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Automotive Fuel Economy: How Far Should We Go? 3 SAFETY IMPLICATIONS OF FUEL ECONOMY MEASURES Of all concerns related to requirements for increasing the fuel economy of vehicles, safety has created the most strident public debate. Some maintain that higher corporate average fuel economy (CAFE) standards will require weight reduction inevitably leading to increased injury and death. Others point to past progress and assert that further substantial gains in fuel economy can be made without sacrificing safety, even if vehicle weight is reduced. Available studies arrive at conclusions that often appear contradictory. Further, because of the differences in the data examined and the analyses conducted, most studies are not directly comparable. In short, although the relationship of fuel economy to safety is of central interest, the issue is surrounded by substantial confusion. BACKGROUND Historically, motor vehicle travel in the United States has become steadily safer, a phenomenon observed in every nation as it motorizes. Deaths per vehicle miles traveled (VMT) have continuously decreased from 1930 to the present (see Figure 3-1). This overall trend of improving safety will no doubt continue regardless of U.S. policy on fuel economy. Many factors have contributed to this trend of reduced fatality rates in the past and are likely to affect future rates. They include support for stricter law enforcement and penalties for driving while intoxicated, increased use of safety belts and increased availability of passive protection, greater urbanization, a lower proportion of very young drivers, reduced motorcycle usage, and continued improvement in the highway environment. Factors operating in the opposite direction include increases in vehicle speeds and increased size and use of large trucks. The aging driving population, with its corresponding increase in crash risk per VMT and vulnerability to injury once a crash occurs, may also be a factor if this age group continues to increase its driving.
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Automotive Fuel Economy: How Far Should We Go? FIGURE 3-1 Death rate per hundred million vehicle miles. SOURCE: National Safety Council (1990). Against the background of improving safety there is the question of the impact of changes in the weight and external size of passenger cars and light trucks and in the vehicle fleet mix. This chapter reviews the historical data on relationships between risk of casualty and vehicle size and weight and draws conclusions as to the likely consequences of any future size and weight reductions. VEHICLE CHARACTERISTICS AND FATALITY RISK The design characteristics of a vehicle influence its safety in two major ways—by altering the risk of being involved in a collision and by altering the risk of occupant injury if a collision occurs. Because fuel economy measures may lead to changes in the design characteristics of given vehicles or to changes in the vehicle mix, they may affect vehicle safety. In multivehicle collisions, the compatibility of the design of the vehicles involved also affects the risk of occupant injury. Risk of Crash Involvement The geometry of a vehicle substantially affects its stability. The propensity of a vehicle to roll over depends on the height of the vehicle's center of gravity and the
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Automotive Fuel Economy: How Far Should We Go? track width (distance between left and right wheels) of the vehicle. The ratio of these two factors provides an approximate index of rollover stability, although the dynamic characteristics of the suspension and handling also have a major influence. Light trucks and vans are less stable than passenger cars because, for a given track width, their center of gravity is higher. Similarly, because utility vehicles have high ground clearance for off-road operation, they have a greater rollover potential than passenger cars. In fact, the actual rollover rate of utility vehicles is some five times greater than that of passenger cars (Insurance Institute for Highway Safety [IIHS], 1987). 1 Although less marked, smaller passenger cars also have a greater potential for rollover than larger cars. The risk of crash involvement can also be affected indirectly through policies affecting the characteristics of the vehicle fleet. For example, older cars have higher crash involvement rates per VMT than newer cars so that policies that lead to extension of the life of the vehicle fleet could have adverse safety implications (Stewart and Carroll, 1980). Similarly, as discussed in Chapter 6, improved vehicle fuel economy could encourage increased VMT, thus increasing exposure to risk. On the other hand, high-performance cars (that is, those with short acceleration times) have high occupant fatality rates, and if fuel economy measures diminish the market for or availability of these cars, there would likely be fuel economy and safety benefits (IIHS, 1990). Of course, despite such measures there will still be high risk drivers. Risk of Injury Once a collision occurs, the basic physics of the event, coupled with the crashworthiness characteristics of the vehicles and the use of safety belts and passive protection, influence the outcome in terms of the severity of occupant injury. Fundamentally, two factors influence injury to the occupant. The first is the severity of the crash, usually expressed in terms of the change in velocity to the occupant that occurs in the impact. The second is the distance over which that change in velocity occurs. A 30-mph change in velocity occurring over 30 feet is hard braking, but is not injurious. A 30-mph change in velocity occurring over 3 feet, the conditions for a severe collision, is normally survivable by a restrained occupant with only moderate and reversible injuries. However, a 30-mph change in velocity occurring over 6 inches would not normally be survivable. These correspond to decelerations of 1, 10, and 60 times the acceleration of gravity, respectively. Newton's laws of motion dictate that when rigid objects of different weights collide, the velocity change of the lighter object will always exceed that of the heavier object, in proportion to their relative weights. A head-on collision between two cars of comparable structure and equal weight and a closing speed of 60 mph will produce a velocity change for each car of 30 mph. If one car is double the weight of the other, 1 Despite the fact that vans have a much greater rollover potential than passenger cars, they actually experience a very low rate of rollover, presumably because of compensating driver behavior and the different trip characteristics associated with these vehicles.
