Links and Updates: Chapter 1

Links and Updates: Chapter 9

No Internet links were published in this chapter.

Update of Appendix 9-1

Vehicle Factors That Prevent Motor Vehicle Fatalities (Adapted from Leon S. Robertson, Prevention of motor-vehicle deaths by changing vehicle factors. Injury Prevention 13:307-310, 2007).

Recent research suggests that a few currently available motor vehicle features would prevent the majority of mortality associated with motor vehicles, if adopted for all vehicles. Electronic stability control (ESC) automatically adjusts braking, throttle or suspension to reduce the likelihood of loss of control of the vehicle. It is estimated to reduce fatalities by about 42 percent.1 Failure to obtain the highest ratings on 40-mile-per-hour offset crash tests 2 is associated with a 25 percent excess deaths and unnecessary weight contributes as much as 28 percent.3 Since changing one or more of the vehicle attributes would likely prevent some of the deaths attributed to others, the percentages cited cannot be added to get a total estimate.

The purpose of this study is to estimate the effect of these and other vehicle factors (side impact crashworthiness, static stability, braking distance from 60 mph to 0, and 0-60 mph acceleration time), each adjusted for the effect of the others, in a comprehensive analysis of preventable motor vehicle mortality. Static stability is the distance between the centers of the tires divided by twice the height of center of gravity (T/2H), a factor that increases risk of rollover when below 1.2.4 I also analyzed the potential for confounding of results by environmental and behavioral factors.

Methods

I selected passenger cars, minivans and “sport utility vehicles” (SUVs) sold from the beginning of the new 1999 model year (beginning in October, 1998) through September, 2005 for which data were available on the mentioned vehicle characteristics. I excluded pickup trucks because their weights and other characteristics vary considerably within make-model designations. If a vehicle was redesigned during the study period, it was treated separately as a new model. In those cases where ESC was added in a given model year without other changes, the vehicle was designated as a new model.

I counted deaths during the years 2000-2005 for each vehicle make-model designation and obtained data on environmental and behavioral factors from the Fatality Analysis Reporting System that contains data on virtually every fatal crash in the U.S. To account for differential exposure, I estimated years of vehicle use by multiplying the monthly sales of a given make and model by years remaining during 2000-2005, discounted by subtracting the estimated percentage scrapped as the vehicles aged.5 One hundred fourteen make-models with more than 100,000 years of use each were selected for analysis. These vehicles were involved in 25,367 crash-related deaths to their occupants or bicyclists and pedestrians.

Data on ESC availability and crash test results by make and model were obtained from the website of the Insurance Institute for Highway Safety. 6 Vehicle specifications and the results of the government’s front and side crash tests were obtained from a vehicle information website.7 Because real-world crashes seldom involve the full front of the vehicle, the Insurance Institute for Highway Safety conducts frontal offset crash tests at 40 mph into a fixed barrier with a 40 percent overlap of the barrier and the driver side of the vehicle. It assigns qualitative ratings of “good”, “acceptable”, “marginal” and “poor” to various aspects of performance on its offset frontal crash tests. I assigned weights of 1 (good) through 4 (poor) to the ratings of four life-threatening elements of the tests – structural integrity, forces on the heads and, separately, the chests of test dummies, and performance of seat belts and air bags. These were averaged as an index of frontal offset crashworthiness.

The U.S. government tests vehicles in full-frontal barrier crash tests at 35 miles per hour and collects data on head and chest injury criteria as well as other body sites. Since head and chest injuries are the most threatening to life, the injury criteria relevant to these injuries were considered in the analysis. The government also tests side crashworthiness by impacting the sides of vehicles with a 3015-pound barrier at 38.5 miles per hour, with “give” in the barrier to simulate the front of a vehicle. Injury criteria measured on driver and passenger test dummies were included in the analysis. Because about 70 percent of occupant deaths occur to drivers, I weighted the injury criteria as 0.7 times driver side plus 0.3 times passenger side when assessing all deaths. When assessing driver deaths, I used the driver side injury criteria. I obtained static stability data from U.S. government measurements 8 as well as the vehicle information website. I classified a vehicle as stable if T/2H was 1.2 or higher.

I obtained data on braking distance from 60 miles per hour to 0 and acceleration time from 0 to 60 miles per hour from the Consumer’s Union road-test data.9 I analyzed the data using least-squares correlation and logistic regression.

