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Consumer Alert:

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     Fatality rates for some vehicles are more than 20x higher than for others.  Consumers, determined to select a "safe" vehicle, and who seek guidance from NHTSA and IIHS crash-test ratings, often find it very difficult to utilize these resources effectively, primarily for the following reasons:
 

(1) Lack of risk apportionment based on the relative importance of each rating category. For example, IIHS presents ratings for frontal, side and rear impact, however 38% of fatalities are associated with frontal impact vs. 26% for side, vs. only 3% for rear. The consumer must judge how to compare vehicles, such as one with "Good" frontal + "Poor" rear impact ratings vs. one with "Acceptable" frontal + "Marginal" rear impact ratings.

(2) Omission by both agencies to factor into their ratings the significant role of vehicle weight in multi-vehicle collisions. Although both agencies warn consumers not to compare frontal crash test results between vehicles varying more than 250 lbs. they do not provide guidance on how you should consider weight.

(3) Both NHTSA and IIHS lump vehicles into very broad rating categories --- for example 65% of '08 vehicles received the best possible frontal ratings by NHTSA (5-star) and 83% received “GOOD” by IIHS. This incorrectly portrays all of these vehicles to be equivalently safe.

(4) Virtually no recognition by either agency that the other agency exists, despite incomplete ratings categories by both agencies. For example, IIHS does not evaluate rollover resistance (NHTSA does); NHTSA does not evaluate head injury risk in their side impact testing (IIHS does). Also, since testing methods differ, ratings between NHTSA vs. IIHS appear inconsistent. For example, 5 model-year '08 vehicles rated 5-stars for side impact by NHTSA received “POOR” side impact ratings by IIHS.

 

     The SCORE (Statistical Combination Of Risk Elements) was created to address these deficiencies by utilizing a mathematical model to combine, on a fatality-weighted basis, all available, quantifiable, validated data published by NHTSA and IIHS, to derive a single, overall measure of fatality risk.
 

The SCORE addresses all 4 deficiencies:

 

(1) In order to assign the appropriate emphasis for each category of rating (frontal, rollover, side, and rear), the SCORE combines ratings in proportion to their total fatalities (38% frontal: 33% rollover: 26% side: 3% rear).

(2) IIHS publishes a regression analysis showing the correlation between vehicle weight vs. fatality rate in multi-vehicle collisions. Their analysis shows, for example, that a 2000 lb. passenger car has 2x the fatality rate vs. a 3300 lb. car (average passenger car weight), in multi-vehicle frontal collisions. The SCORE combines the fatality risk due to vehicle weight with the fatality risk measured by frontal crash testing, to derive the total frontal risk, which enables comparison between vehicles in different weight classes. For example, if due to weight alone, a light-weight vehicle has 2x the fatality rate vs. the average weight vehicle,  and this same vehicle has 1/2x the average vehicle's frontal impact risk of fatality due to its superior crashworthiness ratings by NHTSA and IIHS, then its combined frontal impact fatality risk [(2) x (1/2) = 1] would be the same as an average weight car with average frontal crash-test ratings.

(3) In addition to star ratings NHTSA also publishes the measured forces imparted to the test dummies during crash tests, from which actual risk of injury associated with each vehicle can be derived, enabling differentiation between vehicles, even those with the same star rating. Specifically, for '08 models, 195 vehicles received 5-stars frontal crash ratings, [by definition this means 0-10% risk of serious injury in a 35 mph crash] however 3 of these vehicles have only 5% risk, whereas 33 vehicles have 10% risk (i.e., double the risk), and the remainder are distributed in between. The SCORE utilizes the actual risk values.

(4) Since there are many factors contributing to overall fatality risk, including demographic elements such as driver preferences, it is not surprising that neither NHTSA or IIHS ratings, alone, demonstrate statistically significant correlations vs. fatality rates. The SCORE, however, utilizes the crash test results from both agencies, along with rollover resistance and weight, and results in a metric that demonstrates a strong correlation with fatality rates (see analysis below).

(I) Selecting the Safest Vehicles

     There are three basic components to vehicle safety, and all three are readily quantifiable: (1) accident avoidance, (2) crashworthiness, and (3) vehicle incompatibility. In order to evaluate overall safety all three components must be combined in a statistically meaningful way, and by using actual traffic fatality data it is possible to assign the appropriate “weight” to each of these components. 

 

     Accident avoidance includes agility or maneuverability and is indirectly measured by rollover resistance, as determined by NHTSA. Electronic Stability Control can reduce the risk of rollover by helping to avoid the out-of-control conditions which often trigger a rollover, and must be considered when evaluating risk.           

 

     Crashworthiness, as measured by crash test ratings provided by NHTSA and IIHS, can be subdivided and weighted according to traffic fatalities in each crash mode (frontal, side and rear impacts).                                