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Automotive Fuel Economy: How Far Should We Go? however, the velocity changes will be 20 and 40 mph, respectively. Hence, in vehicle-to-vehicle collisions, occupants of the heavier vehicle will usually fare better because the change in velocity is less than in the lighter car. To reduce the deceleration forces on the vehicle occupant, good crash-protection design seeks to provide a long "ride-down" distance (i.e., the distance over which the deceleration occurs) in a vehicle's structure. The greater the change in velocity, the greater the ride-down distance needed. On the whole, the larger the vehicle the greater the ride-down distance available (vans are an exception). As shown in Table 3-1, which lists occupant fatalities in 1990 for the main crash types (National Highway Traffic Safety Administration [NHTSA], undated c), vehicle-to-vehicle collisions are a large segment of the fatal crash population.2 Weight and ride-down distance are fundamental to occupant protection, and from the standpoint of individual safety, one is better off in the heaviest possible vehicle that has the largest amount of useful ride-down distance built into its structure. However, as will be discussed subsequently, from an overall societal viewpoint, changes in the weight TABLE 3-1 Occupant Fatalities in Passenger Vehicles by Vehicle Type, 1990 Fatal Accident Reporting System Single Vehicle Vehicle Type Vehicle/Vehicle Non-Rollover Rollover Total Passenger cars 13,406a 5,803 4,816 24,025 Vansb 587 259 306 1,152 Pickup trucksb 2,333 1,559 2,337 6,229 Utility vehiclesb 382 194 636 1,212 Total 16,708 (51.2%) 7,815 (24.0%) 8,095 (24.8%) 32,618(100%) a The number of occupant fatalities in two-car crashes is 4,913. Most of the 13,406 fatalities occur to occupants of passenger cars colliding with a vehicle other than a passenger car. b Small and large. SOURCE: NHTSA (undated c). 2 The data in Table 3-1 do not include the approximately 3,200 motorcyclists, 700 medium- and heavy-truck occupants, 6,500 pedestrians, and 850 pedalcyclists who sustained fatal injuries in 1990. Changes in the automotive fleet mix may have some impact on these other fatalities as well.
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Automotive Fuel Economy: How Far Should We Go? distribution of vehicles in the fleet produce winners and losers in two-vehicle collisions, the net effect depending on the precise characteristics of the fleet. Design Compatibility Vehicle design should seek to optimize protection for the greatest number of people at risk. Consequently, design compatibility among vehicles is an important safety issue. In a collision between a heavier car and a lighter car, two major factors contribute to design compatibility. The first concerns the dynamic crash characteristics. To achieve greater between-vehicle compatibility and thus protect the occupants of the lighter weight car, the structure of the heavier car should be "softened up" from what is optimal for that car in an impact with a rigid barrier. The second factor concerns geometric characteristics—that is, the cars should "fit together" in a way that minimizes injury. An example of an incompatible fit is that between a pickup truck and a passenger car. The high bumper on the pickup truck leads to greater intrusion into the car than would be the case if the bumper heights of the two were compatible (Hackney et al., 1989). Thus, measures that increase incompatibility through altered vehicle design or a changed mix of vehicles in the fleet would have safety consequences.3 Such measures could affect occupant safety as well as pedestrian casualties. SAFETY ISSUES IN IMPROVING FUEL ECONOMY Three major issues are related to efforts to improve fuel economy. The first, the one receiving the most public attention, is the impact of vehicle downsizing or downweighting on safety.4 The second is the question of the increased use of light trucks and their safety characteristics. And the third is the question of how much safety can be increased through improved design and technology. Impact of Downsizing and Downweighting on Safety With enactment of the Energy Policy and Conservation Act in 1975, the U.S. automobile industry moved quickly to improve fuel economy in passenger cars. Among other changes, the average weight and external size of the U.S. fleet moved toward that in other motorized countries. 5 A variety of studies have concluded that the occupants 3 The pickup truck's high rigid bumpers, often exacerbated by after-market modifications, are particularly hostile to occupants of passenger cars. 4 Vehicle size and vehicle weight are highly correlated. They are often used interchangeably, and few studies have attempted to measure the effects of each separately. 5 A corresponding response to the fuel economy crisis was a move to larger, heavier trucks. With the reduction in the maximum speed limit to optimize fuel economy, the trucking industry lobbied successfully for increased cargo capacity (i.e., larger, heavier trucks) in order to remain economically competitive.