Results

Logistic regression estimates the odds of an event, in this case death, as a function of specific factors that are assumed to be independent, that is, not significantly correlated. Neither the presence of electronic stability control nor crash test results were correlated significantly to the other factors. Correlations that could bias the assessment of vehicle weight, engine power, size, static stability and braking are displayed in Table 1.

Although excess weight and horsepower is adverse to other road users, size is related to lower risk because it gives occupants more room to decelerate in a crash.10 The weight, horsepower and size variables (wheelbase and turn distance) are correlated to a degree that using more than one could bias the estimates. Because poor fuel economy is highly correlated with these variables, particularly weight and horsepower, and is an important consideration in vehicle purchases, it was chosen as an inverse proxy of weight-power. Braking distance, acceleration distance and static stability are sufficiently independent of one another and the other factors to be used in the regression analysis. The analysis also controlled for types of vehicle (minivan, SUV) because of their differential use compared to cars.

Preliminary analysis indicated that the head and chest injury criteria in the government’s full frontal crash tests and braking distance are not significant factors in relation to odds of mortality, controlling for the other factors. These variables were dropped from the analysis. The logistic regression coefficients of the remaining factors and their 95 percent confidence intervals are presented in Table 2, separately for deaths to all road users, driver deaths and deaths to pedestrians and bicyclists. Lower risk of all deaths is associated with the presence of ESC, particularly as standard equipment, good performance on the offset frontal and side crash tests, static stability of 1.2 or higher, and faster acceleration from 0 to 60 miles per hour. Drivers have lower risk of death when fuel economy is lower but the correlation reverses for all deaths – particularly pedestrian and bicyclist deaths. Vans and SUVs have lower overall death rates when the other factors are controlled.

I calculated the reduction in deaths achievable by changing a given vehicle characteristic as other characteristics remained the same by substituting the value of a given variable in the regression equation for total deaths, applying the rate to the number of vehicles in use for each vehicle, subtracting the result from the actual total deaths, and summing the result across the vehicles. If all vehicles were equipped with ESC, the estimated death reduction would be 11,098, about 42 percent of the total. If all of the vehicles averaged one on the offset frontal crash test index, there would have been approximately 2211 fewer deaths, 8.6 percent of the total. If the vehicles that had injury criteria above average on the side crash tests were improved to the average, 4950 (19.4 percent) deaths would have been prevented. A static stability of 1.2 or higher among vehicles with lower stability would have prevented 2737 deaths, 10.7 percent of the total deaths.

The effects of weight-power, reflected by fuel economy, and acceleration time was much less. If the weight and horsepower of all vehicles that had less than average fuel economy (28.4 miles per gallon) were changed to the average, the death reduction would be 492, 1.9 percent of the total. Achieving average acceleration time (9.4 seconds) for those with more would result in 495 fewer deaths, 1.9 percent of the total. The percentages add to an 85 percent potential reduction in deaths if all vehicles had the best of the mentioned characteristics.

For environmental or behavioral factors to confound these results, they would have to be correlated substantially with the vehicle factors. Since there are no data on the exposure to environmental and behavioral factors by make/model of vehicles, the potential for confounding must be assessed indirectly. If there were potential confounders among major known risk factors, they would be revealed by the correlation of ratios of lower to higher risk in the fatal crashes. Formally, C (L/H) = RL/RH = b(vehicle factor), where

L=low exposure to a risk factor

H=high exposure to a risk factor

C=constant ratio of risk from lower to higher

RL=fatalities in lower risk factor situations

RH=fatalities in high risk factor situations

b=the slope of the correlation.11

Table 3 contains the correlations of the ratios of lower to higher risk environmental and behavioral factors relative to the vehicle characteristics and equipment. Almost all of the correlations are low and are not consistently in the direction of confounding. The two large correlations are opposite from what one would expect if there were confounding. Vehicles with poor scores in side crash tests are more involved in urban areas where risk of fatalities is lower than in rural areas but the specific risk of a side crash at an intersection is higher than in rural areas. The correlation does not suggest confounding but increases confidence in the specification of the effect of side crashworthiness. Drivers of vehicles equipped with ESC are somewhat less likely to have a valid drivers license, the opposite expected from confounding.

Discussion

When the effect of each factor is corrected for the effect of the others, the estimated effect of electronic stability control is similar to the estimate from the cited research comparing vehicles of the same make-model before and after adoption of the technology. The effects of “good” scores on offset crash tests and power-weight reflected by fuel economy are less than expected from previous research. Apparently, ESC would prevent some of the deaths formerly attributed to the other factors.

