Relationships Between Vehicle Weight and Occupant Death Rates

The highest priority for car designs should continue to be improving protection for their own occupants. New study of relationships between vehicle weight and occupant death rates helps put in perspective issue of crash compatibility.

As bigger and heavier utility vehicles get ever more popular, is incompatibility between vehicles in crashes becoming a big safety problem on U.S. roads?

A comprehensive new study by the Insurance Institute for Highway Safety addresses this issue. The study focuses on fatal crashes involving 1990-95 model cars (including minivans), pickups, and utility vehicles. Researchers examined occupant deaths in these vehicles as well as deaths in other vehicles with which these vehicles collided.

For each vehicle type and crash configuration, relationships between vehicle weights and death rates were reviewed to explore the role of incompatibility in passenger vehicle occupant deaths. Examined were deaths inside the study vehicles in two-vehicle and single-vehicle crashes, deaths inside any other vehicles that crashed with the study vehicles, and the relative death risk when study vehicles collided with each other in front-to-front and front-to-side crashes.

Occupant deaths inside 1990-95 vehicles

The basic findings reinforce what's long been known about vehicle size and occupant death rates. As vehicle weight decreases, the number of occupants killed in crashes increases. There also are differences among vehicle types. The lightest utility vehicles have by far the highest death rates, but in the other weight classes, pickups have the highest occupant death rates, and cars have the lowest. These results reflect both single- and two-vehicle crashes, and the latter account for 48 percent of all car occupant deaths, 35 percent of all pickup occupant deaths, and 29 percent of all utility vehicle deaths.

Deaths inside study vehicles in two-vehicle crashes

Lighter vehicles have higher occupant death rates in two-vehicle crashes, and within each weight class, cars and pickups have similar occupant death rates. Utility vehicles have somewhat lower death rates. Incompatibility can play a role in two-vehicle crashes because the characteristics of both vehicles -- especially their weights, crush zone stiffnesses, and geometric designs -- influence crash outcomes.

Other vehicles in two-vehicle crashes with deaths in the study vehicles

In two-vehicle crashes with deaths in 1990-95 model cars, just under 50 percent of the other vehicles also are cars. This percentage ranges from 48 percent for the lightest cars to 40 percent for the heaviest. Pickups are 20-21 percent and utility vehicles 7-8 percent of the other vehicles, regardless of the weight class of the cars with occupant deaths. Medium and heavy trucks are the other vehicles in 17 percent of the deaths in the lightest cars and 26 percent in the heaviest ones. For pickups and utility vehicles with occupant deaths, the other vehicles are less likely to be cars and more likely to be medium and heavy trucks.

Deaths inside other vehicles that collide with the study vehicles

Heavier passenger vehicles have the highest rates of occupant deaths occurring in the other vehicles they collide with. Here, however, there are large differences among vehicle types. In every weight class, pickups have the highest other-vehicle death rates and cars the lowest. Even the heaviest cars have lower other-vehicle death rates than the lightest pickups or utility vehicles.

Deaths in the study vehicles vs. deaths in the other vehicles

In two-vehicle crashes involving cars, there are more deaths inside the cars than in the other vehicles they collide with. Only when the two heaviest classes of cars are involved are there slightly more deaths in the other vehicles. In contrast, in crashes involving pickups or utility vehicles there are many more deaths in the other vehicles. This excess of deaths in other vehicles is most pronounced for crashes with the heaviest pickups.

"These results have particular importance for occupant protection priorities," says Institute President Brian O'Neill. "More people die inside cars than in the other vehicles they collide with, and not all of the other vehicles involved are cars. This means the highest priority for car designs should continue to be improving protection for their own occupants. Designing cars to improve car-to-car crash compatibility should only be considered if it doesn't conflict with this priority. On the other hand, the very high death rates for occupants of other vehicles colliding with pickups or utility vehicles suggest that making future model pickups and utility vehicles more crash compatible, especially in crashes with cars, should be a priority."

Deaths in car-to-pickup and car-to-utility vehicle crashes

The results summarizing the relative risks for occupants when pairs of the study vehicles collided identify particularly high-risk configurations. For all crashes between cars and pickups or cars and utility vehicles, people in cars are about four times more likely to die than people inside pickups or utility vehicles. When pickups or utility vehicles strike cars in the side, the risk of death for car occupants relative to the risk of the pickup or utility vehicle occupants dying is 27-to-1. This compares with about 6-to-1 for people in cars struck in the side by another car and 20-to-1 when the striking car is heavy. People in small cars weighing less than 2,500 pounds struck in the side by pickups or utility vehicles have a relative death risk of 47-to-1.

"These relative risk results highlight some of the severe crash incompatibility problems between cars and light trucks and should help establish priorities for future vehicle design improvements," says O'Neill. "At the same time, it's important to understand that about 10 percent of all car occupant deaths occur in crashes with pickups, and about 4 percent occur in crashes with utility vehicles. Virtually all of the remaining car occupant deaths occur in crashes with other cars, big trucks, or in single-vehicle crashes. Thus, compatibility improvements, even though they're important, are not panaceas."

Occupant death rates in two-vehicle crashes, deaths in 1990-95 model passenger vehicles and other vehicles: deaths per million vehicles per year
weight class
Vehicle Crash Pairs

1990 - 95 Cars

Other vehicles 1990-95
Other vehicles 1990-95 utility
Other vehicles
<2,500 109 35   105 58
2500- 83 43 89 88 54 75
3000- 60 48 56 116 53 75
3,500- 53 57 52 115 37 92
4,000- 49 58 44 144 29 91
4,500-   45 169 29 110
5,000+   38 181 25 112
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There is much debate over loss of control accidents with various opinions from experts and public. The trouble with this is people are not aware that there can be as high as 950 pounds more weight on the front axle of their front wheel drive vehicle than the back. So a car that feels like a limousine on the front holds like a golf cart on the back.

If you analyze single vehicle accidents you will find most of them had better tires on the front than the back or a very large weight difference. In fact the worst balanced cars have 4 times as many fatalities as cars designed with better balance.

How are you going to tell how fast is too fast under these conditions when it is possible for one car to handle fine on ice at 70 kph and another to loose control at 10 kph.

Thanks Harvey McFadden

I have been reading numerous reports on car safety, accident-prone personality disorders (I have a few such disorders of the neurological system), car weight, car size, and US-Canadian road base soils--which dictate uneveness in road surfaces, as well as European versus North American driving patterns. If the United States and/or Canada were to switch to very high miles per gallon micro-cars, in order to save crude oil, traffic fatalities per year would at least quadruple, under the most conservative estimate I can devise. Under some circumstances the death rates could easily increase ten-fold or more. For most North Americans it would be safest to use heavy, long-wheel base, wide-track vehicles, preferably powered by something other than imported petroleum. Nanotube Carbon Paper batteries as developed by Renselear (sp?) Polytechnique Institute, NASA, and Florida State University come to mind. They should be very cheap and last the liffespan of the car. They can be instantly recharged from a larger battery at home hooked up to a rooftop or backyard solar system array. in about five years the price of solar cell technology might make such a system cheaper than buying power from the power company. An alternative to batteries would be liquid nitrogen propulsion systems using a large nickle steel radiator and a small under-the-hood steam engine in lieu of a gas engine. Power might come from a neighborhood fusor to separate and liquify nitrogen from the air. See Wikipedia for details under various terms.

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