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Automotive Fuel Economy: How Far Should We Go? of downsized cars during the late 1970s were at increased risk of injury and fatality in comparison with occupants of larger cars. Although more recent passenger cars have marked improvements in safety, overall differences persist between the safety of smaller and larger automobiles. Some analysts have pointed to these persistent differences and predicted that substantial safety costs will attend further efforts to increase fuel economy. Whether, and to what extent, those claims will hold true cannot be entirely established on the basis of available information. Nonetheless, analyses of historical crash data provide some guidance on the point. Past Studies Most past analyses have focused on passenger cars. Between 1975 and 1990 the average wheelbase (distance between front and rear wheels) in the U.S. new passenger-car fleet, including domestic and imported cars, decreased from 110.9 inches to 101.3 inches, and weight declined by about 900 pounds (NHTSA, undated a). Numerous studies have sought to ascertain the safety consequences of these reductions in size and weight.6 Several analysts have attempted to quantify the relative safety risk for a vehicle's occupants as a function of vehicle size or weight. Robertson and Baker (1976) found that the smaller cars (wheelbase under 105 inches) were involved in three times as many fatal single-vehicle crashes as the largest cars (wheelbase over 120 inches) and twice as many fatal multiple-vehicle crashes. O'Day and Kaplan (1975) reported that smaller cars (compact and sports models) involved a higher risk of fatality than large cars (standard models) for all driver age groups. The relative risk increased with age; drivers over age 55 experienced a threefold risk compared with drivers aged 16 to 24. Mela (1975:1) analyzed crash data from New York and North Carolina and concluded that in two-car crashes, "the chance of serious or fatal injury to an unbelted driver decreased by about five percent for each hundred pounds additional weight in his car and increased about two percent for each hundred pounds increase in the weight of the other car in the collision." Jones and Whitfield (1984) analyzed the effects of belt use and car weight on driver injury in one- and two-car crashes. They found that, "each additional thousand pounds of vehicle mass decreases the odds of a driver injury in a crash by about 34 percent when the driver is not restrained. For restrained drivers, this decrease is 25 percent per thousand (p. 51)." Evans and 6 The early studies focused primarily on unrestrained occupants. Currently, however, more than half of front-seat occupants use safety belts. Moreover, for more recent model cars, with their numerous safety improvements, there is less disparity between the safety of larger and smaller vehicles. (See Robertson and Baker, 1976; Campbell, 1974; Campbell and Reinfurt, 1973; Cerrelli, 1984; Chi et al., 1982; Crandall and Graham, 1988; Evans and Wasielewski, 1987; Joksch and Thoren, 1984; Jones and Whitfield, 1984; Mela, 1975; O'Day and Kaplan, 1975; Partyka, 1990; Partyka and Boehly, 1989; Rihlberg et al., 1964; Robertson and Baker, 1976; Stewart and Stutts, 1978; see also IIHS, 1991c). For example, the occupant fatality rates for 1976-1978 model year (MY) cars were almost 3.5 times as high for cars weighing 2,000 pounds or less compared with cars weighing 4,000 to 4,500 pounds. For MY 1986-1988, the ratio was slightly less than 2.5 (Chelimsky, 1991).
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Automotive Fuel Economy: How Far Should We Go? Wasielewski (1987:119) reported that, "a driver in a 900 kg [1,980 pounds] car crashing head-on into another 900 kg car is about 2.0 times as likely to be fatally injured or killed as is a driver of an 1800 kg [3,960 pounds] car crashing head-on into another 1800 kg car." More recently, Crandall and Graham (1988:22) reviewed a number of studies of automobile safety and estimated that, "the 500 lb [227 kg], or 14 percent, reduction in the average weight of 1985 cars caused by CAFE standards is associated with a 14 to 27 percent increase in occupant fatality risk." Partyka and Boehly (1989:75) reported that the shift to lighter cars between 1980 and 1987 would have resulted in a "5.6 percent increase in the number of moderate and greater injuries to drivers if all other factors remained unchanged." However, they pointed out that other factors, such as vehicle design and use of occupant restraints, have not remained unchanged and must be taken into consideration in arriving at conclusions. In a more recent report, Partyka (1990) found that the fatality rate in minicompact cars was twice that in the largest cars. Recent analyses by NHTSA indicate that the reductions that have occurred in passenger-vehicle size from MY 1970 to 1982 are associated with approximately 2,000 additional occupant fatalities annually (Kahane, 1990; Kahane and Klein, 1991). The additional fatalities identified were in single-vehicle crashes, and approximately two-thirds of them involved rollovers. The primary cause of the rollovers appears to have been the narrower track width of smaller vehicles, a factor that should be resolved independently of fuel economy considerations. In their analyses of two-vehicle crashes, Evans and Frick (1991c:1) reported that, "if a driver transfers to a car lighter by 1 percent that driver's fatality risk in a two-car crash compared to the risk of the other involved driver, increases by between 2.7 percent and 4.3 percent, the specific value depending on other factors such as model year." They further concluded that, "when other factors are equal, (1) the lighter the vehicle, the less risk to other road users, and (2) the heavier the vehicle, the less risk to its occupants." In a second study, Evans and Frick (1991b:1) attempted to calculate the relative risk of driver death in two-car collisions. They concluded that, when cars of the same mass crash into each other, fatality risk is lower when both cars are heavier. If one of the equal cars is replaced by another lighter by any amount, the fatality risk increase to the driver in the car of reduced mass exceeds the fatality risk reduction for the driver in the unchanged car (that is, net risk increases). Net driver fatality risk (or net fatalities) in a car population increases if any car in the population is replaced by a lighter one or if one population of identical cars is replaced by another population of lighter identical cars. Other studies have analyzed overall occupant fatality rates in relation to number of registered vehicles (Ford Motor Company, 1991; IIHS, 1987, 1991b) and reported higher fatality rates for smaller cars. The IIHS analyses were based on MY 1984-1988 vehicles involved in crashes during calendar years 1985-1989. Occupant fatality rates
