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This website describes a comprehensive approach to evaluating the relative safety of vehicles, utilizing ratings and fatality data provided by the National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highway Safety (IIHS). The risk index SCORE (Statistical Combination Of Risk Elements) demonstrates superior correlation with actual driver fatalities than any other safety evaluation method and you can learn its derivation and the SCOREs for many popular vehicles on this website.
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updated 4/22/12.
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What's the SCORE (Statistical Combination Of Risk Elements)? The SCORE for any vehicle is a calculated value that combines 10 known risk elements on the basis of their weighted contribution towards traffic fatalities. These risk elements utilize all known, validated measurements pertaining to vehicle safety, and are comprised of:
Weighting Risk Based on Fatality Modes The SCORE combines the elements of risk, on a weighted basis, in proportion to U.S traffic fatalities: Frontal Impact (52%) + Side Impact (27%) + Rollover (17%) + Rear Impact (4%), [52+27+17+4] = 100. Vehicle SCOREs are therefore proportional to the relative fatality risk between vehicles.
. CONSUMER ALERT:
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sPrimary Fundamental Shortcomings of NHTSA and IIHS Rating Systems . Fatality rates in some vehicles are more than 20x higher than others. Consumers determined to learn which vehicles are safest, 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) 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.
(2) 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 52% of fatalities are associated with frontal impact vs. 27% for side, vs. only 4% for rear, leaving interpretation of the ratings to the consumer. Prior to model year 2011 NHTSA also presented individual ratings (frontal and side impact and rollover risk) without integrating them. However, commencing with model year 2011, NHTSA's new rating system combines these risks into an 'Overall Vehicle Score' (click here for detailed description of NHTSA's new rating system for 2011). [While this change for 2011 is an improvement the new rating system still fails to include the effects of vehicle weight and also ignores the IIHS ratings.] (3) Both NHTSA and IIHS lump vehicles into very broad rating categories --- For example, 94% of 2010 model year vehicles received the best possible frontal rating, “GOOD” by IIHS and 99% received either 5-stars or 4-stars by NHTSA. This incorrectly portrays all of these vehicles to be equivalently safe.
(4) Virtually no recognition by either agency that the other agency exists. The frontal and side impact ratings of these agencies complement each other due to substantial differences in testing protocols, whereas some risk categories are only addressed by a single agency, e.g., only NHTSA rates rollover and only IIHS rates rear impact risk. The SCORE (Statistical Combination Of Risk Elements) was created to address the above 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) 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. Vehicle weight risk is shown in the chart, below, which is based on IIHS's regression analysis showing the correlation between vehicle weight vs. driver fatality rate in multi-vehicle collisions between passenger cars. Their analysis shows, for example, that a 2600 lb. passenger car has 2x the driver fatality rate vs. a 4000 lb. car, in multi-vehicle frontal collisions.
(2) In order to assign the appropriate risk contribution for each category of rating (frontal, rollover, side, and rear), the SCORE combines ratings in proportion to their total fatalities (52% frontal: 17% rollover: 27% side: 4% rear).
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. 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 52% 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. .
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. For example, vehicles with risk index SCORES under 65 represent 21%, 30%, 18%, 16%, 6%, 5%, 3%, and 1% of all vehicles for model years '10, '09, '08, '07, 06, '05, 04, and '03, respectively. This improvement in vehicle safety may have contributed to the 29% decline in driver death rate over this same period of time 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.
Predicting Fatality Rates . 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.
The most recent data available on actual traffic fatalities has been published by IIHS, "Fatality Facts 2009", which provides driver fatality rates by vehicle class for model years 2007 - 2009. As you see from the plot below the risk index SCORE demonstrates a very significant correlation with actual fatality data for size/weight classes of vehicles.
The original evaluation of the risk index SCORE was made based on IIHS's 2005 status report "The Risk of Dying". In this report 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. [2]
. 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 17% of all fatalities, the rollover element for vehicle XYZ would be 2.0 x 17 = 34, and the SCORE would then be calculated as: Vehicle XYZ SCORE = {52 + 27 + 34+ 4} = 117. By comparing a vehicle's ratings for each of the 10 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, whereas for model year 2011 the median SCORE is approximately 70.] Commencing with model year 2011 NHTSA's rating system was revised, so no direct comparison with their previous ratings is possible. This significant reduction in overall risk for newer vehicles may be attributed to a combination of improved crashworthiness, reduced rollover risk and the introduction of Electronic Stability Control and Side-Curtain Airbags.
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Video Demonstrations of Vehicle Incompatibility:
. [VIDEO #1 click here] See how poorly SMALL vehicles -- with GOOD crash-test ratings -- perform when impacting LARGE vehicles: This first video shows 3 head-on collisions between light-weight vehicles and mid-sized vehicles. All 3 of these light-weight vehicles received the best possible crash-test ratings in frontal impact by IIHS, however despite these ratings the drivers of these vehicles would have most likely sustained much more severe injures compared with the drivers of the mid-sized vehicles. .
[Video # 2 click here] See how poorly LARGE vehicles -- lacking GOOD crash-test ratings -- perform when impacting SMALL vehicles: Don't assume that the solution to selecting a safe vehicle is simply to find a large vehicle. This video shows a heavy, older model station wagon, lacking the crashworthiness of modern models, crashing head-on into a light-weight modern vehicle. You see that the driver of the old, heavy station wagon would have most likely sustained severe injures whereas the driver of the lighter, modern car would have been relatively well protected. .
CONCLUSION: You must consider both WEIGHT and CRASH-TEST RATINGS to identify the safest vehicles. Both NHTSA and IIHS have chosen to exclude weight in their rating systems; instead they group their ratings by style and vehicle weight class, leaving it up to the consumer to know the inherent risks of smaller vehicles. This safety handicap, inherent in small vehicles, is the result of a reduced crumple zone forward of the occupant compartment -- providing less energy absorption capacity -- and reduced mass, which results in a faster rate of deceleration when impacted with a heavier vehicle. The risk index SCORE, as presented on this website, combines these risk factors and all other known risk factors to result in a single value to enable comparison of overall risk between all vehicles.
.**************************************** [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.
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