 

     The 3rd component, vehicle incompatibility, is an important consideration since 59% of all traffic accidents involve more than one vehicle, and the laws of physics give the advantage to the heavier vehicle. The relationship between vehicle weight and fatality rate is well documented and must be included in any overall assessment of safety. Heavy vehicles have reduced fatality risk in multiple vehicle accidents compared with light vehicles, however additional weight is not necessarily beneficial since heavier vehicles, especially SUVs and pickups, tend to ride higher above the ground with an attendant higher risk of rollover.

     Only by combining all three components, and their elements, on a fatality-weighted basis, can overall fatality risk be compared between vehicles.

 

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(II) Vehicle Age

 

  

    Generally, the older the vehicle the less likely it is that rollover stability testing and comprehensive crash testing were conducted, and therefore your ability to screen those vehicles is limited. For example, prior to 2003 IIHS did not rate vehicles for side impact, nor did NHTSA prior to 1997. Also, prior to 2004, NHTSA did not perform dynamic stability testing on vehicles to evaluate tipping resistance.

     Over the past decade manufacturers have greatly improved the crashworthiness of vehicles, and the availability of certain very effective safety equipment has greatly increased since 2003, including side-curtain airbags and Electronic Stability Control (ESC). Therefore, recent model vehicles are generally "safer" than older vehicles. Vehicles with risk index SCORES under 65 are in the lowest risk category, representing the safest 18%, 16%, 6%, 5%, 3%, and 1% of all vehicles for model years '08, '07, 06, '05, 04, and '03, respectively.

     To evaluate vehicle safety for model years prior to 2003 your best resource is actual fatality rates. The "safest" vehicles can be considered to have demonstrated fatality rates less than 1/2 the average fatality rate (i.e., under 45 driver deaths per million vehicle-years), and represent approximately 15% of all vehicles (see Ranking Lists).

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(III) What's the SCORE (Statistical Combination Of Risk Elements?)

     The SCORE for any vehicle is a calculated value that combines 11 known risk elements on the basis of their weighted contribution towards serious injury or fatality. These risk elements utilize all known, validated measurements pertaining to vehicle safety, and are comprised of:

 

(I)   Frontal impact	a. NHTSA ratings	b. IIHS rating
(II)  Vehicle Weight Class
(III) Side Impact	a. NHTSA ratings	b. IIHS rating	c. Side-Curtain Airbags
(IV) Rear Impact	a. IIHS rating
(V)  Rollover	a. NHTSA rating	b. Electronic Stability Control

 

 

(IV) Weighting Risk Based on Fatality Modes^[1]

     By definition, the “average” passenger car has a SCORE = 100 which represents the sum of the percent contribution towards traffic fatalities for each of the 4 fatality modes: Frontal Impact (38%) + Side Impact (26%) + Rollover (33%) + Rear Impact (3%), {38+26+33+3} = 100, and the relative importance of risk data for any vehicle is assumed to be in proportion to these fatality ratios. 

 

 

 

Reference: IIHS's Highway Loss Data Institute publication: "Fatality Facts 2004: Occupants of Cars, Pickups, SUVs and Vans" chart of "Passenger vehicle occupant deaths in all crashes by impact point and vehicle type, 2004", adjusted to be consistent with the chart "Passenger vehicle occupant deaths in rollover crashes by impact point and vehicle type, 2004"

  

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(V) Predicting Fatality Rates^[2]

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     The best indicator of the predictive value for any measurement system is past performance. By applying the SCORE calculation to prior model-year vehicles for which fatality data have been compiled, the degree to which a vehicle's SCORE correlates to its real-world fatality rates can be observed.

     In IIHS's 2005 status report "The Risk of Dying" driver fatality rate data are provided for 199, 1999-2002 model year vehicles, for which statistically significant crash test rating data are available to enable meaningful SCORE calculations. As can be observed from the data plot below, a significant correlation does exist. This is not the case when attempting to correlate individual risk factors, such as IIHS's frontal impact rating, or NHTSA's frontal impact rating or vehicle weight (see the Elements of Risk page). Only through combining these risk elements in a weighted manner does a significant correlation appear.

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(VI) NHTSA and IIHS Ratings

     Data provided by NHTSA and IIHS enable comparison between the average passenger car risk and any specific vehicle's risk, for each fatality mode. For example, if a vehicle (call it vehicle XYZ) receives "average" SCOREs for testing in all modes except rollover, which was determined by NHTSA to have 24% risk of rollover (vs. the average passenger car which NHTSA determined has a rollover risk of 12%) then the rollover element of risk for vehicle XYZ is considered to be double, i.e., 24%/12% = 2.0. Therefore, since rollover fatalities account for 33% of all fatalities, the rollover element for vehicle XYZ would be 2.0 x 33 = 66, and the SCORE would then be calculated as: Vehicle XYZ SCORE = {38 + 26 + 66 + 3} = 133.

