(I) Frontal Impact

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a.   NHTSA Frontal Ratings

     Because of the high incidence of front-end accident caused deaths NHTSA^[1] began its crash testing program in 1978 to evaluate vehicles for frontal crashes. They use crash-test dummies representing an average-sized adult placed in driver and front passenger seats and secured with the vehicle’s seat belts.^[2]

     Vehicles are crashed into a fixed barrier at 35 miles per hour, which is equivalent to a head-on collision between two similar vehicles each moving at 35 mph. Instruments measure the force of impact to each dummy’s head, neck, chest, pelvis, legs, and feet. Frontal star ratings indicate the chance of a serious head and chest injury to the driver and right front seat passenger. A serious injury is one requiring immediate hospitalization and may be life threatening.

     NHTSA’s rating scale is shown below. One significant observation is that small changes in the number of stars represent large differences in risk. This point is overlooked by many people, who apparently never read the definition of these ratings. Some people mistakenly believe that a four star rating is 4/5ths as good as a five star rating – whereas the average risk for 4-stars (15%) is actually 3x the average risk vs. 5-stars (5%).

                            NHTSA Frontal Crash ratings represent Risk of Serious Injury

Reviewing the distribution of these frontal ratings for 2004 vehicles shows that:   

    Out of 382 vehicles, 252 vehicles received NHTSA frontal ratings for both driver and passenger.

 Ø        29% of these received 5 stars for both.

 Ø        22% received 4 stars for both.

 Ø       49% received 3 stars or less for either driver or passenger.  

 

     By model-year 2006, the distribution had shifted dramatically, with approximately 95% of all vehicles rated either 4 or 5 stars, and no vehicle receiving less than 3 stars.

     With so many vehicles receiving the same 5 or 4 star ratings, and in order to differentiate the best among these vehicles, the risk index score utilitzes actual crash test loads measurements, as published by NHTSA, to identify the frontal impact risk of serious injury. For example, both the 2007 Mercury Milan and 2007 Chrysler PT Cruiser received 4-star driver-side, frontal impact ratings by NHTSA since both vehicles are within the 11% - 20% risk band. However, based on the crash test dummy loads measurements the risk of serious injury to the driver for the PT Cruiser is 19% whereas it is only 10% for the Milan.

     It is reasonable to expect to see a strong correlation between the NHTSA frontal crash test ratings and driver fatality rates, i.e., vehicles with high star ratings generally experiencing lower fatality rates than those with low star ratings, however this is not the case based on the driver fatality rates for approximately 200, 2002 vehicles^[3]. The data plot shown below does not indicate any significant correlation between driver side frontal crash star rating and driver death rate, which appears to defy logic, but should be interpreted as demonstration that there are many elements which comprise the causes for the 30:1 variation  in driver death rates from vehicle-to-vehicle. This observation reinforces the principle that many safety elements must be included to predict vehicle safety and selecting a single variable, as logical as it may appear, is inadequate as a safety screening method.

b.   IIHS Frontal Rating

     Although NHTSA frontal ratings provide useful frontal crashworthiness indicators, they are not the only considerations for evaluating front-end collision crashworthiness. NHTSA’s frontal test crashes the full width of the front of a vehicle into a rigid barrier. This maximizes the energy absorbed by the front of the vehicle so that the occupant compartment is more likely to remain intact than if the same collision occurred hitting a smaller portion of the test vehicle, thereby concentrating the impact forces.  

     IIHS conducts an offset frontal crash test, where only the driver’s side of a vehicle’s front end is hit, so a smaller area of the structure is available to absorb the energy from the crash. Offset crashes are more demanding on the structure of a vehicle. IIHS conducts its offset test at 40 mph and only 40 percent of the total width of each vehicle strikes a barrier on the driver side. The barrier is constructed of aluminum honeycomb, which simulates the rigidity of another vehicle, so that the forces developed in the test approximate two vehicles of the same weight, colliding at approximately 40 mph.  

     IIHS has been performing offset frontal crash tests since 1995 and they use a qualitative rating system consisting of 4 scores: GOOD, ACCEPTABLE, MARGINAL, and POOR. For offset impact evaluation they derive an overall crashworthiness rating based on structural performance, injury measurements, and restraints/dummy kinematics.  

