CAFE Rules, Lightweight Vehicles & Questions of Safety

TASA ID: 3190

Recently adopted fuel efficiency rules in the USA require that passenger vehicles must achieve an average of 54.5 miles per gallon by the year 2025. This “average” is further detailed into different goals for different sizes and class of vehicles and does not mean that all automobiles have to achieve the average 54.5 mpg. However, it still represents a big jump from today’s Corporate Average Fuel Economy (CAFE) numbers, especially because the phased march towards this goal must begin with the 2017 vehicles. This significant challenge for the automakers has led to the study of many technologies for increasing automobiles’ fuel efficiency, and one of the promising ones is that of reducing a vehicle’s mass while maintaining its usable space. Such mass reduction will require that vehicle designs be optimized for efficiency and that they use more “alternate materials” such as aluminum, magnesium and ultra-high strength steel.

This outlook of future vehicles being of relatively lighter mass raises several questions, including the often-repeated one – Will such mass reduction lead to reduced safety of the occupants in crashes? This question also applies to current fleet because the passenger vehicles on the road today range widely in size and mass, from the smallest mini-compact to full-size SUVs and pickup trucks. In addition, automakers are continuously reducing their vehicle’s mass with each design cycle. The question as applied to current vehicles then becomes – Are lighter vehicles less safe than heavier vehicles?

One prevailing opinion regarding the effect of mass reduction on the safety of vehicle’s occupants holds that a reduced-mass vehicle will provide less safety for its occupants in crashes. The example cited to support this opinion is that of a crash between two vehicles of unequal mass (but; otherwise, identical) along with the statement that occupants in lighter vehicles will likely have more injuries than those in the heavier ones. However, the reality is that actual crashes cover a wide range in terms of their severity, crash mode, struck objects, safety system use, occupant size, etc.  When this complexity is taken into account, the effect of mass reduction on automotive safety becomes more difficult to project and a realistic estimate of future vehicle safety requires that all the relevant and important factors be taken into account. Some of these factors that must be considered in comparing present- to future-vehicles are (a) the ability of the vehicle to avoid a crash, (b) the parameters of the crash (severity, direction, struck object, etc.), and (c) the vehicle’s design for occupant protection (structure, restraint systems, etc.).

In order to obtain such realistic estimates of mass-reduction effect, several well-planned studies have involved the design of reduced-mass vehicles by using computer-aided-engineering (CAE), followed by the evaluation of the crash performance of these virtual vehicles in several simulated crashes. Since it is possible to achieve varying levels of mass reduction by proper choice of the amounts of the alternate materials, a meaningful comparison between a current vehicle and its mass-reduced future version requires that some criterion be imposed to restrict the amount of alternate materials. The criterion used in the two undermentioned studies is that these near-future vehicles should remain “affordable.” These studies were sponsored by (a) the US National Highway Traffic Safety Administration (NHTSA) and (b) the Environmental Protection Agency (EPA) and underwent peer reviews (I served as one of the peer reviewers). The full report of the NHTSA study is available from the Department of Transportation as are the reviews. The EPA-sponsored study and its peer reviews are not yet published but a summary is available as a technical paper.[iii]

Both of the aforementioned studies utilized finite element analysis (FEA) methodology to develop highly-detailed designs of passenger vehicles that meet the constraints of cost and size.  The NHTSA study used a 2010 midsize passenger car as “base” and redesigned it to achieve the specified reduction in mass. The report showed the feasibility of 22% reduction in vehicle mass within the constraint of 10% increase in direct manufacturing cost. This study concluded that the reduced-mass vehicle would be able to maintain the same safety ratings in NHTSA and IIHS crash tests as the current (2010) model if safety-relevant components, e.g., airbags, sensors, seatbelts are properly designed as an integrated system with the rest of the vehicle.

