balance 

desired vehicle attributes with economic and technical risk. In the absence of challenging 

regulatory requirements, a substantial technology change is often implemented in stages, starting 

with lower volume pilot production before a commitment is made to the infrastructure and 

supply chain modifications necessary for inclusion on a high-volume production model. Joining, 

damage characterization, durability, repair, and significant uncertainty in final component costs 

are also concerns. Thus, for example, the widespread implementation of high-volume composite 

or magnesium structures might be problematic in the short or medium term when compared to 

relatively transparent aluminum or high strength steel implementations. Regulatory changes will 

affect how these tradeoffs are made and these risks are managed. 

Koichi Kamiji presented data showing in increased use of high strength steel in their 

Honda product line to reduced vehicle mass and increase vehicle safety. He stated that mass 
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Page 27 of 32

reduction is clearly a benefit in 42% of all fatal crashes because absolute energy is reduced. He 

followed up with slides showing the application of certain optimized it designs can improve 

safety even when controlling for weight and size. 

A philosophical theme developed that explored the ethics of consciously allowing the 

total societal harm associated with mass reduction to approach the anticipated benefits of 

enhanced safety technologies. Although some participants agreed that there may eventually be 

specific fatalities that would not have occurred without downsizing, many also agreed that safety 

strategies will have to be adapted to the reality created by consumer choices, and that “We will 

be ok if we let data on what works – not wishful thinking – guide our strategies.” 

5. How have the agencies estimated safety effects for the proposed standards? 

a. What was the agencies’ methodology for estimating safety effects for the 

proposed standards? 

As explained above, the agencies consider the 2011 statistical analysis of historical crash 

data by NHTSA to represent the best estimates of the potential relationship between mass 

reduction and fatality increases in the future fleet. This section discusses how the agencies used 

NHTSA’s 2011 analysis to calculate specific estimates of safety effects of the proposed 

standards, based on the analysis of how much mass reduction manufacturers might use to meet 

the proposed standards. 

Neither the proposed CAFE/GHG standards nor the agencies’ analysis mandates mass 

reduction, or mandates that mass reduction occur in any specific manner. However, mass 

reduction is one of the technology applications available to the manufacturers and a degree of 

mass reduction is used by both agencies’ models to determine the capabilities of manufacturers 

and to predict both cost and fuel consumption/emissions impacts of improved CAFE/GHG 

standards. We note that the amount of mass reduction selected for this rulemaking is based on 

our assumptions about how much is technologically feasible without compromising safety. 

While we are confident that manufacturers will build safe vehicles, we cannot predict with 

certainty that they will choose to reduce mass in exactly the ways that the agencies have 

analyzed in response to the standards. In the event that manufacturers ultimately choose to 

reduce mass and/or footprint in ways not analyzed or anticipated by the agencies, the safety 

effects of the rulemaking may likely differ from the agencies’ estimates. 

NHTSA utilized the 2011 Kahane study relationships between weight and safety, 

expressed as percent changes in fatalities per 100-pound weight reduction while holding 

footprint constant. However, as mentioned previously, there are several identifiable safety trends 

already occurring, or expected to occur in the foreseeable future, that are not accounted for in the 

study. For example, the two important new safety standards that were discussed above for 

electronic stability control and head curtain airbags, have already been issued and began phasing 
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Page 28 of 32

in after MY 2008. The recent shifts in market shares from pickups and SUVs to cars and CUVs 

may continue, or accelerate, if gasoline prices remain high, or rise further. The growth in vehicle 

miles travelled may continue to stagnate if the economy does not improve, or gasoline prices 

remain high. And improvements in driver (and passenger) behavior, such as higher safety belt 

use rates, may continue. All of these will tend to reduce the absolute number of fatalities in the 

future. The agency estimated the overall change in fatalities by calendar year after adjusting for 

ESC, Side Impact Protection, and other Federal safety standards and behavioral changes 

projected through this time period. The smaller percent changes in risk from mass reduction 

(from the 2011 NHTSA analysis), coupled with the reduced number of baseline fatalities, results 

in smaller absolute increases in fatalities than those predicted in the 2010 rulemaking. 

