Q2: How many vehicles receive the "best" and "worst" ratings from the ratings agencies?

 A2: Below is a summary of ratings for 2006 model-year vehicles.

Q3: How different are the crash test ratings between NHTSA and IIHS?

A3: The differences can be very significant since the test methods are different and ratings are based on different criteria.

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, NHTSA does not rate the resulting injury to the head whereas IIHS does.

The following vehicles received the best possible NHTSA side-impact rating (5-stars) whereas IIHS rated these same vehicles as "POOR" for side impact (their lowest rating):

The following vehicles received the best possible IIHS side-impact rating (GOOD) whereas NHTSA rated these same vehicles as either 3 or 4 STARS for side impact.

Q4: Why aren't big and heavy vehicles always the safest choice?
 

Q5: 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?

A5: 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, Side-curtain AirBags (SAB) can have a dramatic effect on the side impact ratings provided by IIHS since their side-impact 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 model-year (e.g. prior to January for a given model-year) since very few “new” models are tested prior to January and often the review to determine equivalency from year-to-year 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 model-year, where appropriate. Caution: you should not assume that models of the same name have equivalent ratings from year-to-year.

Q6: How does NHTSA calculate the % risk of serious injury for frontal and side impact star ratings?

A6: NHTSA publishes a complete database for all of the vehicles they test. To acces the database go to:

ftp.nhtsa.dot.gov/

then select NCAP, then select the file: NCAP.db.mdb

Below is NHTSA paper describing NHTSA's rating formulae.

You can see the entire NHTSA paper by clicking here.

FRONTAL IMPACT

For head injury, the equation

      P_head = {1+exp[5.02-0.00351*HIC]}^-1                                   

relates the probability of an AIS $ 4 head injury to HIC. The injury risk curve is shown in Figure 1.
  
For chest injury, the equation    P_chest = {1+exp[5.55-0.0693*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;

 P_combined = P_head + P_chest - P_head * P_chest                                                                

where P_combined = 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.

5-stars    =          10% or less chance of any serious injury to the head or chest

4-stars    =          11% to 20% chance of serious injury

3-stars    =          21% to 35% chance of serious injury

2-stars    =          36% to 45% chance of serious injury

1-star     =          46% or greater chance of serious injury

  SIDE IMPACT

IV. 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.

5-stars   =          TTI # 57

4-stars   =          57 < TTI # 72

3-stars   =          72 < TTI # 91

2-stars   =          91 < TTI # 98

1-star    =          TTI > 98

From the probability values, a star ratings for the front and rear seat occupant is developed. The following levels are used to designate the stars:

5-stars    =          5% or less chance of serious thoracic and upper abdominal injury

4-stars    =          6% to 10% chance of serious injury

3-stars    =          11% to 20% chance of serious injury

2-stars    =          21% to 25% chance of serious injury

1-stars    =          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 one-half 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 2-door 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 “G_R is the greater of the peak acceleration of either the upper or lower rib expressed in g’s.

 
2.      The term “G_LS “ 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(G_R + G_LS).

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 dyanmic 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 tip-up in the dynamic test and one for vehicles that do not tip up.

Q7. Where can I learn about the best way to protect children/baby passengers?

A7. NHTSA summarizes their recommendations for child protection at http://www.nhtsa.dot.gov/people/injury/childps/ParentGuide2005/pages/WhenDoYou.htm

Q8. Where can I see the presentation provided to the Glastonbury Conneticut Exchange Club on 3/1/07?

A8. Click here to open the PowerPoint Presentation. WARNING: ths file is approx. 16 mb

Q9.  Where can I see the April '07 IIHS status report on driver death rates?

A9. Go to http://www.iihs.org/sr/pdfs/sr4204.pdf

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