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Automotive Fuel Economy: How Far Should We Go? for the smaller cars (wheelbase under 95 inches) were approximately twice those of the largest cars (wheelbase over 114 inches). The differential rates were observed for single-vehicle, non-rollover crashes; single-vehicle, rollover crashes; and multivehicle crashes. While downsizing will typically involve downweighting, the use of appropriate lightweight materials would allow a reduction in vehicle weight without a proportionally similar reduction in size. Because reductions in weight promise fuel economy gains, it is of interest whether the downweighting that is not accompanied by downsizing has a safety penalty. Unfortunately, weight and size are highly correlated in historical data, and it is difficult to disentangle any differential effects. Several analyses have concluded that it is vehicle size, as indicated by wheelbase, rather than weight that is the more important (Ford Motor Company, 1991; IIHS, 1991b; Robertson, 1991). But another study, which attempted specifically to separate the effects of changes in wheelbase from changes in vehicle weight (Evans and Frick, 1991a), concluded that, "mass is the dominant causative factor in the large dependence of driver fatality risk on mass in two-car crashes, with size playing at most a secondary role (p. 1)."7 A recent report by the U.S. General Accounting Office (GAO) concluded that automobile weight reductions have not led to increased fatalities (Chelimsky, 1991). Certain assumptions and conclusions in the report are subject to question, however. First, the GAO report concluded that, because the overall number of automobile deaths per 100,000 registered vehicles has gone down since the 1970s and passenger cars have become lighter, the weight reductions "have had virtually no effect on total highway fatalities" (p. 2). As pointed out in IIHS's (1991a) response to this report, analyses of the data indicate that the decrease in the overall death rate would have been even greater if the weight reductions had not occurred. Second, the GAO report assumed that a move toward smaller cars will automatically lead to a decrease in occupant fatalities in two-car crashes because of the "reduced aggressiveness of relatively light cars" (p. 24).8 Some analyses of two-vehicle crashes have shown, however, that at a given speed a collision between two smaller cars results in greater occupant injury than a similar collision between two larger vehicles (Campbell and Reinfurt, 1973; Evans and Wasielewski, 1987). If that is the case, removing large cars from the vehicle fleet does not necessarily remove the 7 The studies noted are based on somewhat different measures. Evans and Frick, Ford, and IIHS all based their analyses on Fatal Accident Reporting System (FARS) data, but Evans and Frick examined two-car crashes, and Ford and IIHS considered all occupant fatalities in passenger cars. Evans and Frick used the relative risk of driver fatality in two-car collisions, while Ford and IIHS considered the occupant fatality rates per 10,000 registered vehicles. Evans and Frick used very broad ranges for vehicle model years and included much older vehicles; Ford and IIHS considered only more recent models and used much narrower vehicle age ranges. Consequently, while the conclusions reached may appear contradictory, it is entirely possible that each analysis is correct, but that each analysis focuses on a different aspect of the issue. See Appendix D for analyses of hypothetical data sets involving the effects of changes in the weight distribution of cars in the fleet on two-car fatalities and a discussion relevant to the issue. 8 Aggressiveness or aggressivity refers to the potential for inflicting damage to the other vehicle and its occupants.
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Automotive Fuel Economy: How Far Should We Go? incremental risk to the occupant of the smaller car (Evans and Frick, 1991b). As noted earlier, however, the difference in occupant injury risk between large car-large car collisions and small car-small car collisions has been reduced in more recent model year cars. Third, the GAO (and other analysts) assumed that drivers in fatal crashes are representative of all drivers. Numerous studies have reported that drivers in fatal crashes are different from other drivers and cannot be considered as representative of the driving population (Baker, 1970; Evans, 1987, 1991; Garretson and Peck, 1982). The GAO report also pointed out that many of the factors that contributed to the greater risk potential of smaller cars in the past do not necessarily have to persist in the future—for example, the greater propensity to roll over. Thus, while the GAO report recognized the possibility of a trade-off between safety and fuel economy, it concluded that such a trade-off may not be necessary, and that at the very least the predicted safety costs that have been claimed by some are greater than would likely occur. A recent report by the Office of Technology Assessment (1991) acknowledged that pressures to achieve significant downsizing in a relatively short time period would likely result in a safety cost. However, the report pointed out that if sufficient time is allowed, and if measures other than downsizing are pursued, it should be possible to maintain safety while improving fuel economy. The report emphasized the importance of maintaining a vehicle's exterior dimensions and structural integrity, as well as providing sufficient interior volume for occupants to survive the ''second crash"—the collision of occupants with the interior of the vehicle. However, the report concluded that reductions in weight need not incur a safety cost, a view not shared by all investigators. The overall conclusion of previous analyses is that the historical changes in the fleet—downsizing and/or downweighting—have been accompanied by increased risk of occupant injury. Not all studies have found completely consistent relationships between weight change and occupant injury, however. The data do show that the difference between the safety of large and small cars has diminished. Data Issues The analysis of safety issues is complicated by a variety of data problems. The use of the FARS data limits analyses to the extreme end of the injury distribution—motor vehicle crashes that result in the death of at least one participant. It is well established that fatal crashes are not representative of the entire spectrum of crashes and do not necessarily provide a valid basis for broad generalizations about crash characteristics.9 9 For example, alcohol consumption is much more likely to be a factor in fatal crashes, somewhat less so in injury crashes, and much less so in crashes resulting in property damage only.