NHTSA examined the impacts of identifiable safety trends over the lifetime of the 

vehicles produced in each model year. An estimate of these impacts was contained in a previous 

agency report.28 The impacts were estimated on a year-by-year basis, but could be examined in a 

combined fashion. Using this method, we estimate a 12.6 percent reduction in fatality levels 

between 2007 and 2020 for the combination of safety standards and behavioral changes 

anticipated (ESC, head-curtain air bags, and increased belt use). Since the same safety standards 

are taking effect in the same years, the estimates derived from applying NHTSA fatality 

percentages to a baseline of 2007 fatalities were thus multiplied by 0.874 to account for changes 

that NHTSA believes will take place in passenger car and light truck safety between the 2007 

baseline on-road fleet used for this particular safety analysis and year 2025. 

To estimate the amount of mass reduction to apply in the rulemaking analysis, the 

agencies considered fleet safety effects for mass reduction. As previously discussed and shown 

in Error! Reference source not found., the Kahane 2011 study shows that applying mass 

reduction to CUVs and light duty trucks will generally decrease societal fatalities, while applying 

mass reduction to passenger cars will increase fatalities. The CAFE model uses coefficients 

from the Kahane study along with the mass reduction level applied to each vehicle model to 

project societal fatality effects in each model year. NHTSA used the CAFE model and 

conducted iterative modeling runs varying the maximum amount of mass reduction applied to 

each subclass in order to identify a combination that achieved a high level of overall fleet mass 

reduction while not adversely affecting overall fleet safety. These maximum levels of mass 

reduction for each subclass were then used in the CAFE model for the rulemaking analysis. The 

agencies believe that mass reduction of up to 20 percent is feasible on light trucks, CUVs and 

 

ls technologies, JOM Journal of the Minerals, Metals and Materials Society, Volume 59, Number 10, 21-32. 

Available at http://msl.mit.edu/pubs/docs/Field_KirchainCM_StratEvalMatls.pdf (last accessed Aug. 22, 2011). 

28 Countermeasures could potentially involve improved front end structure, knee bags, seat ramps, buckle

pretensioners, and others. 

28 Blincoe, L. and Shankar, U, “The Impact of Safety Standards and Behavioral Trends on Motor Vehicle Fatality 

Rates,” DOT HS 810 777, January 2007. See Table 4 comparing 2020 to 2007 (37,906/43,363 = 12.6% reduction 

(1-.126 = .874). Since 2008 was a recession year, it does not seem appropriate to use that as a baseline. We believe 

this same ratio should hold for this analysis which should compare 2025 to 2008. Thus, we are inclined to continue 

to use the same ratio. 
*** E.O. 12866 Review – Draft – Do Not Cite, Quote, or Release During Review ***

Page 29 of 32

minivans, 29 but that less mass reduction should be implemented on other vehicle types to avoid 

increases in societal fatalities.. For this proposal, NHTSA used the mass reduction levels shown 

in Table Error! No text of specified style in document.-4. 

Table Error! No text of specified style in document.-15 Mass Reduction Levels Applied 

in CAFE Model 

Absolute

% 

Subcompact 

and 

Subcompact 

Perf. PC

Compact 

and 

Compact 

Perf. PC

Midsize PC 

and Midsize 

Perf. PC

Large PC 

and Large 

Perf. PC

Minivan LT

Small, 

Midsize and 

Large LT

MR1* 0.0% 2.0% 1.5% 1.5% 1.5% 1.5%

MR2 0.0% 0.0% 5.0% 7.5% 7.5% 7.5%

MR3 0.0% 0.0% 0.0% 10.0% 10.0% 10.0%

MR4 0.0% 0.0% 0.0% 0.0% 15.0% 15.0%

MR5 0.0% 0.0% 0.0% 0.0% 20.0% 20.0%

Notes:

*MR1-MR5: different levels of mass reduction used in CAFE model 

After applying the mass reduction levels in the CAFE model, Table Error! No text of 

specified style in document.-5 shows the results of NHTSA’s safety analysis separately for each 

model year.30 These are estimated increases or decreases in fatalities over the lifetime of the 

model year fleet. A positive number means that fatalities are projected to increase, a negative 

number (indicated by parentheses) means that fatalities are projected to decrease. The results are 

significantly affected by the assumptions put into the Volpe model to take more weight out of the 

heavy LTVs, CUVs, and minivans than out of other vehicles. As the negative coefficients only 

appear for LTVs greater than 4,594 lbs., CUVs, and minivans, a statistically improvement in 

safety can only occur if more weight is taken out of these vehicles than passenger cars or smaller 

light trucks. Combining passenger car and light truck safety estimates for the proposed standards 

results in an increase in fatalities over the lifetime of the nine model years of MY 2017-2025 of 4 

fatalities, broken up into an increase of 61 fatalities in passenger cars and 56 decrease in fatalities 

in light trucks. NHTSA also analyzed the results for different regulatory alternatives in Chapter 

IX of its PRIA; the difference in the results by alternative depends upon how much weight 

reduction is used in that alternative and the types and sizes of vehicles that the weight reduction 

applies to. 

 

29 When applying mass reduction, NHSTA capped the maximum amount of mass reduction to 20 percent 

for any individual vehicle class. The 20 percent cap is the maximum amount of mass reduction the agencies believe 

to be feasible in MYs 2017-2025 time frame. 

30 NHTSA has changed the definitions of a passenger car and light truck for fuel economy purposes between the 

time of the Kahane 2003 analysis and this final rule. About 1.4 million 2 wheel drive SUVs have been redefined as 

passenger cars instead of light trucks. The Kahane 2011 analysis continues with the definitions used in the Kahane 

2003 analysis. Thus, there are different definitions between Tables IX-1 and IX-2 (which use the old definitions) 

and Table IX-3 (which uses the new definitions). 
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Page 30 of 32

Table Error! No text of specified style in document.-5 NHTSA Calculated MassSafety-Related Fatality Impacts of the Proposed Standards over the Lifetime of the 

Vehicles Produced in each Model Year 

MY 

2017

MY 

2018

MY 

2019

MY 

2020

MY 

2021

MY 

2022

MY 

2023

MY 

2024

MY 

2025 Total

Passenger 

cars 

(2) (0) 11 16 15 13 12 1 (2) 63

Light 

trucks 3 (7) 0 (13) (8) (15) (12) (8) 0 (56)

Total 1 (8) 12 3 7 (2) 0 (8) (2) 4

Using the same coefficients from the 2011 Kahane study, EPA used the OMEGA model 

to conduct a similar analysis. After applying these percentage increases to the estimated weight 

reductions per vehicle size by model year assumed in the Omega model, Table Error! No text of 

specified style in document.-6 shows the results of EPA’s safety analysis separately for each 

model year. These are estimated increases or decreases in fatalities over the lifetime of the 

model year fleet. A positive number means that fatalities are projected to increase; a negative 

number means that fatalities are projected to decrease. For details, see the EPA RIA Chapter 3. 

Table Error! No text of specified style in document.-6 EPA Calculated Mass-SafetyRelated Fatality Impacts of the Proposed Standards over the Lifetime of the Vehicles 

Produced in each Model Year 

MY 

2017

MY 

2018

MY 

2019

MY 

2020

MY 

2021

MY 

2022

MY 

2023

MY 

2024

MY 

2025 Total

Passenger 

cars 

-3 -5 -8 -11 -14 8 32 58 86 143

Light 

trucks -1 -1 -2 -2 -4 -35 -67 -99 -133 -343

Total -3 -7 -10 -13 -18 -27 -34 -41 -47 -201

b. Why might the real-world effects be less than or greater than what the agencies 

have calculated? 

As discussed above the ways in which future technological advances could potentially 

mitigate the safety effects estimated for this rulemaking: lightweight vehicles could be designed 

to be both stronger and not more aggressive; restraint systems could be improved to deal with 

higher crash pulses in lighter vehicles; crash avoidance technologies could reduce the number of 

overall crashes; roofs could be strengthened to improve safety in rollovers. As also stated above, 

however, while we are confident that manufacturers will strive to build safe vehicles, it will be 
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Page 31 of 32

difficult for both the agencies and the industry to know with certainty ahead of time how crash 

trends will change in the future fleet as lightweighted vehicles become more prevalent. We will 

have to continue to monitor the crash data going forward. 

