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May 9, 2011

Size Matters


Electric vehicle (EV) and clean energy advocates rejoice! The first mainstream EVs to hit dealerships across the U.S., the Chevrolet Volt and Nissan Leaf, both received top safety ratings from the Insurance Institute for Highway Safety. As the Associated Press reported:

“While both the Leaf and Volt are classified as small cars, the institute said their heavy battery packs put their weight closer to large sedans. The Volt, for example, weighs 3,760 pounds, which is close to the weight of the Chevrolet Impala. The Leaf weighs 3,370 pounds, which is similar to a Nissan Altima midsize car. That extra mass helps protect their occupants, since heavier cars are less likely to be pushed around in a crash.” [emphasis added]

But there’s a problem with this last thought: weight is conflated with size.

The perception that weight is necessary for vehicle safety initially stemmed from studies conducted over the past several decades by the National Highway Traffic Safety Administration (NHTSA) that determined making autos 100 pounds lighter would kill an extra 400–1,300 Americans a year.(i) This, combined with consumer demand for SUVs, created a kind of “mass arms race” where, in the name of safety, you drive an Expedition, your neighbor drives a Hummer, and the guy down the street drives a tank.

But the NHSTA studies made a critical mistake. They, like many consumers, assumed weight and size meant the same thing. Peer-reviewed analysis of the same data showed that making autos 100 pounds lighter would actually save 1,500 lives. What increases crash safety is not weight, but size: larger vehicles have more crush space to absorb impacts.(ii)

However, larger doesn’t have to mean heavier. Strong but lightweight materials such as aluminum (able to absorb about twice as much crash energy per pound than steel) or carbon fiber (only 1/3 the density of steel but able to absorb up to six times more crash energy than aluminum) enhance safety and reduce vehicle weight.

But why should we make cars lighter in the first place? For starters, lighter vehicles allow auto manufacturers to use smaller battery packs, potentially decreasing the total cost of EVs. As explained in RMI’s forthcoming book, Reinventing Fire, designing a sufficiently sized car with lightweight but strong materials and a downsized propulsion system could be a game changer for the automotive industry.

We should applaud the safety ratings received by these vehicles given their importance to the future of EVs. But, the long held misconception that weight = safety has encouraged decades of inefficient car design. Lets not let this trend continue as we transition to the next generation of vehicles. Lightweighting is a key enabler for more efficient, affordable cars that run on cleaner and cheaper fuels. In pursuit of safety, electric vehicles shouldn’t pack in more batteries to make a car heavier. The reality is that stronger, lightweight materials incorporated into similarly sized cars can lead to both enhanced safety and better fuel economy. 

RMI last wrote about lightweighting as a key to overcoming barriers to electric vehicle adoption in GreenBiz.

(i)Kahane, C.J. “Vehicle Weight, Fatality Risk and Crash Compatibility of Model Year 1991–99 Passengers Cars and Light Trucks.” DOE HS 809 662. Washington, DC: National Highway Traffic Safety Administration. October, 2003.


(ii)Van Auken, R.M. and J.W. Zellner. “A Further Assessment of the Effects of Vehicle Weight and Size Parameters on Fatality Risk in Model Year 1985–98 Passenger Cars and 1985–97 Light Trucks.” DRI-TR-03-01. Torrance: Dynamic Research, Inc. January, 2003.

Join the Discussion

Showing 1-2 of 2 comments

June 30, 2011

my bicycle is a lot safer than any SUV. I can't run into you and kill you like 40,000 people do every year. I won't back over my pets or children becauase I can't see out the back of my oversized vehicle.
I also don't use an many resources llike 50% imported OIL and don't make any pollution.

In fact when you add up what I normally eat and drink I my bicycle runs on perpetual energy, I eat less, am healthier and get faster the more I use it. My bicycle costs a lot less to iown and operate too.

Anonymous User

November 17, 2011

Weight matters certainly, but the winds in places like Chicago and Garden of the Gods , Colorado can and do impose hazards. For example, a VW bug was blown right off the road in The Garden of the Gods. Another VW was blown from lane to lane by a cross wind in Chicago with no steering wheel or front wheel movement right or left, with an airline captain driving the vehicle! The solution to the problem has to be aerodynamics combined with weight distribution. Windtunnel tests, followed by real driving tests under the worst possible conditions, including ice on the road, must be done with utmost thoroughness. Design of the bottom of the vehicle must be done with wind effects in mind, then tested extensively.