I recently spoke with Tom Wenzel, a Research Scientist at the Lawrence Berkeley National Laboratory. Mr. Wenzel works in transportation energy and environmental policy analysis and has written papers on this very subject. According to Mr. Wenzel, the analysis used by Max Schultz is wrong, “We have come to the conclusion that the claim that lighter vehicles are inherently dangerous to those riding in them is flawed.”

There is little, if any, trade-off between improvements in fuel economy and in safety in light-duty motor vehicles, particularly if priority is given to both of these goals. A reduction in vehicle mass is an important technique for improving fuel economy, but certainly not the only one.

In crashes with a stationary object, frontal crush space, rather than mass, provides protection to the vehicle occupants. Mass offers protection in head-on car-car, and perhaps truck-truck, crashes when there is a substantial difference in mass between the two vehicles; however, a substantial majority of casualties in motor vehicle crashes are unrelated to the masses of the vehicles involved.

Lighter cars aren't necessarily more dangerous. A lighter car is more at risk when it collides with a heavier automobile. But what is the cause and effect? Are lighter cars more dangerous, or is it the fact that these cars have to share the road with enormous SUV's and trucks. We certainly would all be safer if we drove tanks, but is that a viable objective?

According to Mr. Wenzel, “NHTSA research indicates that the risk of fatality of a car driver is much higher when struck by a pickup or truck-based SUV, particularly in the car’s side, than when struck by another car, even after controlling for the striking vehicle’s weight”

So, are more fuel efficient vehicles more dangerous to their drivers? Let’s ask the Wall Street Journal.

Sponsors of the studies are cautious about drawing hard conclusions about what's causing fatalities and fatality rates to decline. They don't want to confuse coincidence with cause and effect.

This is something that Max Schultz, and others that continue to mislead about the relationship between fuel-efficiency and car safety, should keep in mind.

Tom Wenzel’s full email response (with references) is below.

In his recent article in City Journal, Max Schultz of the Manhattan Institute wrote that the effect of an Obama Administration’s granting to California a waiver to implement tailpipe greenhouse-gas emission standards would be that cars would “be made of lighter, less crash-resistant materials” resulting in “more highway fatalities nationwide.” We have spent the past several years examining the research underlying the misconception that lighter vehicles are inherently less safe than heavier ones, and more recent work, including ours, challenging it. We have come to the conclusion that the claim that lighter vehicles are inherently dangerous to those riding in them is flawed.

A recent workshop of experts on vehicle safety and fuel economy, including representatives from auto manufacturers, academia, the materials industry, and non-governmental organizations, found that there is little, if any, trade-off between improvements in fuel economy and in safety in light-duty motor vehicles, particularly if priority is given to both of these goals (Gordon et al., 2007). A reduction in vehicle mass is an important technique for improving fuel economy, but certainly not the only one. A host of existing technologies can be used to increase the fuel economy of light-duty vehicles by over 50%, and are cost effective at recent gas prices ($3 per gallon; EEA, 2006). In addition, new powertrains (such as hybrid electric, plug-in hybrid electric, homogenous charge compression ignition, and fuel cells) or fuels (such as clean diesel or other low-carbon fuels) can achieve even further increases in fuel economy without reducing vehicle mass. The engineering modeling for the recent 35-mile-per-gallon CAFE standard only considered up to a 3 percent reduction in vehicle mass, for only light trucks weighing over 5,000 lbs (NHTSA, 2008).

Conversely, several strategies can be employed to increase vehicle safety with little effect on fuel economy, other than a small increase in vehicle mass. Electronic stability control (ESC), in which the vehicle computer automatically brakes individual wheels to inhibit a dangerous skid, better seat belts, stronger roofs and pillars, and vehicle-to-vehicle communication, all can make vehicles more safe. Our research by vehicle model indicates that there is essentially no relationship between a car’s mass and the fatality risk to its driver, even in frontal crashes (Wenzel and Ross, 2005).

