Working with staff at the U.S. National Highway Traffic Safety Administration (NHTSA) and Environmental Protection Agency (EPA), Berkeley Lab researchers use statistical analysis of real-world crash data to estimate the recent historical relationship between vehicle weight, size, and safety. The results inform the technical analyses conducted by the agencies in setting standard levels that maximize efficiency, reduce emissions, and save consumers money while maintaining safety.

Researchers estimate the effect of a 100-lb reduction in vehicle weight on societal fatality risk; that is, the risk of any fatality in a crash, including pedestrians and cyclists, per 10 billion miles of travel. These results indicate that every 100-lb reduction in vehicle mass, holding vehicle size (i.e., footprint, or the area between the four wheels) constant, results in a 0.3% to 1.2% increase in societal fatality risk in cars and lighter-than-average light trucks, but a 0.2% to 0.6% decrease in societal fatality risk in heavier-than-average light trucks and crossover utility vehicles (CUVs) and minivans. Only the 1.2% increase in risk from making lighter cars lighter, and the 0.6% decrease in risk from making heavier trucks lighter, are statistically significant.

Comparison with previous statistical analyses indicates that the effect of mass reduction on both lighter- and heavier-than-average cars has consistently declined over time, as safety technologies such as airbags and electronic stability controls have mitigated any inherent disadvantage of lighter vehicles.

Although mass reduction in lighter-than-average cars is associated with increases in societal fatality risk, these increases are quite small compared to those associated with other vehicle attributes, driver characteristics, and crash circumstances. For instance, crashes that occur at night, in rural counties, or on high-speed (55 mph or higher speed limit) roads are associated with a societal fatality risk two orders of magnitude higher than a 100-lb increase in car mass. Similarly, crashes involving young male or elderly drivers, and occurring in two-door cars, are associated with much higher fatality risk. In contrast, vehicles with full-torso side airbags, automated braking systems, and electronic stability controls are associated with much lower fatality risks than mass reduction in cars. (Results are similar for light trucks and CUVs/minivans.)

On average, societal fatality risk tends to decrease as vehicle mass increases, even after accounting for vehicle footprint and other attributes, driver characteristics, and crash circumstances. However, the correlation between adjusted fatality risk and the curb weight of cars is low, on the order of 0.30, suggesting that the design of individual car models can mitigate any safety penalty from making cars lighter. For example, the adjusted societal fatality risks of a Honda Fit, Toyota Prius, and Volkswagen Passat are half that of the Dodge Neon, Chevrolet Cobalt, and Pontiac G6, which are of comparable weight. And societal fatality risk in a Fit is comparable to that in a Lincoln Town Car or Mercury Grand Marquis, which weigh over 1500 lbs. more.

Wenzel, Thomas P. Assessment of NHTSA’s Report “Relationships Between Fatality Risk, Mass, and Footprint in Model Year 2004-2011 Passenger Cars and LTVs” (LBNL Phase 1). 2018. LBNL-2001137.

One unintended consequence of new vehicle efficiency standards is that, by reducing the operating (i.e., fuel) cost of driving, consumers are likely to drive more miles, thereby somewhat reducing the effectiveness of the standard in lowering fuel use. Such a “rebound” in vehicle miles of travel also may have implications for overall safety, and other effects. Using a dataset of annual odometer readings from millions of vehicles in Texas, Berkeley Lab estimated that consumers are likely to decrease their driving by 0.09% in response to a 1.0% increase in the price of gasoline, and by 0.17% from a 1.0% increase in the cost of driving (price of gasoline times vehicle efficiency). Consequently, a decrease in the cost of driving, from implementing more stringent fuel economy standards, is expected to increase the number of miles driven.

Wenzel, Thomas P., and K. Sydny Fujita. Elasticity of Vehicle Miles of Travel to Changes in the Price of Gasoline and the Cost of Driving in Texas. 2018. LBNL-2001138.

The mission of NHTSA is to save lives, prevent injuries, and reduce economic costs due to road traffic crashes, through education, research, safety standards, and enforcement.

NHTSA's research offices are the Office of Vehicle Safety Research and the Office of Behavioral Safety Research. The Office of Vehicle Safety Research's mission is to strategize, plan, and implement research programs to continually further the agency's goals in reduction of crashes, fatalities, and injuries. Its research is prioritized based on potential for crash/fatality/injury reductions and is aligned with Congressional Mandates, along with U.S. Department of Transportation and NHTSA goals. The Office of Behavioral Safety Research studies behaviors and attitudes in highway safety, focusing on drivers, passengers, pedestrians, and motorcyclists, and uses that research to develop and refine countermeasures to deter unsafe behaviors and promote safe alternatives.

The Office of Air and Radiation (OAR) is the scientific research arm of EPA. Its leading-edge research informs Agency decisions and supports the emerging needs of EPA stakeholders, including the Agency’s state, tribal, and community partners. On September 30, 2019, OAR restructured for the first time in 25 years to better address the increasingly complex environmental challenges of the 21st century.  EPA, along with the U.S. Department of Transportation (DOT), develop and implement the Combined Average Fuel Economy (CAFE) standards. The relevant websites within the agencies include the EPA's Regulations for Emissions from Vehicles and Engines page and DOT's CAFE page.