FAQFrequently Asked Questions
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* Q1: Why did the criteria for "Safest" vehicles change in 2018? . Q2: Prior to 2018, what were the requirements for a vehicle to be listed as "safest"? . A2: SAFEST means all crash ratings by IIHS (medium overlap frontal, small overlap frontal, side and rear impact) and ratings by NHTSA (frontal, side impact and rollover) are known and in the top quartile, and Compatibility Fatality Factor <1.00. . LEAST SAFE means one or more rating in the bottom quartile, and/or NHTSA static rollover risk greater than the average risk of an SUV class vehicle, and/or Compatibility Fatality Factor above 1.20. . REDUCED SAFETY means one (or more) rating is between the top quartile and bottom quartile, and/or compatibility fatality factor between 1.001.20. . SAFETY INDETERMINATE means one or more rating is unknown however known ratings are in the top quartile . Of 4392 vehicles for modelyears 20112016: 2% meet SAFEST criteria; 20% have REDUCED SAFETY; 29% are LEAST SAFE; 48% have INDETERMINATE SAFETY. .* Q3: Why has the risk index SCORE presented on this website between 20032012 been superseded by the Safest/Least Safe categories ? . A3:When the risk index SCORE was initially developed in 2003 there existed a very wide distribution of risk among vehicles and both agencies utilized their full range of ratings, i.e., "POOR" to "GOOD" by IIHS and "1Star" to "5Stars" by NHTSA. Since that time vehicle design has greatly improved reflected by the excellent crash test results for most vehicles. This does not mean that all vehicles are equivalently safe but rather that the rating systems are not refined enough to distinguish the differences in crashworthiness. For modelyear 2012, IIHS rated 63% of all vehicles as "Top Pick", which means they received their best rating possible in all 4 categories, and NHTSA rated 24% as "5Star Overall", their best possible rating. While the rating systems continue to be useful in isolating the least crashworthy vehicles, there were too many vehicles with vitually identical best ratings . In order to overcome this distortion and to focus on the objective of this website which is to identify the "safest" vehicles, the methodology needed to change. Vehicle weight has always been the missing risk element in the ratings systems of both agencies. The new methodology which requires the double selection of "Top Pick" plus "5Star Overall" to create a short list of "Safest", then focuses on vehicle weight in order to isolate the best of the best safe choices.
* Q4: How does NHTSA’s new rating system (commencing with model year 2011) compare with Informed for Life's risk index SCORE method of calculating vehicle safety? . A4:
* Q5: Are new vehicles becoming safer and have traffic fatalities been reduced? . A5: The chart, below, shows the that since 2003 there has been a significant reduction in vehicle risk index SCORE and approximately a 30% reduction in driver fatality rates. Although there may be multiple factors reducing fatality rates, including reduction in alcohol related accidents and increased use of seat belts, the significant improvement in vehicle crashworthiness, as evidenced by the reduction in SCOREs, appears to be a significant contributor. Note that the reduction in fatality rates resulting from safer vehicles will lag the actual reduction in SCOREs due to the relatively small number of "new" model vehicles added to the highway traffic each year. .
* Q6: How different are the crash test ratings between NHTSA and IIHS? . A6: The differences can be very significant since the test methods are different and ratings are based on different criteria. In order to evaluate a vehicle’s crashworthiness both sets of ratings must be included. . For example, the side impact crash testing performed by IIHS utilizes an impact sled that simulates an SUV colliding with the test vehicle which is higher off the ground than NHTSA's test sled. Also, not until the 2011 rating system did NHTSA include head trauma in their side impact rating whereas IIHS has done so consistently.
* Q7: Why aren't big and heavy vehicles always the safest choice? . A7: Although increased weight is an advantage in multivehicle accidents that advantage can be quickly overcome with the higher risk of rollover typically associated with many "large and heavy" vehicles due to their high centerofgravity. Below is a listing of heavy passenger vehicles (over 4,500 lbs.) and their risk of rollover, and despite their heavierthanaverage weight their risk index SCORES are above 100 (greater than average passenger vehicle risk) . The average passenger car has a 12% risk of rollover and many of these heavy vehicles are at significantly higher risk of rollover than average. Their rollover risk, when combined on a fatalityweighted basis with the higherthanaverage weight, results in a net increase in risk compared with the average passenger car.
* Q8: Why does the risk index SCORE for a vehicle change significantly, from one year to the next, when the vehicle appears to be virtually unchanged? . A8: There are several possible valid reasons for differences in SCORE between models which appear to be identical, or for models with the same name from one year to the next. .