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Automotive Fuel Economy: How Far Should We Go? Because crashes do not appear in FARS unless they result in at least one fatality, there is a potential bias in the calculation of relative fatality risk in two-car crashes. Based on relative fatality rates derived from FARS, Evans and Frick (1991b) reported that greater homogeneity of the vehicle fleet increases fatality risk. This finding could be an artifact of including only crashes in which at least one fatality occurs. To illustrate, in a fatal crash involving a large car and a small car, the probability is greater that the driver of the smaller car will be fatally injured rather than the driver of the larger car. However, in a fatal crash involving two vehicles of equal size (either large or small), the crash severity must be sufficiently great to result in fatal injury to at least one of the drivers in order for the crash to be recorded in FARS. But, if the crash is sufficiently severe to result in fatal injury to one of the drivers, it is more likely to be severe enough to be fatal to the other driver as well, since the two cars, being of nearly equal weight, should experience similar forces.10 Consequently, if only fatal crashes are considered, greater homogeneity in the size of automobiles will appear to exacerbate the consequences of similarity in size. Measures of exposure to risk pose additional problems in interpreting available studies. Some of the analyses by Evans and Frick (1991b) are based on the assumption that, for purposes of estimating vehicle exposure to accidents, one can use the distribution of vehicle weights for those vehicles involved in pedestrian fatalities. Yet this measure of exposure requires the assumption that vehicles involved in pedestrian fatalities are representative of the distribution of the exposure of the entire vehicle fleet. What is known about the times and places in which pedestrian fatalities occur, as well as the populations at highest risk for fatal pedestrian injury, raises serious questions about the validity of such an assumption.11 While some of the analyses attempt to take some account of driver variables, most do not. Drivers in fatal crashes are not a random sample of drivers in general, and hence the relative risk of fatality attributable to vehicle characteristics per se must take into account driver variables and vehicle-use variables. Individual versus Societal Risk It is evident that in two-car crashes, other things being equal, the occupants of the heavier car will fare better than the occupants of the lighter car. However, as has been pointed out by a number of investigators, the heavier car, while affording more protection to its occupants, also inflicts more injury on the occupants of the smaller car. Thus, a reduction in the weight of the larger vehicle, while perhaps increasing the risk 10 Obviously, the fatality risk is also affected by other factors, such as use of occupant restraints, crash geometry, and driver age. 11 Pedestrian fatalities involve three major groups—children, the elderly, and the alcohol impaired. Using the distribution of car sizes involved in pedestrian fatalities as a measure of exposure of all cars requires two major assumptions. The first is that the exposure of fatally injured pedestrians is representative of the exposure distribution of all car sizes. The second is that once a pedestrian is hit, there is no effect of car size on the probability of fatal injury. Available data regarding the second assumption indicate that smaller cars are less likely to result in fatal injury to a struck pedestrian (e.g., Robertson and Baker, 1976).
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Automotive Fuel Economy: How Far Should We Go? for its occupants, carries some benefit for the occupants of the other vehicle. In the case of two-car collisions, it is not clear how these two opposing effects are balanced, at least in the current fleet. Although in the past it has been shown that small car-small car collisions are more injurious than large car-large car collisions (Campbell and Reinfurt, 1973; Evans and Wasielewski, 1987), the difference has diminished over time. Earlier analyses suggested that the safety gains associated with increased vehicle weight were greater than the safety losses associated with the greater aggressivity of heavier cars in collision with other vehicles (Mela, 1975). However, early analyses were based on the assumption that most occupants were unrestrained. Greater use of occupant restraints and improved vehicle design have enhanced occupant safety in more recent model years in ways that could affect the outcome of collisions. In any event, it is important to distinguish individual risk from societal risk. Estimates of societal risk must take into account the net effects of the safety gains to the occupants of the heavier car and the safety losses that the increased weight imposes on the occupants of the struck car, as well as other road users (e.g., pedestrians, pedalcyclists, and motorcyclists). Whether, and how much, total societal risk is modified by shifts in the mix of vehicle weights is not clear. Based on consideration of two-car collisions using hypothetical estimates of accident variables, Appendix D considers the change in fatalities from various changes in distribution of vehicle sizes in the fleet. 12 The appendix suggests that, in principle, downsizing could increase, decrease, or leave unchanged total deaths and injuries in two-car collisions, depending on the changed size distribution of cars in the fleet. The more aggressive large cars are relative to small and midsize cars, the more beneficial it will be to get large cars off the road. The less crashworthy small cars are relative to larger cars in collisions with cars of equal size, the more beneficial it will be to get small cars off the road. These analyses do not consider other types of crashes—single vehicle into roadside obstacles, passenger car-truck collisions, and collisions of cars and light trucks with pedestrians and cyclists.13 Moreover, additional analyses are needed, based on more recent model years and actual data, to reflect improved vehicle design and increased use of occupant restraint systems. Presumably, crashes into anything but a rigid barrier would still favor the occupant of the large vehicle, but nonetheless, the shifts in total societal costs that may be anticipated from downsizing passenger vehicles are not clearly established.14 Light Trucks While the reasons for the shift to light trucks are not clear and may have little to do with fuel economy, their increased usage raises concerns about both safety and fuel 12 Appendix D contains a set of sample calculations illustrating this point. 13 Analysts have suggested that the size of passenger vehicles is related to the probability of fatality even in crashes with large trucks (e.g., Cerrelli, 1984). 14 The National Highway Traffic Safety Administration declined the committee's request that NHTSA conduct a fuller assessment of this issue. Letter from Administrator J.R. Curry, NHTSA, to R.A. Meserve, chairman, Committee on Fuel Economy of Automobiles and Light Trucks, November 25, 1991.