The EPA-sponsored study applied similar technique to the redesign of a full-size pickup truck for the 2020-2025 timeframe. The future design was required to maintain the current level of safety and truck-relevant functionalities, e.g., closure performance, durability, vehicle handling, as well as meet the cost constraints. The study showed that such lightweight designs are feasible with various levels of mass reduction achieved by using alternate materials to different degrees. It was demonstrated that mass reductions of up to 9% are possible without any cost increase while greater reductions in mass can be achieved at higher costs, e.g., 511 kg reduction in mass at cost increase of $2,228.00 or $4.36/kg) while maintaining safety and performance.

Other researchers have also concluded that “lighter mass” doesn’t mean “less safe.” A statistical analysis of accident data found that the estimated effects of passenger car weight reduction “on fatalities per accident are small (not statistically significant) or to decrease fatalities.” The Oak Ridge National Laboratory Review (volume 41, no.3, 2008) similarly called it a myth that “lighter cars are less safe than heavier vehicles” and stated that “new materials can make cars lighter and as safe as heavier vehicles.”

Of course, any estimation of future trends is subject to and limited by the assumptions made in that study. One of such assumptions usually made in the estimating the safety of automobiles is that this parameter is represented by the vehicle’s performance in the high-speed crash tests conducted by NHTSA and IIHS. However, detailed examinations of US crash statistics show that most crashes and injuries happen at speeds far lower than the speed in the aforementioned tests. Recent NHTSA-sponsored research has tried to address this by creating “fleet based models” of vehicle safety. This research proposed statistical estimation of the safety of future designs by using CAE simulations (as stated above) combined with statistical approximations. But this research is in its infancy, needing much more work before these concepts can be considered practical (I was a peer reviewer for this research as well).

Additionally, the term “vehicle safety” may mean different things to different segments of society, ranging from total societal harm (total number of injuries and fatalities nationally per year), to a driver’s definition of personal safety, to the socially important safety of pedestrians (concentrated among young children and older adults). Another point to consider is that the long-term effects of the environment on our health and longevity are becoming better understood and quantified. Since automobile tailpipe emissions are a significant component of air quality, it is logical to ask if the effects of an automobile’s emissions should be included in the evaluation of its overall safety. Such additions of immediately perceptible variables (speed, direction, structure, airbags, post-crash rescue, etc.) to the longer-term variables (environmental effect on health) may lead to different conclusions regarding safety of future designs.

The actual assessment of any vehicle’s safety can only be made after such vehicles have been in use for a number of years and after large amounts of accident data have been collected. Although such data for 2017-2025 vehicles will not be available for many years, the research shows that “reduced mass vehicles” do not necessarily equate to “less safe vehicles” and that technologies exist to achieve high levels of occupants’ safety for all sizes and types of passenger vehicles. Of course, when evaluating a vehicle’s safety in a specific event, it is necessary to only consider the specifics of that vehicle’s design and of the performance of its components under the given operating (or accident) parameters.


This article discusses issues of general interest and does not give any specific legal or business advice pertaining to any specific circumstances.  Before acting upon any of its information, you should obtain appropriate advice from a lawyer or other qualified professional.

This article may not be duplicated, altered, distributed, saved, incorporated into another document or website, or otherwise modified without the permission of TASA.

“Agency Sponsored Mass Reduction Studies” by Bolon and Zhao, SAE 2015.

“Peer Review for Mass Reduction for Light Duty Vehicles for Model years 2017-2025, US Department of Transportation, July 26, 2012.

Cost-Effectiveness of a Lightweight Design for 2020-2025: An Assessment of a Light-Duty Pickup Truck” by Caffrey et al, SAE 2015-01-0559.

“Updated Effects of the Analysis of Passenger Vehicle Weight and Size on Safety,” Report DRI-TR-13-04, May 2013, Dynamic Research Inc.

“Preliminary Evaluation of Pre-crash Safety System Effectiveness,” by Verma & Goertz, SAE paper 2010-01-1042.

“Methodology for Evaluating Fleet Protection of New Vehicle Designs,” Report DOT HS 812 051, August 2014.

“Life cycle air quality impacts of conventional and alternative light-duty transportation in the United States,” by Tessum et al., Proceedings of National Academy of Sciences, December 2014.

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