First, there may be a difference in the safety equipment installed in the vehicle. Specifically, Sidecurtain AirBags (SAB) can have a dramatic effect on the side impact ratings provided by IIHS since their sideimpact testing measures trauma to the head and SAB often change unacceptable "POOR" ratings to "GOOD" ratings. Also, Electronic Stability Control (ESC) can significantly change the rollover risk points calculated for a vehicle since ESC can significantly improve your ability to avoid a rollover from ever occurring. . Second, the amount of rating information provided by NHTSA and/or IIHS may be different between the two vehicles. This is especially common early in a modelyear (e.g. prior to January for a given modelyear) since very few “new” models are tested prior to January and often the review to determine equivalency from yeartoyear is not completed until several months after a new model is in the showroom. Once the agency completes its review of equivalency – it then publishes the prior year ratings for the new modelyear, where appropriate. Caution: you should not assume that models of the same name have equivalent ratings from yeartoyear. * Q9. Where can I learn about the best way to protect children/baby passengers? . A9. NHTSA summarizes their recommendations for child protection at http://www.nhtsa.dot.gov/people/injury/childps/ParentGuide2005/pages/WhenDoYou.htm * Q10. Which vehicle colors are safest? . A10. There is presently no scientific evidence supporting the selection of a particular color for vehicle safety. Go to this link for the AAA Foundation for Traffic Safety study results. * Q11: Does the requirement to display NHTSA star ratings on showroom window stickers help the consumer select safer cars? . A11: Unfortunately the window sticker information is incomplete, and therefore misleading!
What the sticker should also say but doesn’t:
1.4STARS is typically 3x risk vs. 5STARS . 2.in 2006, 56% of all vehicles received 5STARS (+ 40% received 4STARS) . . 3.Side impact star ratings EXCLUDE head injury risk . 4.Another independent agency (IIHS) may have crash tested this same vehicle in a different manner with different results. IIHS does evaluate head injury due to side impact. . 5.Lightweight vehicles experience approx 2x fatality rate vs. average weight vehicles . 6.SideCurtain Airbags [mandated for 2013] reduce sideimpact fatalities by approx 45% . 7.Electronic Stability Control [mandated for 2012] reduces rollover fatalities by approx 43%) . 8.To properly evaluate the safety of this vehicle relative to any other, all of the above considerations must be weighed. * Q12: How does NHTSA calculate the % risk of serious injury for frontal and side impact star ratings?
A12: For model years prior to 2011 NHTSA publishes a database for all of the vehicles they tested. To access the database go to: ftp.nhtsa.dot.gov/ and then select NCAP, then select the file: NCAP.db.mdb
Below are highlights from NHTSA’s paper describing NHTSA's rating formulae applicable prior to model year 2011.
FRONTAL IMPACT
For head injury, the equation Phead = {1+exp[5.020.00351*HIC]}1 . relates the probability of an AIS $ 4 head injury to HIC. . For chest injury, the equation Pchest = {1+exp[5.550.0693*Chest G]}1 relates the probability of an AIS $ 4 injury to chest G’s. This chest injury risk curve is in Figure 2. . Injury risk functions depend on complex biomechanical and real world collision investigations. Limitations exist relative to correlations between engineering measures of trauma and AIS levels. Research and analysis activities are continuing to examine existing biomechanical data and real world collision data and to develop additional data which may lead to changes and improvements in methods of determining injury risk. . Basic Assessment Criteria . NHTSA uses the injury risk functions as defined by the equations for the head and chest to develop a star rating system. It was decided that a star rating system could better communicate the safety protection of vehicles than the older NCAP safety presentations. Consumers could better understand such a system, with five stars indicating the lowest probability of an AIS $ 4 injury and one star indicating the highest probability. . NHTSA concluded that a combined effect of injury to the head and chest should be used, since it is well documented that an individual who suffers multiple injuries has a higher risk of permanent disability or death. Therefore, a combined probability of an AIS $ 4 injury is calculated from the equations for the head and chest as; . Pcombined = Phead + Pchest  Phead * Pchest . where Pcombined = combined probability. This equation is applied to the HIC and chest G responses of the driver and passenger dummies in each frontal impact NCAP test. . From these combined probability values, the safety performance is provided for each vehicle that is tested for the frontal direction in NCAP with the driver and right front seat passenger. The agency uses a simplified nonnumeric format, the five star rating, for the frontal NCAP results. NHTSA wanted to give the US consumer easily grasped vehicle safety performance information. This nonnumeric format is based on the use of injury risk functions, that relate the Hybrid III dummy measurements to injury probabilities. The head and chest injury data is combined into a single rating, reflected by the number of stars. . 5stars = 10% or less chance of any serious injury to the head or chest 4stars = 11% to 20% chance of serious injury 3stars = 21% to 35% chance of serious injury 2stars = 36% to 45% chance of serious injury 1star = 46% or greater chance of serious injury . SIDE IMPACT . . Lateral Impact Assessment criteria and limit values:
