Q&A: Vehicle size and weight

March 2013

1 How do a vehicle’s size and weight affect safety?

All other things being equal, occupants in a bigger, heavier vehicle are better protected than those in a smaller, lighter vehicle. Both size and weight affect the forces people inside a vehicle experience during a crash. The magnitude of those forces is directly related to the risk of injury.

In the case of size, the longer distance from the front of the vehicle to the occupant compartment gives a bigger vehicle an advantage in frontal crashes, which account for 41 percent of passenger vehicle occupant deaths. The longer that distance, the bigger the crush zone, and the lower the forces on the occupants.

Weight comes into play in a collision involving two vehicles. The bigger vehicle will push the lighter one backward during the impact. As a result, there will be less force on the occupants of the heavier vehicle and more on the people in the lighter vehicle. Heavier vehicles also fare better in some single-vehicle crashes because they are more likely to move, bend or deform objects they hit. 

IIHS demonstrated the role of size and weight in a series of crash tests in 2009 in which a microcar and two minicars were each crashed into a midsize car from the same manufacturer. The Smart Fortwo, Honda Fit and Toyota Yaris all had good ratings in the Institute’s moderate overlap frontal test, but all three performed poorly in the crashes with midsize cars. Insurance Institute for Highway Safety. 2009.  Car size and weight are crucial. Status Report, Vol. 44, No. 4.

2 Are people less likely to be killed or injured in a bigger, heavier vehicle?

Yes. Driver death rates calculated by IIHS illustrate the real-world advantages of bigger, heavier vehicles. For example, as a group, in 2011 very large cars 1-3 years old had 24 deaths per million registered vehicles, while minicars had 65. Of the 26 vehicles with the lowest driver death rates from the 2005-08 model years during 2006-09, all but three were midsize or larger. More than half of those with the highest rates were small vehicles or minicars. Insurance Institute for Highway Safety. 2011. Dying in a crash. Status Report 46(5)5:1-7.

Improvements in crash protection have made vehicles of all sizes safer, but, as illustrated in the figure below, an advantage persists for bigger vehicles, as measured by shadow, or length times width. Size and weight are highly correlated, so the figure would look similar if weight were used instead.

Car driver deaths per million registered vehicle years

3 What’s more important, size or weight?

It’s difficult to separate the effect of weight from the effect of size in the real world. However, a recent HLDI analysis did this to some extent by comparing hybrid vehicles with their conventional, nonhybrid twins. Highway Loss Data Institute. 2011. Injury odds and vehicle weight comparison of hybrids and conventional counterparts. HLDI Bulletin 28(10). Arlington, VA.  These pairs are identical except for the battery packs that give the hybrids extra mass. The analysis of insurance claims found that the odds of being injured in a crash are 25 percent lower for people in hybrids than for people in the nonhybrid versions of the same vehicles. While other factors, including how, when and by whom hybrids are driven also may contribute to their advantage, HLDI concluded that the extra weight is likely a key factor. 

Size also confers its own advantages, independent of mass. For example, among 2005-08 models, SUVs weighing 3,001-3,500 pounds had 39 driver deaths per million registered vehicles, compared with 51 for cars in the same weight range. Insurance Institute for Highway Safety. 2011. Dying in a crash. Status Report 46(5)5:1-7.

The advantage for SUVs may be connected to their taller profile. For one thing, the taller a vehicle is, the less likely it is to underride another vehicle in a crash. In addition, crash testing suggests that sitting higher makes occupants less likely to have head and chest injuries when their vehicles are struck in the side by shorter vehicles because the point of impact is lower.

This SUV advantage is a relatively new phenomenon. In the past, these vehicles had some of the highest death rates because they were prone to rollover crashes, and that tendency outweighed any advantage from their bigger size. That’s no longer a factor because of less top-heavy designs and the increasing prevalence of electronic stability control, which prevents rollovers and is now required on all new vehicles.

4 Do bigger vehicles pose a threat to occupants of smaller vehicles?

A lighter vehicle will always be at a disadvantage in a collision with a heavier vehicle. Beyond weight differences, SUVs and pickups historically have posed a danger to people in cars because their energy-absorbing structures didn’t line up with those of lower-profile vehicles. As a consequence of such incompatible designs, cars would often underride SUVs and pickups, resulting in more severe damage and a higher risk of injury and death. An analysis of crashes in 2000-01 involving 1997-99 model vehicles showed that SUVs and pickups were much more likely than cars or minivans of the same weight to be involved in crashes that killed occupants of other cars or minivans. O'Neill, B. and Kyrychenko, S.Y. 2004. Crash incompatibilities between cars and light trucks: issues and potential countermeasures. Paper No. 2004-01-1166. SAE 2004 World Congress & Exhibition. SAE International: Warrendale, PA.

This mismatch has faded as a problem in recent years, thanks to new designs of SUVs and pickups. The energy-absorbing structures of these vehicles are now lower so that they line up better with those of cars. The new designs came out of a voluntary agreement forged by automakers, the government and IIHS in 2003. A more recent study of 2008-09 crashes involving 2005-08 models showed that SUVs were no more likely than cars to be in crashes in which occupants of other cars died. Teoh, E.R. and Nolan, J.M. 2012. Is passenger vehicle incompatibility still a problem? Traffic Injury Prevention 13(6): 585-91.  Pickups still killed people in other vehicles at a higher rate, but the difference was much smaller than it was previously. 

5 How can fuel economy be improved without sacrificing safety?

Technological innovations, including electric vehicles, hybrids and more efficient internal combustion engines, are the best way to improve fuel efficiency without affecting occupant protection. These improvements don’t come with the safety tradeoffs that downsizing brings, and in the case of electric and hybrid vehicles, the extra weight from their batteries can provide a safety boost.

Still, automakers rely on a variety of strategies to meet fuel economy standards and that includes making vehicles lighter. In the past, manufacturers met fleetwide fuel targets by selling more small cars to balance out the bigger gas guzzlers. The current standards, in effect for 2012-16 models, have addressed this problem by tying fuel economy requirements to a vehicle’s footprint, roughly defined as the square footage outlined by a vehicle’s wheels. In the new system, simply replacing a big vehicle with a small one wouldn’t help meet targets because the smaller vehicle has to meet a more stringent target. This framework will continue under standards that have been finalized for 2017-21.

A government analysis of potential weight reductions in vehicle fleets found that the risk of fatalities doesn’t go up as long as the reductions are concentrated in the heaviest vehicles. Kahane, C.J. 2012. Relationships between fatality risk, mass, and footprint in model year 2000-2007 passenger cars and LTVs – final report. Report No. DOT-HS-811-665. Washington, DC: National Highway Traffic Safety Administration.  In contrast, an across-the-board cut of 100 pounds from every vehicle would cause fatalities involving cars weighing less than 3,106 pounds to rise 1.6 percent. The calculations included not only fatalities of people inside a given vehicle, but also occupants of other vehicles and pedestrians.