Additionally, we note that the total amount of mass reduction used in the agencies’ 

analysis for this rulemaking were chosen based on our assumptions about how much is 

technologically feasible without compromising safety. Again, while we are confident that 

manufacturers are motivated to build safe vehicles, we cannot predict with certainty that they 

will choose to reduce mass in exactly the ways that the agencies have analyzed in response to the 

standards. In the event that manufacturers ultimately choose to reduce mass and/or footprint in 

ways not analyzed by the agencies, the safety effects of the rulemaking may likely differ from 

the agencies’ estimates. 

The agencies acknowledge it is possible that a manufacturer could theoretically “game” a 

footprint-based standard by redesigning a vehicle to increase wheelbase and/or track width. 

However, as NHTSA explained in promulgating MY 2008-2011 light truck CAFE standards and 

MY 2011 passenger car and light truck CAFE standards, and as the agencies jointly explained in 

promulgating MY 2012-2016 CAFE and GHG standards, the agencies believe that such 

engineering changes are significant enough to be unattractive as a measure to undertake solely to 

reduce compliance burdens. Similarly, the agencies acknowledge that a manufacturer could, 

without actually reengineering specific vehicles to increase footprint, shift production toward 

those that perform well compared to their respective footprint-based targets. However, as 

NHTSA and, more recently NHTSA and EPA have previously explained, because such 

production shifts would run counter to market demands, they would also be competitively 

unattractive. Based on this regulatory design, the analysis assumes this proposal will not have 

either of the effects described above. 

As discussed in Chapter 2 of the Draft Joint TSD, the agencies note that the standard is 

flat for vehicles smaller than 41 square feet and that downsizing in this category could help 

achieve overall compliance, if the vehicles are desirable to consumers. The agencies note that 

only xxxx percent of MY2008 passenger cars were below 41 square feet, and due to the overall 

lower level of utility of these vehicles, and the engineering challenges involved in ensuring that 

these vehicles meet all applicable federal motor vehicle safety standards (FMVSS), we expect a 

significant increase in this segment of the market in the future is unlikely. Please see Chapter 2 

of the Draft Joint TSD for additional discussion. 

We seek comment on the appropriateness of the overall analytic assumption that the 

attribute-based aspect of the proposed standards will have no effect on the overall distribution of 

vehicle footprints. Notwithstanding the agencies’ current judgment that such deliberate 

reengineering or production shift are unlikely as pure compliance strategies, both agencies are 

considering the potential future application of vehicle choice models, and anticipate that doing so 

could result in estimates that market shifts induced by changes in vehicle prices and fuel 
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economy levels could lead to changes in fleet’s footprint distribution. However, neither agency 

is currently able to include vehicle choice modeling in our analysis. 

c. Do the agencies plan to make any changes in these estimates for the final rule? 

As discussed above, the agencies have based our estimates of safety effects due to the 

proposed standards on Kahane’s 2011 report. That report is currently undergoing peer review 

and is docketed for public review;31 the peer review comments and response to peer review 

comments, along with any revisions to the report in response to that review, will also be 

docketed there. Depending on the results of the peer review, our calculation of safety effects for 

the final rule will also be revised accordingly. The agencies will also consider any comments 

received on the proposed rule, and determine at that time whether and how our estimates should 

be changed in response to those comments. Additional studies published by the agencies or 

other independent researchers as previously discussed will also be considered, along with any 

other relevant information. 

 

31 Kahane, C. J. (2011). “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 2000-2007 

Passenger Cars and LTVs,”, July 2011. The report is available in the NHTSA docket, NHTSA-2010-0152. You can 

access the docket at http://www.regulations.gov/#!home by typing ‘NHTSA-2010-0152’ where it says “enter 

keyword or ID” and then clicking on “Search.”