The relatively high rate of light-truck rollover fatalities can be seen in Table 3. Kahane’s (1997) regression analysis also found that weight reduction in cars would increase the rollover risk, because, for 1980s model years, light weight was correlated with fatal rollovers. But this tendency largely disappeared in the 1990s as manufacturers improved car designs. The correlation of rollovers with weight is not inherent, but depends on features such as height of center of mass, track width, and stiffness of suspension. With rollover standards, changes will also be made to reduce rollovers for light trucks.

Table 3. Number of Vehicles in Fatal One-Vehicle Crashes, by "First Harmful Event", 1999

Seat belts are the most important safety technology. Their importance is evident from comparing belt use by drivers killed in crashes with that observed in the careful National Occupant Protection Use Survey, Table 4. Seat belt use among drivers killed is roughly half as great as among all drivers! Part of the reason is that careless drivers, who are more likely to be involved in a crash, are less likely to use seat belts.

Table 4. Seat Belt Use by Drivers in Percent

Historical trends. The total number of fatal crashes and fatalities declined 18% from 1979 to 1999, even though the number of vehicles in use increased 55% and estimated vehicle miles traveled increased 75%. There was a dramatic decline of fatalities in car-to-car collisions, accompanied by a sharp rise in the fatalities involving light trucks. While car-to-car crashes became much safer, the number of light trucks and dangerous car-to-light truck crashes increased.

Analysis of fatalities in one- or two-year-old cars which crash with cars of any age reveals patterns for late-1990s models. Dividing by the number of new cars, one obtains the deaths in car-to-car crashes per million new cars. We find the extraordinary result that deaths in new cars from car-to-car head-on collisions were only a third as great in 1998 as one decade earlier, and only 20% as great as in 1980. The improvement shown in Table 5 is partly due to standardized testing programs which have emphasized crash types in the order shown. These developments suggest a point of departure: Driving cars in an environment of cars has become much safer, and there are great opportunities for further progress. Before building on this concept, consider some background.

Table 5 Decline in Driver Fatalities per Car, for Cars One and Two Years Old, 1988 to 1998

Peak deceleration correlates with severity of injury. One way to mitigate peak deceleration is to lengthen crash duration by increasing crush space. (A serious collision lasts roughly one-tenth of a second.) Another is vehicle design that spreads the deceleration more evenly during the crash. However, all fatalities cannot be eliminated; and one cannot usually protect adequately at high speeds.

This means (for an average case) that if car H is 1.6 times as heavy as car L, then 4 times as many are killed in car L as those in car H. (For current four-door sedans, Cadillac Seville and Honda Civic, the mass ratio is 1.58.) Improving safety technology has affected these mass-risk relations. For example, average results for model years 1995-99 imply a fatality risk ratio of 2.3, rather than 4, for a mass ratio of 1.6. Of course, this is still a large ratio. Although presented in terms of mass, these are dependences on mass and correlated dimensions.

The risk to passengers in both vehicles. While it is more risky to be in a light/small vehicle than in a heavy/large vehicle if the two collide, let us ask: What is the risk to society considering the occupants of both vehicles? And how does that risk change as masses/sizes are changed?

This analysis is straightforward and convincing because it is based on fatality ratios.

Cars vs light trucks. Fatalities in car-to-light truck head-on collisions are about 5 times higher in the car than in the light truck (Joksch 1998, Gabler & Hollowell 1998). In collisions where an SUV strikes the left side of a car, there are 30 driver fatalities in the struck car for each fatality in the striking SUV! Today’s light trucks are incompatible with cars.

Table 6 enables comparison of the fatality risk for both drivers in car-to-car and car-to-light truck collisions (Joksch 2000, pp 9-10). This approach shows, for example, that the ratio of fatalities in SUV-to-car collisions to those in car-car collisions is 2.32/1.28 = 1.8. Based on the 1013 driver fatalities in SUV-to-car collisions (Table 7), the ratio 1.8 leads to an estimate that about 450 excess driver’s lives were lost in 1999 due to the use of SUVs as substitutes for cars. On this basis, an annual excess of about 2200 deaths may be associated with the use of light trucks as car-substitutes.

Table 6 Drivers Killed per 1000 Drivers Involved: Car-to-Light Duty Vehicle Collisions,1991-1997. The "denominator" is 1000 times the number of police-reported crashes.

Table 7 Driver Fatalities in Car-to-Car and Car-to-Light Truck Collisions, 1999 FARS

Ford recently announced design changes to improve the compatibility of SUVs and pickups with cars, starting in MY2004. This is a promising, if modest, beginning. Presumably others are taking similar steps.

A Safety-Fuel Economy Scenario. In order to address both fuel economy and safety, we propose making heavier vehicles lighter but not smaller, and making lighter vehicles larger but not lighter. These changes would be enabled by mass-reduction technologies. Two or three major kinds of changes could be made to reduce mass, independent of size:

To illustrate, we assume that the mass range (curb weights) of most light duty vehicles would be reduced to between 2400 and 3300 lbs (the present distribution is 2400 to 5000 lbs). Mass reductions of up to 33% are projected. About half of light-duty vehicles would be reduced in mass, with an average 1000 lb reduction. To achieve such major mass reductions while maintaining the size and performance characteristics that attract customers would be a major engineering challenge. In addition, safety technologies would continue to be developed and applied.

We estimate this scenario would save about 2200 lives a year in two-vehicle crashes. This estimate is conservative, based on the excess of deaths in light truck-to-car crashes, for only those light trucks used as car substitutes, and using 1999 fatality rates. There would also be many lives saved in car-to-car crashes, but some extra lives lost in heavy truck-to-light duty vehicle crashes. We also estimate an increase of less than 400 deaths per year in one-vehicle crashes with stationary objects.

We project improvement in combined city-highway fuel economy of 55% (DeCicco et al. 2001). Fuel economy improvement in gas guzzlers is especially important for fuel savings and CO2 reduction.

There would be increases in manufacturing cost-per-unit in this scenario if the lightweight materials aluminum and engineering plastics are substantially involved. However, ultralight steel techniques are not costly, and can reduce the mass of steel bodies and associated parts 15% to 20%. While there would be substantial development and re-tooling costs, distinct from the per-unit manufacturing costs, in the larger picture of automotive model changes and safety they appear reasonable (DeCicco et al. 2001).