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Automotive Fuel Economy: How Far Should We Go? economy. Light trucks are clearly more likely to roll over than automobiles because their design dictates a higher center of gravity than that for passenger cars.15 Although the findings from a NHTSA crash-testing program clearly show that recent model light trucks are safer than their earlier counterparts, they still have poorer safety performance than passenger cars.16 The rapid expansion of light trucks as a proportion of market share has not been accompanied by a clear understanding of their use. They now represent close to one-third of the market, and according to the industry, about 70 percent are purchased for personal rather than commercial use. In effect, light trucks are being used as a substitute for the passenger car. In the past, however, light trucks have not been subject to the same standards as passenger cars for safety (or fuel economy), nor have the safety standards for light trucks been established in the same manner. Because they have become such a significant portion of the passenger-vehicle fleet, more aggressive efforts to improve the safety and fuel economy of light trucks should be taken. Otherwise, any gains made through improvements in passenger cars may be offset by the increased use of light trucks. Potential for Enhancing Safety through Design and Technology Just as estimates of fuel economy improvements are based in part on what is technologically feasible, so too safety should be evaluated with an eye to technological opportunities. For vehicle occupants, future benefits will come from the application of new technologies for crash avoidance and for occupant protection once a collision occurs. Some safety improvements are already scheduled to be implemented, others are being seriously considered, and still others will occur because of market pressures. Safety modifications are expected to add somewhat to vehicle weight and thus result in some reduction in fuel economy levels. The safety standards already scheduled for implementation include expanded installation of automatic restraints and improved side- and head-impact protection. Additional padding of vehicle interiors may also be required. Antilock brake systems, although not mandated by federal standards, will likely be incorporated into most of the vehicle fleet over the next several years. Based 15 There are anomalies in the data, however. The inherent stability of minivans is much lower than that of passenger cars, which suggests that minivans should have a higher rate of rollover than automobiles, but the actual rollover experience is much lower than that for automobiles. Presumably, the explanation lies in the types of drivers of vans and the types of trips they make (IIHS, 1987). 16 The National Highway Traffic Safety Administration operates the New Car Assessment Program (NCAP), in which vehicles undergo a standard test crash into a rigid barrier with belted and unbelted dummies. Measurements are made of injury potential, including head injury criteria (HIC) and chest accelerations (chest Gs). The lower the measurements, the less the potential for injury. Light trucks were not included in NCAP testing until MY 1983, and the number of such vehicles tested to date is small (N = 41 as of 1989). Analyses indicate, however, that the measurements for the light-truck fleet are approximately 20 percent higher (less safe) on HIC and 10 percent higher on chest Gs than for passenger cars. When light trucks are further subdivided into light-duty trucks, vans, and multipurpose vehicles, the number within a test category becomes very small. Nevertheless, it is clear that the vans are less safe from the viewpoint of the driver (Hackney et al., 1989).
