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Status Report, Vol. 46, No. 10 | November 17, 2011 Subscribe

Hybrid advantage: Hybrids' injury odds are lower than their gas-powered twins

Hybrids have a safety edge over their conventional twins when it comes to shielding their occupants from injuries in crashes, new research by the Highway Loss Data Institute (HLDI), an Institute affiliate, shows. On average, the odds of being injured in a crash are 25 percent lower for people in hybrids than people traveling in nonhybrid models.

"Weight is a big factor," says Matt Moore, HLDI vice president and an author of the report. "Hybrids on average are 10 percent heavier than their standard counterparts. This extra mass gives them an advantage in crashes that their conventional twins don't have." He notes that other factors, such as how, when, and by whom hybrids are driven, also may contribute. Researchers included controls to reduce the impact these differences may have had on the results.

The new finding is more good news for green-minded drivers who don't want to trade safety for fuel economy. Not so long ago, car buyers had to choose between the two because fuel-efficient cars tended to be smaller and lighter. Now, consumers have more options than ever when it comes to picking an environmentally friendly — and crashworthy — vehicle.

"Saving at the pump no longer means you have to skimp on crash protection," Moore says.

In the study, HLDI estimated the odds that a crash would result in injuries if people were riding in a hybrid versus the conventional version of the same vehicle. The analysis included more than 25 hybrid-conventional vehicle pairs, all 2003-11 models, with at least 1 collision claim and at least 1 related injury claim filed under personal injury protection or medical payment coverage in 2002-10.

Collision coverage pays to repair or replace an at-fault driver's vehicle after a crash with an object or another vehicle. Personal injury protection, or PIP, pays medical expenses for injuries insured drivers and other people in their vehicles sustain in a crash, no matter who is at fault in the collision. Medical payment, or MedPay, covers treatment costs when insured drivers or their passengers are hurt in crashes when the driver is at fault. PIP coverage is sold in states with no-fault insurance systems, and MedPay coverage is sold in tort states.

Hybrids' injury odds were 27 percent lower than their standard counterparts for collision claims with a related PIP claim and 25 percent lower than their twins for collision claims with a related MedPay claim.

Crash physics

It's well known that size and weight influence injury likelihood. In a crash involving two vehicles that differ in size and weight, the people in the smaller, lighter vehicle will be at a disadvantage. The bigger and heavier vehicle will push the smaller, lighter one backward on impact. This means less force on people in the heavier vehicle and more on the people in the lighter one. Greater force means greater risk, so people in the smaller, lighter vehicle are more likely to be injured. Even in single-vehicle crashes, heavier vehicles have an advantage because they are more likely to move, bend, or deform objects they hit (see Status Report special issue: car size, weight, and safety, April 14, 2009).

Even with advances in occupant protection, larger vehicles still are safer choices than smaller ones. That's why downsizing vehicles to improve fuel efficiency has traditionally resulted in safety trade-offs. The trend among automakers nowadays is to boost fuel economy by designing more efficient internal combustion engines and by adding hybrids to their fleets.

Although hybrids share the same footprint and structure as their conventional counterparts, they outweigh them because of the added heft of battery packs and other components used in dual-power systems. At about 3,600 pounds, a hybrid Honda Accord midsize sedan, for example, can weigh as much as 480 pounds more than a conventional Accord. A hybrid Toyota Highlander, a midsize SUV, weighs about 4,500 pounds, compared with about 4,170 pounds for the conventional Highlander.

"Park any of these pairs side by side and you can't tell the hybrid from the traditional vehicle just by looking at them," Moore points out. "But you could if you were to put them on a scale."

Curb weight in pounds by model, hybrid vs. conventional counterpart

Graph image

Disentangling size and mass

Moore says the study is unique in its ability to examine how vehicle mass affects injury rates. One question safety advocates long have grappled with is, how much does size affect crashworthiness as compared with weight? These two factors have separate effects, but they are highly correlated. Disentangling them requires keeping one variable constant while modifying the other.

"What we ended up with was a study that may be as good a natural experiment as can be done to examine the effects of mass independent of size," Moore says.

Researchers excluded the Toyota Prius and Honda Insight from the study because they are only sold as hybrids. The analysis controlled for calendar year, rated driver age and gender, marital status, vehicle density (number of registered vehicles per square mile), garaging state, vehicle series, and vehicle age.

Hybrids' driver death rates

HLDI's findings complement other Institute research into how vehicle size and weight affect crash injuries. A recent study found that driver death rates for 2005-08 models with at least 100,000 registered vehicle years decreased as vehicle weight increased (see "Dying in a crash," June 9, 2011).

Every hybrid in the analysis had a lower driver death rate than its conventional twin. For example, the Honda Civic Hybrid had an overall driver death rate of 53 per million registered vehicle years, compared with 55 for the nonhybrid Civic. The hybrid Toyota Camry had a driver death rate of 36, while the standard Camry's rate was 10 points higher.

SIDEBAR
Hybrids more likely to injure pedestrians

Hybrids may be as much as 20 percent more likely to injure pedestrians than nonhybrid vehicles because of their quiet motors.

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