     In February 2004 IIHS published a report^[4] showing the results of their statistical correlation between traffic fatalities which occurred over a 10-year period and the offset frontal ratings which had been assigned to vehicles resulting from IIHS’s offset frontal test.

Reviewing the distribution of these ratings for 2004 vehicles:  

Out of 382 vehicles, 250 vehicles received IIHS frontal ratings

54% received “GOOD” ratings

23% received “ACCEPTABLE” ratings

 

     For model-year 2006 vehicles approximately 95% are rated either "GOOD" or "ACCEPTABLE", and no vehicle rated "POOR".

    With so many “GOOD” and “ACCEPTABLE” choices there is no need to select vehicles rated “MARGINAL” or “POOR”. The typical rating is considered to be "ACCEPTABLE", and this is the value incorporated into the SCORE for the average vehicle, and assumed to be applicable to vehicles which have not been tested.

     Despite the 4-to1 variation in fatality rates explained by IIHS frontal ratings, when IIHS frontal ratings are compared with fatality rates for individual vehicles (see plot below) there appears to be only a weak correlation. This is the same conclusion was reached when evaluating the NHTSA frontal crash ratings and emphasizes that there are other significant risk elements which are contributing to fatalities.

     In order to combine the frontal impact ratings provided by NHTSA and IIHS the SCORE assumes equal weight to each agency's ratings. Therefore, since frontal impacts are responsible for 38% of all U.S. traffic fatalities (i.e., 38 points out of 100), the SCORE allocates 19 points to IIHS and 19 points to NHTSA (which is further subdivided into 9.5 points for NHTSA's driver rating and 9.5 points for NHTSA's front passenger rating).

(II)   Vehicle Weight Class 

     Fatality data^[11] show that 57% of all fatal crashes involve more than one vehicle. The laws of physics require that the momentum of the heavier vehicle impart higher deceleration forces to the lighter vehicle and experience correspondingly slower deceleration itself. Consequently, the occupants of the lighter vehicle experience larger forces. This is why both IIHS and NHTSA advise not to compare frontal crash ratings for vehicles of different weights. Accordingly, when two vehicles have identical frontal crash ratings, the heavier vehicle generally is safer than the lighter one. Fatality data demonstrate this to be the case and in a head-on collision a 1 percent weight advantage corresponds to more than a 5 percent reduction in the driver’s fatality risk, relative to the driver of the lighter vehicle.^[12]

     By examining fatality data in 2-vehicle fatal accidents, provided by the NHTSA’s Fatality Analysis and Reporting System[13], it is noted that there is an inverse correlation between weight and fatalities (i.e., more weight is associated with lower fatalities), as would be expected from the laws of physics. The SCORE utilizes these data by factoring the frontal impact risk by the ratio of driver death rate for a particular vehicle's weight divided by the driver death rate for the average (3,300 lb.) passenger car. The 41 driver deaths per million vehicle-years associated with the average weight passenger car (see plot below) is therefore representative of the average number of frontal impact driver fatalities.   In the real world a vehicle's weight has the effect of amplifying or attenuating the risks obtained from crash test ratings, and since 57% of all frontal collisions involve more than one vehicle, weight is a significant risk factor. 

     The weight risk factor for heavy vehicles has the effect of offsetting frontal impact risk, derived from crash test data alone. For example, an "average" (3,300 lb.) passenger car with "average" frontal crash test ratings (4-1/2 stars by NHTSA + "ACCEPTABLE" by IIHS) has the equivalent SCORE as a 2,500 lb. passenger car rated 5-stars by NHTSA + "GOOD" by IIHS. In effect the improved crash test ratings compensated for the lighter weight, and the driver fatality rate stayed the same. This is the reason both NHTSA and IIHS warn consumers to only compare frontal impact ratings between vehicles within +/- 200 lbs of each other. Unfortunately, these agencies don't give you the guidance to compare the relative safety of vehicles in different weight classes. The SCORE utilizes the weight fatality ratio (vehicle fatality rate divided by the average passenger car fatality rate) to modify the frontal crashworthiness risk obtained from crash test ratings alone. Total frontal impact fatality = weight fatality ratio x crashworthiness fatality rate.