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Automotive Fuel Economy: How Far Should We Go? on the proportion of the new vehicle fleet that would be affected by these changes, there would be an estimated increase of 67 pounds over the fleet's average weight for MY 1990 passenger cars. Similar anticipated changes in light trucks would result in an estimated average weight increase of 133 to 167 pounds above 1991 models. These anticipated weight increases apply to safety improvements only; they do not take into consideration any weight increases associated with technology required for improved emissions control (Bischoff, 1991). The National Highway Traffic Safety Administration has issued a notice of proposed rulemaking to address the rollover stability of passenger cars and light trucks (Federal Register, 1992). Several options are being discussed, including increasing the stability of the vehicle, improving the crashworthiness of the vehicle if certain levels of stability are not achieved, and providing information on rollover risks to consumers to enable them to make more informed choices. Within each of these categories of action, a variety of options are being considered, as well as possible combinations of the various options. In light of the uncertainty as to the eventual regulation, no attempt has been made here to estimate the weight implications of improvements in rollover stability. It is also anticipated that increased use of antilock brake systems may reduce the incidence of rollover crashes. Rollovers are usually initiated by a lateral component of movement of the vehicle, and antilock brake systems improve the stability of the vehicle in braking situations, thereby reducing the probability of rollover. As noted, roughly two-thirds of the increased risk of fatality attributed by NHTSA to the downweighting of cars results from rollover (Kahane and Klein, 1991). The incorporation of antilock brake systems into the vehicle fleet might gradually eliminate some of the additional fatalities attributable to downweighting. As noted, light trucks are not currently subject to the same safety standards as passenger cars. Given the increased use of these vehicles as passenger vehicles (as opposed to cargo carriers), NHTSA is implementing a program of rulemaking that broadly requires the same crash performance for both classes of vehicle. In particular, the extension of occupant crash-protection requirements and the application of mandatory safety belt use laws to occupants of light trucks are likely to diminish the risk to occupants of light trucks and to reduce fatalities (Kee, 1991). Improvements in vehicle handling, stability, and brakes should lead to significant safety gains for utility vehicles. From the standpoint of societal costs, attention must be given to the lack of design compatibility between light trucks and passenger cars—that is, the geometrical and structural mismatches discussed above. Proposals have been formulated, but the rulemaking process has not yet been initiated on this important safety issue. Future technologies hold promise for crash avoidance. The use of on-board systems to provide drivers with information not only about vehicle conditions (e.g., underinflation of tires, impending equipment failures) but also about upcoming traffic hazards can enable drivers to avoid otherwise high-risk situations. Intelligent vehicle highway systems include crash avoidance as a major objective.
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Automotive Fuel Economy: How Far Should We Go? Once a collision occurs, improved occupant protection can be achieved through advanced restraint systems. Driver- and passenger-side airbags are currently entering the fleet of cars and light trucks. Advanced safety belts with pre-tensioning and loadlimiting capabilities will enhance protection, particularly for the more vulnerable segments of the population (e.g., the elderly). Side-impact airbags are under development. The greater acceptance of mandatory safety belt use laws and their extension to all occupants, in front and rear seats, offer significant crash-protection gains. Better understanding of the behavior of materials subject to impact, through the use of analytic techniques, high-speed computers, and the techniques of computer-aided design, now allow bodyshell designs to be optimized much more efficiently. Thus, technology offers the promise of better compatibility between vehicles of varying weight for varying crash configurations and speeds. Because safety is becoming increasingly important to the public, it is likely that over the next decade the automotive industry will move beyond the requirements imposed by federal regulations and will offer enhanced safety as a positive marketing attribute. Safety adds value to the product, and the public's growing recognition and appreciation of safety features may well result in greater safety gains than predicted from future regulatory activity alone. STRATEGIES FOR IMPROVING SAFETY AND FUEL ECONOMY A number of measures can improve both safety and fuel economy. They can be categorized into three major areas: control of exposure to accidents, crash avoidance, and behavior modification (Trinca et al., 1989). Exposure Control To the extent that exposure (i.e., VMT) is reduced, safety and fuel economy can be enhanced.17 Current examples of measures to reduce exposure include substitution of electronic for physical communication, the provision of low-risk modes of travel (e.g., urban bus networks), and zoning of urban development to diminish commuting distances. Indeed, because discretionary driving often presents the greatest risk, reduction in VMT may offer disproportionate safety benefits (Transportation Research Board, 1984). However, it should be noted that in the United States, efforts to reduce VMT have been singularly unsuccessful—VMT has increased more rapidly than the population of licensed drivers (Highway Statistics, 1987-1990). 17 The primary purpose of the highway system is mobility, hence exposure. All motor vehicle injury could be prevented if exposure were reduced to zero. Thus, for exposure control to be an effective strategy, it must fulfil the purpose of travel while reducing it.
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Automotive Fuel Economy: How Far Should We Go? Crash Prevention Collisions not only increase the risk of injury, but also contribute to fuel consumption because of the traffic disruptions they cause. Improved highway design and maintenance, as well as improved traffic operations, contribute to safety and fuel economy. While fuel costs increase with road roughness, other operating costs, such as maintenance and repair, increase much more markedly (Wyatt et al., 1979). The potential safety benefits of improved pavement maintenance are reviewed in a summary by Cleveland (1987). Effective traffic management through computerized control of traffic signal networks can optimize traffic flow and minimize speed variance and delays, thereby reducing fuel consumption as well as conflicts and the incidence of collisions. Improved vehicle design can also contribute to crash avoidance. Daylight running lights, centered high-mounted brake lights, improved truck brakes, and antilock brake systems are examples of such improvements. Behavior Modification Driver behavior affects both safety and fuel economy in that fuel-efficient driving is associated with safer performance. An example is the 55-mph National Maximum Speed Limit (NMSL), a measure that was enacted primarily to increase fuel economy. The measure led to significant fuel savings, but the safety benefits were striking as well (National Research Council, 1984). SAFETY AS A SOCIETAL VALUE Safety is only one of many societal values that are considered in connection with automotive transportation. Traditionally it has been assumed that "safety does not sell," but recent trends in consumer preference for passenger cars with airbags and for vehicles with good safety records suggest a major shift in the public's concern for safety.18 Yet the fact remains that the highway transportation system was developed primarily for purposes of mobility, not safety. Decisions must be made regarding the system trade-offs between safety and other values. Currently, society accepts approximately 45,000 deaths and several million disabling injuries annually as the safety costs for highway transportation in the United States. The public supported the lifting of the 55-mph speed limit on rural interstate highways, although analyses indicated that it would cost several hundred lives annually; subsequent studies have confirmed those predictions (e.g., Baum et al., 1990; National Research Council, 1984; Wagenaar et al., 1990). Right-turn-on-red was promoted in some states as a fuel economy measure, although the evidence suggests it saves little fuel and also results in an increase in pedestrian deaths (Preusser et al., 1982; Zador 18 Ford Motor Company, in its presentation to the committee's Impacts Subgroup on September 16, 1991, provided survey data indicating the rising importance of safety to car buyers between 1980 and 1990.