     To see a complete explanation of the risk rating system published by NHTSA and IIHS for each fatality mode go to The Elements of Risk page on this website. The weighted risk assigned to each element of the SCORE is shown in the table below, including the value assigned to the "average passenger car" (shown in the right-hand column). Note that vehicle weight, per se, is not a fatality mode; rather it is part of the frontal impact fatality mode and the SCORE includes the effect of weight in the frontal impact risk factor. The average passenger car is weighs approximately 3,300 lbs., and based upon the published relationship between weight and fatalities, a vehicle's frontal risk factor is adjusted up or down, based on its deviation from 3,300 lbs.

 

 

(VII) How to Select a Vehicle that May Save Your Life

     By comparing a vehicle's ratings for each of the 11 risk elements vs. the "average" passenger car it is possible to calculate the combined fatality risk for any vehicle, compared with the "average" passenger car. Since very few vehicles have been rated in all risk modes it is necessary that the SCORE system accommodate missing data by filling the blanks using the "average" value for that missing data. This results in a concentration of vehicles with SCOREs near the average vehicles' SCORE (100). [Note that the SCORE = 100 is actually the median SCORE for model-year 2005 passenger cars.]

     Vehicles with very high (or very low) SCOREs can only achieve these levels as a result of data specific to that vehicle which are significantly above (or below) the average rating. Therefore, this approach achieves the desired result, which is to isolate vehicles with the lowest risk! Annually, in the United States, there are, persistently, approximately 42,000 traffic fatalities and 3,000,000 injuries. These statistics provide the grim measurement of the consequences of driving vehicles with average safety, i.e. SCORE = 100.

     Vehicles with above average SCORES (>100) can be expected to result in greater than average fatalities. For model-year '08 approximately 26% of all vehicles have SCORES above 100, compared with approximately 65% for '03 model-year vehicles. This significant reduction in overall risk for newer vehicles is attributed to a combination of improved crashworthiness, reduced rollover risk and the introduction of Electronic Stability Control and Side-Curtain Airbags.

     Note, the "average" passenger car weight associated with the average SCORE = 100 is 3,300 lbs., whereas the average SUV, Van or Pick-up weighs approximately 4,300 lbs. The increased weight for these vehicle classes reduces their risk index SCORE however these same vehicle classes also typically have increased risk of rollover which often overcomes their weight advantage. NHTSA publishes the "average" risk of rollover for each class of vehicle and compared with a risk of rollover = 12% for passenger cars, Vans are 23%, SUVs and Pickups are 28%. The net effect of combining the weight advantage with the increased risk of rollover produces a SCORE of 116 for the average Van, 130 for SUVs, and 140 for Pickups. These average risk levels demonstrate that Vans, SUVs and Pickups are inherently more prone to rollover and therefore good rollover ratings are essential for these vehicles to "earn" low SCOREs. For example, if an average weight Van (4,300 lbs.) achieves a NHTSA risk of rollover rating = 12% (the same as the average passenger car), it would have a SCORE = 85 (assuming all other ratings for the the average VAN are "average").

     Vehicles with better than average SCORES (<100) are the safest. Although the (theoretical) best SCORE = 0, for model year '08 the lowest SCORE is 46 which means that the fatality risk associated with this low risk vehicle is approximately 46% x the average vehicle's risk. It also means the compared with the '08 vehicle with the highest risk index score = 164 it inherently represents 46/164 x the risk of that highest score vehicle (46/164 = 28%).  Approximately 18% of all '08 model-year vehicles scored 65 or below. Generally, to achieve such low SCOREs, these vehicles have superior crash test ratings and are equipped with ESC and SABs, enabling them to reduce risk points and drop below average. It is projected that vehicles with SCOREs below 65, will result in below average fatality rates, and these are the vehicles recommended as your "safest" choice. However, vehicles with individual ratings by NHTSA or IIHS that are exceptionally poor in frontal, side or rollover ratings, such as receiving only 3 stars in crash tests by NHTSA or "POOR" or "MARGINAL" by IIHS, should be avoided---regardless of their risk index SCORE.

 

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Risk Index SCORE Distribution for model-year 2006 Vehicles
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[1]  For model year 2005 the median passenger car SCORE = 100.

[2]  IIHS's 2005 status report "The Risk of Dying" http://www.iihs.org/srpdfs/sr4003.pdf.
[3]  The average risk for rollover, based upon vehicle class: SUV/Pickup = 28%, Van = 23%, Pass. car = 12%
[4]  ESC reduces loss of vehicle control and reduces rollover fatalities by 43%.
[5]  Head-protecting side-airbags are assigned a 45% reduction in fatalities vs. no airbag.
[General Note] The effect of vehicle class and weight is identified as Risk Element #1

 

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