. Similarly for the NCAP lateral impact results, a star rating system is being used based on the thoracic injury function curve that was developed for Thoracic Trauma Index (TTI). This thoracic injury function curve is contained in the final regulatory evaluation for Federal Motor Vehicle Safety Standard (FMVSS) No. 214 and is shown in Figure 4. This function relates the probability of an AIS >= 4 thoracic and upper abdominal injury to TTI in a lateral impact. The TTI value is determined for the side impact dummy (SID) based on the signals recorded in the lateral impact NCAP crash test. Probability of injury is determined from the injury function curve. From the probability values, the star ratings can be determined for each laterally impacted car for the front and rear seat occupant. The following levels are used to designate the stars: Using the risk curve, the star ratings correspond to a range of TTI values. . 5stars = TTI # 57 4stars = 57 < TTI # 72 3stars = 72 < TTI # 91 2stars = 91 < TTI # 98 1star = TTI > 98 . From the probability values, star ratings for the front and rear seat occupant is developed. The following levels are used to designate the stars: . 5stars = 5% or less chance of serious thoracic and upper abdominal injury 4stars = 6% to 10% chance of serious injury 3stars = 11% to 20% chance of serious injury 2stars = 21% to 25% chance of serious injury 1stars = 26% or greater chance of serious injury . Since the lateral impact probabilities are based on only one injury, to the thorax, (whereas, frontal impact probabilities are based on the combination of head and chest injury), the lateral impact star levels are onehalf of the levels for the front impact at the five, and four star levels. A further deviation from the frontal impact star levels is used for the three, two, and one star levels. The break for the three star level for frontal impact rating was based on the condition that any vehicle that met FMVSS 208 (i.e., HIC not greater than 1000 and chest g's not greater than 60) in the NCAP condition would receive at least three stars. To follow this same procedure, the level is set such that any vehicle that meets FMVSS 214 (i.e., TTI not greater than 90 as applicable to 2door passenger cars) would receive three stars. Subsequently, establishing this level requires some changes at the two and one star levels to provide a spread between the levels. . Steps in calculating injury probability: . 1. The term “GR is the greater of the peak acceleration of either the upper or lower rib expressed in g’s. . 2. The term “GLS “ is the lower spine (T12) peak acceleration expressed in g’s. . 3. The TTI is calculated in accordance with the following formula . TTI = 0.5(GR + GLS). . Rollover Risk: . The formula for calculating rollover risk is described in the October 2003 Federal Register notice (68 FR 59250) that established NHTSA's current NCAP methodology which uses a logistic regression technique and includes the effect of the dynamic test ("fishhook) result as well as SSF. To access that document, go to the DOT docket website (http://dms.dot.gov) and, using the "Simple Search" feature, type in the docket no. "9663". The notice is item no. 84 in that docket, and the formulae are listed near the end of the document in Appendix II, on p. 59291 of the notice as it appeared in the Federal Register (assumes you are viewing the PDF version). There are two formulae, one for vehicles that tipup in the dynamic test and one for vehicles that do not tip up.
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Q13. Where can I see the definition of the terms used in NHTSA's test database ? . A14. Go to the Federal Motor Vehicle Safety Standards. Everything is defined in the CFR. You can view the CFR online using the links below. *
Q14: Is one factor more important safety wise than the other when considering the following: a car that is heavier and thus a lower weight effect fatality factor or a higher safety rating? For example: the Mazda Cx5 has a weight of 3300 pounds and weight effect fatality factor of .61, but is 5 star all around and iihs top pick plusearning a good on the small overlap. Is this comparable to a 4 star car that weighs over 4100 pounds with a weight effect of .34? Or is there no comparison? . A14:
.Crashworthiness performance, as evaluated by crash testing, is independent of weight. Frontal crash test ratings are indicative of a vehicle colliding headon with an identical vehicle which produces the same forces as that vehicle colliding headon into a barrier. Generally speaking the likelihood of fatalities/injuries doubles going from 5star to 4star.
Since 57% of all accidents involve more than one vehicle the crash test ratings alone are insufficient to evaluate safety. For this reason I have created the weighteffect fatality factor.
When you compare a vehicle that received 4stars frontal impact rating vs. one receiving 5stars, the risk of injury doubles for all frontal impact collisions whereas when the weighteffect fatality factor doubles it only applies to multivehicle accidents.
Since there are now so many “5star”+ ”Top pick” vehicles available to choose from there is no need to compromise on crashworthiness, whereas the size/weight of your vehicle selection has practical limitations and you should limit your exposure to crashes involving other vehicles by limiting the acceptable weighteffect fatality rate . (Note that there are many very large and heavy vehicles that do not have 5star crashworthiness. In addition, large vehicles tend to have a higher centerof gravity resulting in lower rollover resistance. Selecting the largest and heaviest vehicle therefore does not identify the safest vehicle.)
The bottom line is that you shouldn’t tradeoff crashworthiness with size/weight. Both are important and there is no need to compromise crashworthiness!
Q15: What are your safety criteria for finding a safe car that is older than 2011?
. A15:To identify the safest pre2011 vehicles follow these steps:
1. Limit your choices to vehicles weighing more than 3300 lbs. and which include "Sidecurtain airbags" and "Electronic Stability Control" (not mandatory until 2013).
2. Lookup the NHTSA ratings at: http://www.safercar.gov/Vehicle+Shoppers/5Star+Safety+Ratings/19902010+Vehicles [Avoid vehicles with less than 5stars in any crash mode and less than 4 stars in rollover. Note that NHTSA's testing/rating system was completely revised between modelyears 2010 and 2011 and ratings between those periods cannot be compared.]
3. Lookup the IIHS ratings at: http://www.iihs.org/ratings/default.aspx [Avoid vehicles that are not "Top Safety Picks"]
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