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Automotive Fuel Economy: How Far Should We Go? et al., 1982). The use of motorcycles on public highways is associated with greatly increased crash rates per VMT, as well as extremely high rates of serious injury. The licensing of very young and very old drivers is another area in which safety costs are paid in exchange for mobility. It may be inevitable that significant increases in fuel economy can occur only with some negative safety consequences. Should that be the case, it does not necessarily follow that increased fuel economy should automatically be rejected. Rather, the choice should be made on the basis of the most complete information available and an evaluation of the relative cost and the relative benefit of the change. FINDINGS AND CONCLUSIONS Some maintain that significant increases in fuel economy will require vehicle downweighting which, in turn, will increase injury and death. Others claim that these impacts can be avoided. The fact that both sides are able to make credible arguments attests to the ambiguity of the evidence available and the complications in attempting to forecast the impacts of future changes in vehicle characteristics. Although there are no conclusive answers to the question of whether, and to what extent, safety may be compromised by improvements in fuel economy, the following points may be made: Safety, as measured by fatalities per hundred million VMT, has steadily improved since 1930, and it is likely that this general trend will continue. In evaluating the safety consequences of fuel economy measures requiring vehicle modifications, this overall trend must be taken into consideration. Otherwise, safety improvements are likely to be erroneously attributed to changes that are unrelated or even detrimental to safety. Numerous studies conducted over the past 30 years show there is a relationship between reduced vehicle size/weight and increased occupant fatality risk, but the difference in safety between large and small cars for recent model years has diminished. Significant improvements have been made in small and large cars, although proportionately greater improvement has been made in small cars, a result that might be anticipated given that small cars had considerably more room for improvement. Changes in design or the application of technology, such as a slight increase in track width or the use of antilock brake systems, can differentially reduce the rollover tendency of smaller cars. These changes, however, could increase vehicle weight, with some resulting fuel economy penalty. In certain crash types—for example, a collision with a nonrigid fixed object—vehicle weight is a major determinant of the forces experienced by the vehicle occupants. Any major reduction in vehicle weight carries the potential for reduced safety in such collisions. Consequently, any such weight reductions must be accompanied by measures to maintain safety through improved vehicle design or other means.
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Automotive Fuel Economy: How Far Should We Go? While decreased vehicle weight may increase risk in collisions with nonrigid fixed objects, lighter weight vehicles will pose less hazard to occupants of the other vehicle in two-vehicle collisions. Thus, in such collisions, vehicle weight reductions have opposing effects; they decrease the safety of the occupant of the lighter weight vehicle, but increase the safety of occupants of the struck vehicle. It is not clear to what extent these two opposing effects offset each other—that is, the extent to which changes in vehicle weight will have an impact on overall societal costs incurred in two-car collisions. Those studies specifically examining such changes conclude that there is an increase in overall risk with downsizing of the passenger-car fleet. Currently scheduled and anticipated vehicle safety modifications—for example, passenger-side airbags, improved side-impact protection, antilock brake systems—will enhance safety at the cost of increased vehicle weight and thus reduce fuel economy. Improved design and the incorporation of new technology, however, can enhance both crash avoidance and crashworthiness potential, while improving fuel efficiency. Light trucks have special safety problems, and because they constitute almost one-third of the new vehicle fleet, they warrant careful scrutiny. Moreover, light trucks are particularly aggressive to passenger cars in passenger car-light truck collisions. Some measures to reduce such aggressivity—for example, lowering bumpers—should not affect fuel economy. Reducing exposure, improving crash avoidance, and increasing driving efficiency improve both safety and fuel economy. These measures should be pursued independently of technological measures to increase fuel economy. There is likely to be a safety cost if downweighting is used to improve fuel economy (all else being equal), although the available information is insufficient to make a specific estimation of the impact. Safety is an important consideration in fuel economy deliberations, but it must be considered in relation to other important societal values that are affected by improved fuel economy. Concern for safety alone should not be allowed to paralyze the debate on the desirability of enhancing the fuel economy of the fleet. Because of the importance of the safety issue, coupled with the equivocal interpretation of existing and projected data, a comprehensive study of the effects of vehicle weight and size on safety should be conducted and should examine the full range of crash severity and crash types. While such a study will not provide definitive answers regarding the future safety performance of the vehicle fleet, it should be valuable in clarifying the dimensions of the safety aspects of measures to improve fuel economy in automobiles and light trucks.
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Representative terms from entire chapter: