Safety in Lighter Cars

Are Light Cars Safe?

The mass of a vehicle plays an important role in determining its fuel efficiency. In general, a lighter vehicle is more fuel-efficient than a heavier one this efficiency has well-known advantages – it will consume less fuel and emit lower amounts of pollution. The energy efficiency of automobiles sold in the USA is governed by federally mandated Corporate Average Fuel Economy (‘CAFE’). While these CAFE regulations do not mandate the fuel efficiency of a specific vehicle or model, they set the requirement for the average fuel efficiency of all the vehicles produced by a manufacturer (designated as the ‘corporate fuel efficiency average’). These standards have increased over the years. The most recent 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 would have represented a big jump from today’s Corporate Average Fuel Economy numbers. However, this proposed rule has recently been set aside by the US government (vehicles sold in California have a more stringent set of rules) and US automobiles continue to be governed by the previous requirements for energy efficiency.

But the need to continually reduce the fuel consumption of cars will continue under the present circumstances. Since the preference of customers is of maintaining the usable space inside their vehicles, the engineering challenge is to use all available technologies as well as to make automobiles as light as possible by optimizing their designs and using ‘alternate materials’ such as aluminum, magnesium and ultra-high strength steel in their construction. Thus, the US fleet will continue to have vehicles of widely varying masses in the future and this variation will become even more pronounced as newer cars are made lighter while the older, heavier versions are still in use. This leads to the question – are lighter vehicles less protective of their occupants when involved in crashes? Actual crashes on roads vary widely in terms of severity, type of crash, occupants’ sizes, restraint systems, etc. and it is necessary that all these factors be taken into account in answering these questions.

There have been several well-planned studies in the past for generating realistic estimates of mass-reduction effects on traffic safety.  Two of the more recent studies [1]were based on developing highly detailed designs of passenger vehicles by utilizing Finite Element Analysis (‘FEA’) methodology and then using computer simulation of various crash scenarios to analyze their occupants’ protection. 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 was one of the peer reviewers)The full report of the NHTSA study and the reviews[2]are available from the Department of Transportation. The EPA-sponsored study has its summary available as a technical paper[3]. 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 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 $2228 or $4.36/kg) while maintaining safety and performance.

 Other researchers have also concluded that “lighter weight” doesn’t mean “less safe”. A statistical analysis of accident data[4] 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.’ Many of these estimates of a vehicles’ safety are based on its performance in high-speed crash tests conducted by the NHTSA and the IIHS. However, most crashes and injuries happen at much lower speeds [5]. Other NHTSA-sponsored research[6] has addressed this by creating ‘fleet based models’ of vehicle population and generating statistical estimates of the safety of future designs (I was a peer reviewer for this research as well).

It should also be noted that ‘automotive safety’ has a broader meaning than just protecting its occupant(s) in crashes and can include multiple factors e.g. –‘societal harm’ (the total number of injuries and fatalities nationally per year), safety of pedestrians (concentrated among young children and older adults) and bicyclists, as well as the long-term effects of emissions on overall health in the society [7]. Since automobile tailpipe emissions are a significant component of air quality which affects everyone, it is logical that these emissions be included in the evaluation of overall automotive safety. When all these factors are taken into account, the effect of automobiles’ mass reduction appear to be positive.

Of course, all these studies are projections and are based on certain assumptions. The actual assessment of automobiles’ mass reductions can only be made after such vehicles have been in use for a number of years and after large amounts of accident and societal data have been collected. The research to date shows that “reduced-mass vehicles” do not necessarily equate to “less safe vehicles” and that additional technologies exist to achieve even higher levels of occupants’ safety for all sizes and types of automobiles.

[1] “Agency Sponsored Mass Reduction Studies” by Bolon and Zhao (SAE 2015)

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

[3] “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.

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

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

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

[7] “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