Lightweight, fuel-efficient cars not necessarily less safe

Rocky Mountain Institute (RMI) has been advocating the entry of fuel-efficient cars into the market for years, for security and economic reasons as well as environmental. On May 19, 2009, President Obama announced a "historic agreement to help America break its addiction to oil." The centerpiece of that announcement was a new 35.5-mpg CAFE standard for 2016. This is a pivotal time to envision innovative transportation solutions as we work to rebuild the domestic auto industry.

One of the barriers to American adoption of fuel-efficient vehicles is the common-yet-misleading line of logic that fuel efficiency equates to smaller, less safe cars.

This misperception is often fed by the media headlines, including an April 14, 2009, New York Times story, "Study Says Small-Car Buyers Sacrifice Safety for Economy" and a May 22, 2009, Wall Street Journal piece, titled "Light Cars Are Dangerous Cars."

Yet the relationship is not that simple. What is left out of this "quick-and-easy" debate is the role of engineering design. Sure, based purely upon a typical vehicle's design, geometry, and occupant position, it's true that larger -- but not necessarily heavier -- vehicles can offer considerable safety advantages to passengers within.

Rocky Mountain Institute's survey of existing studies indicates that lightweight vehicles can be as safe, or even safer than, the cars on the road today through the use of stronger, lighter materials when engineering design is focused on crash safety. Additionally, lightweight cars have great advantages when it comes to fuel-efficiency.

Designing for passive safety on par with current NHTSA five-star ratings demands not only using lightweight materials, but also new vehicle geometries and components that can act as energy-absorbing crumple zones. Indeed, a lighter vehicle can achieve the performance of a conventionally designed vehicle, but because it can do so with a smaller engine, there is more room in the engine compartment for crush space and, ultimately, a better crumple zone design.

There is another aspect of safety to consider: the safety of the people outside the car. A lightweight vehicle will be less aggressive (less likely to injure the occupants of another vehicle, bicyclists, or pedestrians during a collision). For objects moving at a given speed, less mass reduces the energy that an object brings into a collision. Thus, a lightweight fleet (traveling at the same speeds) can statistically reduce the overall number of traffic deaths compared to a conventional (heavy) fleet.

RMI continues to encourage manufacturers to design and build efficient, lightweight cars -- and to support legislation that helps focus our automakers on priorities that benefit us as a society.[1] We hate to see the benefit that comes from fuel efficiency thwarted by misconceptions about safety trade-offs. Both fuel efficiency and safety can be achieved, and both are good for the health of the country in more ways than just safety on the road.

RMI continues to do more research on this topic and is seeking partners to work with. Preliminary results of a recent RMI study are expected to be released in July. For additional information, please visit www.rmi.org.

The authors are all employees of Rocky Mountain Institute, where Mike Simpson is a transportation analyst, Kristine Chan-Lizardo is Interim Director of the Mobility and Vehicle Efficiency Team (MOVE), Cory Lowe is a public relations manager, and Cameron M. Burns is Senior Editor.

[1] As part of our research on vehicle lightweighting, RMI reviewed all the existing studies and commentary about the relationship between size, safety, and weight, including reports published by the National Academy of Sciences and the National Highway Traffic and Safety Administration, and the Insurance Institute for Highway Safety (IIHS). Many reports link size and weight as being responsible for safety reductions, while the most recent studies separate the two, and find size alone to blame.

In a January 2007 article, IIHS stated "a way to improve fuel economy and maintain vehicle crashworthiness is to use lighter materials that reduce vehicle weight but not size." IIHS also noted "... some weight reduction, especially among very heavy vehicles, could improve total safety by lowering the risk to other people on the road."

The fact is that today, there are not many examples of vehicles that have been designed to be both lightweight and large. This is changing as overseas automakers, and hopefully our domestic automakers as well, explore designs and materials that are both fuel-efficient and safe.