Electronic stability control is the most important innovation in reduction of vehicle-related mortality in decades, perhaps the single most effective innovation since the invention of seat belts. If all vehicle purchasers bought only vehicles with ESC and good offset frontal and side crash test ratings, deaths would be reduced by more than half after the older vehicles were scrapped. A list of 2007 and subsequent model vehicles that have the ESC system, as well as the top scores on crash tests and good static stability, will be updated on the Internet as new models are tested.12 Although pickup trucks were not included for technical reasons, the results should apply to them as well. Pickups as a class have higher deaths rates than passenger cars, vans and SUVs. Few pickups on the market in the U.S. have ESC or do well on crash tests.

The effect of low static stability is substantial despite the effect of ESC. Apparently the installation of ESC does not negate the need to achieve a minimum static stability of 1.2 or higher.

A surprise in the results is the lack of effect of braking distance. Since ESC works by selectively applying brakes to wheels and could account for some of the same variance, a regression of the other factors and braking distance was done excluding vehicles with standard or optional ESC. No effect of braking distance was found among these vehicles either. The measurement of braking distance is somewhat subjective, dependent on the ability of the test driver to apply brakes fully while controlling the vehicle. It may not be possible to obtain an objective measure of braking distance that is applicable to drivers in panic situations.

Despite the evidence that vehicles with higher weight-power and lower fuel economy contribute to excess total deaths, the Insurance Institute for Highway Safety continues to promote such vehicles based solely on driver death rates with no consideration of the net losses related to weight-power. The Institute calculates only driver death rates to obtain a rate with known exposure because every vehicle in motion has a driver but the number of passengers may vary among vehicle make-models. 13 While driver death rates are lower in vehicles with more weight power, their excess involvement in bicyclist and pedestrian deaths more than offsets the advantage to drivers and occupants in such vehicles. In addition, heavy vehicles are over involved in deaths to children backed over in home driveways, deaths that are not reported in FARS because they do not happen on public roads.14 They dump more carcinogenic polycylic aeromatic hydrocarbons and greenhouse gasses in the environment and deplete oil supplies. If vehicles have ESC and perform well on crash tests, there is little advantage in risk reduction to drivers who select a vehicle based on heavier weight and far more harm to others. Vehicles that are too small to protect occupants do poorly on crash tests and can be avoided on that basis.

The major threat to the validity of the conclusions of this study is the potential selectivity by risk conscious vehicle buyers who select vehicles based on crashworthiness tests and ESC. The lack of correlation of the major known behavioral risk factors with vehicle characteristics suggest that such selectivity did not occur to the extent that selectivity is manifested in well-known indicators of relative risks among drivers. Seat belt use is not included in the study because police and occupant reports of belt use in crashes have proved unreliable when crash recorder data are compared to reported belt use.15 Nonuse of belts is highly correlated to illegal alcohol concentrations among drivers.16 The lack of correlation of alcohol and the vehicle characteristics studied here suggest that there is no systematic choice of less safe vehicles by higher risk drivers.

The significant correlation of reductions in pedestrian and bicyclist deaths with crash test results suggests some degree of selectivity in buying vehicles that do well on crash tests by drivers less likely to hit other road users or drive in environments where there is less exposure to pedestrians and bicyclists. There is no reason to expect that front and side crashworthiness would reduce pedestrian and bicyclists deaths. Yet, when the regression results on other road users are used to estimate death reductions, pedestrian and bicyclists are 22.5 percent of the reduction in deaths attributed to good offset frontal crash tests and 9.4 percent attributed to better than average side protection. Even if 25 percent of the effects of each of the vehicle factors is attributable to selectivity, however, total deaths would have been 64 percent lower if each of the vehicles met the criteria mentioned on each factor.

This is a study of the effects of preventive measures, not causation. When such research is reported, vehicle manufacturers and others often comment that the main cause of vehicle crashes is behavior. The inference in such comments is that preventive efforts should be directed at the major causes. In fact, changing only necessary conditions for harmful results substantially prevents a variety of diseases and injuries.17 For example, the lack of barriers on windows in high-rise buildings does not cause children to crawl out of windows but the presence of barriers prevents them from doing so.18 A simple barrier negates the effect of the diverse causes of parental inattention that results in lack of supervision of the child. Similarly, numerous factors contribute to drivers losing control of their vehicles. ESC detects when the vehicle is nearing loss of control and adjusts throttle, braking and suspension accordingly. While changing vehicles does not preclude efforts to change behavior, the results of this study indicate that a substantial majority of vehicle-related deaths can be prevented by full adoption of changes in vehicle characteristics that are preventative, whatever the complex mix of factors that lead to serious crashes.

