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Status Report, Vol. 39, No. 3 | March 6, 2004 Subscribe

NHTSA report confirms overall safety disadvantage of lighter vehicles

Based on analyses of deaths in passenger vehicles during 1996-2000, the National Highway Traffic Safety Administration (NHTSA) has published an exhaustive report that confirms the overall safety disadvantages of riding in vehicles of lighter weight.

Published late last year amid ongoing policy debates about fuel economy and vehicle compatibility in crashes, the NHTSA report by Charles J. Kahane finds that making all passenger vehicles 100 pounds lighter could cost 1,000 or more lives per year, mostly in crashes of vehicles that already are lightweight. But Kahane points out that the disadvantages of vehicle weight reductions don't necessarily carry though to the heaviest passenger vehicles: "There may have been some weight above 3,870 pounds beyond which overall fatality rates tended to increase, rather than decrease, as weight increased."

Further analyses led Kahane to suggest that reducing the weights of pickups and SUVs weighing more than about 5,000 pounds would be beneficial. Some added deaths would occur in these vehicles because of their reduced weights, but these deaths would be more than offset by reductions in deaths of others on the road who crash with the heavy vehicles.

When Kahane considered only deaths inside vehicles of various sizes (that is, when he didn't factor in deaths of others in crashes with these vehicles), he found the highest driver death rates by far in the lightest cars (11.6 driver deaths per billion miles of travel). The lowest rates are in minivans (2.8 deaths per billion miles), followed by the heaviest cars (3.3). Death rates in pickup trucks follow this pattern. The rate in lighter pickups is higher than in heavier ones (6.8 deaths per billion miles compared with 4.1).

Exception to the pattern

The heaviest SUVs have lower death rates than ones of lighter weight. However, there's an interesting deviation from the pattern of lower death rates in heavier vehicles — the lightest SUVs have lower rates (5.7) than midweight ones (6.7).

The occupant death rate in crashes that don't involve rollover is somewhat lower in midweight SUVS, but the rollover death rate in the lightest SUVs is less than half of the rate in midweight SUVs (1.1 deaths per billion miles compared with 2.7 per billion). In discussing this exception, Kahane points out that lighter SUVs and pickups used to be especially rollover prone but by 1996-99 several new models with improved rollover stability had been introduced.

"Small four-door SUVs of 1996-99 may have been the beginning of a new generation of more stable, less aggressive vehicles with lower fatal crash rates. This trend appears to have continued and expanded since 1999," Kahane says.

Driver deaths per billion miles by vehicle size, 1996-99 models during 1996-2000
Cars 4-door Curb weight range Death rate
Very small 1,950-2,274 lbs. 11.6
Small 2,208-2,878 lbs. 7.8
Midsize 2,566-3,567 lbs. 5.3
Large  3,035-4,690 lbs. 3.3
Minivans 3,354-4,819 lbs. 2.8
SUVs 4-door
Small 2,636-3,437 lbs. 5.7
Midsize 3,476-4,484 lbs. 6.7
Large  4,332-5,899 lbs.  3.8
Lighter 2,625-4,178 lbs. 6.8
Heavier 3,404-5,268 lbs. 4.1
Estimated changes in numbers of driver deaths during 1999 if vehicles had been 100 pounds lighter
Cars 4-door
Lighter than 2,950 lbs. +226 to +715
2,950 lbs. or heavier  +129 to +303
SUVs, pickups, and vans
Lighter than 3,870 lbs. +59 to +296
3,870 lbs. or heavier -156 to +241
All vehicles +258 to +1,555

Vehicle weight and fuel economy

The overall fuel economy of passenger vehicles on U.S. roads has deteriorated in recent years, largely because of the surge in popularity of bigger, heavier vehicles. This has prompted calls for more stringent fuel economy requirements. At the same time, requiring improved fuel economy could have adverse safety consequences because the vehicles that get the most miles per gallon typically are the lightest and least protective of their occupants in crashes.

NHTSA's new report quantifies the safety cost of downweighting vehicles to meet tougher fuel economy requirements. If the weights of all passenger vehicles were reduced by 100 pounds, on average, the result would be an increase in highway fatalities ranging from about 250 to 1,500 per year —and this increase could approach 3,000 more deaths per year if passenger vehicle weights were reduced by 200 pounds instead of 100.

The biggest share of the increase would be among people riding in the lightest cars. Downweighting only these vehicles by 100 pounds would result in an estimated 226 to 715 more deaths per year. However, downweighting the heaviest SUVs and pickup trucks by 100 pounds might not increase crash deaths. In fact, it could reduce deaths.

These results come as NHTSA considers changing the structure of fuel economy requirements. A problem with the current structure is that auto manufacturers can reduce the weights of their vehicles and/or increase sales of lightweight vehicles to comply with the federal standards. The revisions NHTSA is considering would take into account the recommendations of the National Academy of Sciences, which has urged the agency to index fuel economy requirements to passenger vehicle weight but to level off the index at about 4,000 pounds (see "Improve fuel economy without negative safety consequences," April 6, 2002).

Indexing means federal fuel economy requirements would be less stringent for a vehicle weighing 4,000 pounds than for one weighing 2,500 pounds. However, leveling off the index means a 6,000 pound vehicle would have to meet about the same requirements as a 4,000 pound vehicle.

"This approach would accomplish two purposes," Institute chief operating officer Adrian Lund points out. "It would acknowledge the safety disadvantages of downweighting passenger vehicles across the board — the very disadvantages that Kahane quantified in the NHTSA report. At the same time, leveling off the index would encourage auto manufacturers to downweight the heaviest passenger vehicles. This would benefit not only fuel economy but also safety because in a two-vehicle crash the heavier vehicle poses added risks to the people in the other vehicle."

Vehicle weight and crash compatibility

In crashes involving two passenger vehicles of differing weights, two effects are at work. The extra weight of the heavier vehicle reduces the risks for its occupants but also inflicts extra risks on the people in the lighter vehicle. The safety benefits of heavier vehicles to their own occupants diminish as the vehicles get heavier and heavier.

Institute research has shown that the extra weight of vehicles weighing 4,000 pounds or more actually has a small negative effect on society because the additional harm they inflict on people in lighter vehicles in two-vehicle crashes more than offsets the benefits for their own occupants (see Status Report special issue: vehicle incompatibility in crashes, April 26, 2003).

The new NHTSA report supplies evidence of this. The worst safety consequences of downweighting vehicles by 100 pounds would occur in light cars (those already weighing less than about 3,000 pounds) in crashes with SUVs or pickups. In contrast, there would be a net safety benefit in all kinds of two-vehicle crashes if the weights of the heaviest SUVs and pickups (those weighing more than 5,000 pounds) were reduced by 100 pounds.

"So it would serve both safety and fuel economy to downweight these very heavy SUVs and pickups," Lund says.

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