In crashes with a stationary object, frontal crush space, rather than mass, provides protection to the vehicle occupants. Mass offers protection in head-on car-car, and perhaps truck-truck, crashes when there is a substantial difference in mass between the two vehicles; however, a substantial majority of casualties in motor vehicle crashes are unrelated to the masses of the vehicles involved. In head-on crashes between a car and a truck, differences in frontal height and/or stiffness, rather than mass, are the most important factor. NHTSA research indicates that the risk of fatality of a car driver is much higher when struck by a pickup or truck-based SUV, particularly in the car’s side, than when struck by another car, even after controlling for the striking vehicle’s weight (Kahane, 2003). The aggressivity of a striking truck can be mitigated either by lowering the bumper or the stiff parts of the truck, or by making the front of the truck less stiff. More detailed tests are needed to determine which characteristic is more important in defining a truck’s aggressivity. Raising the car bumper height (in head-on crashes) or door sill height (in side impact crashes), reinforcing the occupant compartment, and installing side curtain airbags, may increase protection to car occupants struck by a light truck.

Increased mass improves neither the crash avoidance ability or crashworthiness of a vehicle in a rollover crash. Although SUVs and pickups are more likely to roll over than passenger cars which are generally lighter, the height of a vehicle’s center of gravity has a substantially greater impact than mass on the propensity of a vehicle to roll over. The center of gravity can be lowered by lowering the overall vehicle height, or by increasing its track width. The widespread adoption of electronic stability control (ESC) in vehicles, which provides automatic braking separately at the four wheels to inhibit vehicle skidding, should substantially reduce rollovers.

In statistical analyses, mass has been shown to have a protective effect in crashes with an object and with another vehicle. However, more recent research indicates that mass is merely a proxy for other characteristics that are more important for crashes between cars and trucks, such as frontal height and stiffness (Wenzel and Ross, 2006).

References

Energy and Environmental Analysis, Inc., Technologies to Reduce Greenhouse Gas Emissions from Light-duty Vehicles, Draft Final Report prepared for Transport Canada, Arlington VA: Energy and Environmental Analysis, Inc., 2006.

D. Gordon, D. L Greene, M. H. Ross and T. P. Wenzel, Sipping Fuel and Saving Lives: Increasing Fuel Economy without Sacrificing Safety: A report informed by an October 3, 2006 Experts Workshop on Simultaneously Improving Vehicle Safety and Fuel Economy through Improvements in Vehicle Design and Materials, Washington DC: the International Council on Clean Transportation, 2007.

C.J. Kahane, C.J., Vehicle weight, fatality risk and crash compatibility of model year 1991-99 passenger cars and light trucks. NHTSA DOT HS 809 662. U.S. Department of Transportation, National Highway Traffic Safety Administration, Washington, D.C., 2003.

National Highway Traffic Safety Administration, Corporate Average Fuel Economy for MY 2011-2015 Passenger Cars and Light Trucks, Preliminary Regulatory Impact Analysis, Office of Regulatory Analysis and Evaluation, National Center for Statistics and Analysis, April 2008.

T. P. Wenzel and M. Ross, “The effects of vehicle model and driver behavior on risk,” Accident Analysis and Prevention 37, 479-494 (2005). LBNL-55417

T. P. Wenzel and M. Ross, Increasing the Fuel Economy and Safety of New Light-Duty Vehicles: White paper prepared for The William and Flora Hewlett Foundation Workshop on Simultaneously Improving Vehicle Safety and Fuel Economy Through Improvements in Vehicle Design and Materials, Berkeley CA: Lawrence Berkeley National Laboratory, 2006. LBNL-60449

T. P. Wenzel and M. Ross, “Safer Vehicles for People and the Planet.” American Scientist, 96:2 (March-April 2008), 122-128. LBNL-325E

T. P. Wenzel and M. Ross, “Safer Vehicles for People and the Planet: Response to a Letter to the Editor.” American Scientist, 96:3 (May-June 2008), 179. LBNL-311E

T. P. Wenzel and M. Ross, “The Relationship between Vehicle Weight/Size and Safety,” in The Physics of Sustainable Energy: Using Energy Efficiently and Producing It Renewably. Hafemeister, David, Barbara Levi, Mark Levine, Peter Schwartz, editors. The American Institute of Physics: Melville, New York, 2008.