Table 1. Least-squares correlation of selected vehicle factors

Weight Wheelbase Turn Braking Horsepower Acceleration Static

Distance Distance distance Stability

Weight 1.00

Wheelbase 0.64 1.00

Turn 0.60 0.65 1.00

Distance

Brake 0.20 0.17 0.06 1.00

distance

Horsepower 0.70 0.59 0.50 0.04 1.00

Accel. -.22 -0.19 -0.25 0.24 -0.50 1.00

distance

T2H -0.34 0.02 0.15 -0.18 -0.06 -0.14 1.00

Fuel -0.76 -0.44 -0.39 -0.21 -0.65 0.10 0.47

economy

Table 2. Logistic regression estimates of the preventive effects of vehicle factors

All road users Drivers Pedestrians and

Bicyclists

Coefficient 95% CI Coefficient 95% CI Coefficient 95% CI

Intercept 0.2468 -1.7855 -2.9905

ESC standard -0.5764 -0.669 -0.484 -0.7806 -0.943 -0.618 -0.4813 -0.617 -0.345

ESC optional -0.2551 -0.297 -0.213 -0.3304 -0.397 -0.264 -0.2014 -0.270 -0.133

Front crash test 0.1329 0.105 0.161 0.3758 0.339 0.413 0.0928 0.045 0.141

Side Crash test 0.0111 0.010 0.012 0.0090 0.007 0.011 0.0030 0.001 0.005

T/2H<1.2,

else 1.2 -8.7614 -9.559 -7.964 -8.258 -9.532 -6.985 -6.0460 -7.313 -4.779

Acceleration 0.0477 0.034 0.062 0.0687 0.048 0.089 0.0483 0.024 0.073

Fuel economy -0.1100 -0.114 -0.106 0.0036 -0.002 0.009 -0.0260 -0.032 -0.020

Van -0.3266 -0.394 -0.259 -0.9167 -1.034 -0.799 -0.3283 -0.435 -0.221

SUV -0.2994 -0.369 -0.230 -0.5262 -0.633 -0.419 -0.3548 -0.465 -0.244

_________________________________________________________________________________________

Table 3. Correlation of vehicle characteristics, environmental and behavioral variables

Front Crash Static ESC ESC Side Acceleration Fuel

Test stability optional standard Crash Time Ecnomy

Test

Environment

Urban/Rural 0.035 0.292 0.081 -0.200 0.602 -0.180 0.206

Interstate/other -0.159 0.178 0.285 0.300 -0.036 -0.291 -0.086

Onroad/Off -0.194 0.013 0.005 -0.030 -0.268 0.050 -0.227

3+ lanes/2 lanes -0.160 0.189 0.033 0.386 -0.102 -0.121 -0.204

Speed limit<55/

55+ -0.136 0.126 0.086 0.324 -0.092 -0.193 -0.128

Straight/Curve -0.174 0.240 0.159 0.319 -0.025 -0.254 -0.123

Level/grade -0.160 0.044 -0.112 0.168 -0.064 -0.016 -0.232

Concrete/Blacktop -0.220 0.152 0.122 -0.032 0.101 -0.027 -0.141

Dry/Wet -0.141 0.062 0.193 -0.081 0.069 -0.003 -0.051

Daylight/other -0.132 0.001 0.073 0.228 0.001 -0.112 -0.193

Behavior

Valid License/

Other 0.202 0.349 -0.371 -0.572 -0.209 -0.148 -0.206

No prior crash/1+ -0.046 0.050 0.133 0.199 0.100 0.149 -0.048

No prior

Suspension/1+ 0.115 -0.034 -0.087 -0.007 -0.056 -0.074 -0.189

No prior DWI/1+ 0.002 0.040 0.089 0.150 0.102 0.103 -0.170

No prior

Speeding/1+ 0.145 -0.020 -0.044 0.045 0.133 0.073 -0.248

No other

Conviction/1+ 0.017 0.017 0.063 0.167 0.170 0.152 -0.215

No blood

alcohol/.01+ 0.046 -0.015 -0.042 0.147 0.126 0.097 -0.211

No illegal blood

Alcohol/1+ 0.028 0.037 -0.024 0.101 0.133 0.115 -0.239

Age 25+/<25 0.028 0.029 0.051 0.171 0.198 0.110 -0.173

Women/Men -0.043 0.056 0.181 0.294 0.296 0.234 -0.127

Regerences