     NHTSA categorizes vehicles by vehicle class using “curb” weight. Curb weight represents the weight of a vehicle with standard equipment including the maximum capacity of fuel, oil, coolant and air conditioning, if so equipped. Passenger cars are subdivided as shown in the table, below. Therefore, based on weight alone, the SCORE for "heavy' passenger cars is lower than the average car ; for "mini", "light and "compact' cars it is higher.

NHTSA Weight Classification 

     Similar to the discussion of frontal impact crash test ratings the raw data plot, below, does not show significant correlation between vehicle weight and driver fatality rates,  based on driver fatality rates for approximately 200, model year 2002 vehicles^[13]. Again, this provides further evidence that there are many elements which comprise the causes for the 30:1 variation in driver death rates from vehicle-to-vehicle, and selecting a single variable, as logical as it may appear, is inadequate as a safety screening method.

(III)   Side Impact

a. NHTSA  Ratings

     In frontal impacts, the vehicle’s front-end structural volume behaves as an effective crumple zone, to absorb impact energy thereby decelerating the vehicle. Side impact, however, typically has much less crumple zone available, making side protection fundamentally a greater engineering challenge. Beginning in 1997, NHTSA began testing vehicles for side impact, using crash-test dummies representing an average-sized adult placed in the driver and rear passenger seat (driver’s side) and secured with the vehicle’s seat belts. The side crash test represents an intersection-type collision with a 3,015 lb. barrier moving at 38.5 mph into a standing vehicle. The moving barrier is covered with material that has “give” to replicate the front of a vehicle. Instruments measure the force of impact to each dummy’s head, neck, chest, and pelvis.

     Side-collision star ratings (1–5 stars) indicate the chance of serious chest injury for the driver, front seat passenger, and the rear seat passengers. As with the frontal crash test ratings, a serious injury is one requiring immediate hospitalization and may be life threatening.  

     Head injury is not currently included in NHTSA’s star rating. This is a significant shortcoming, since some vehicles may receive high star ratings yet may expose the occupants to serious or fatal head injuries. As you will see below, IIHS’s side impact rating is necessary to evaluate risk of head injury. 

NHTSA Side Crash Ratings Predict Risk of Serious Injury

     Note that small changes in the number of stars for NHTSA’s side impact ratings represent large differences in risk, similar to what was observed for frontal impact star ratings.

Reviewing the distribution of these ratings for the 2004 Vehicles shows:   

Ø     Out of 382 vehicles, 197 received NHTSA side impact ratings for both driver and rear passenger

Ø       38% received 5-stars for both

Ø       35% received 4-stars for both   

     With so many vehicles rated at 4 or 5 stars it is not necessary to select vehicles rated over 10% risk of serious injury (1, 2 or 3-stars). The "typical" rating is approximately 4-1/2 stars, and this is the score incorporated into the SCORE for the “average” vehicle and assumed to be applicable to vehicles which have not been tested.

b.   IIHS Ratings

     In crashes between cars and other passenger vehicles during 2000- 01, almost 60 percent of the driver deaths in the cars struck on the driver side were hit by SUVs or pickups. Due to the height incompatibility between the large size SUV-type vehicles and smaller vehicles, especially passenger cars, the occupants in the cars are much more exposed to head injury, compared with car-to-car side impacts. Often, when an SUV crashes into a passenger car, the impact point is near the occupant’s head of the passenger car.^[5]

     In 2003, IIHS began a side impact crash test program to specifically address this height incompatibility. They designed a moving 3300 lb. barrier to represent the front-end geometry of a typical SUV or pickup. Compared with NHTSA’s barrier, this one is higher off the ground, taller, and impacts at 31 mph perpendicular into the driver’s side of a passenger vehicle. Dummies are positioned in the driver seat and the rear seat behind the driver. There are no direct correlations available yet comparing side impact ratings with traffic fatalities. However, according to Brian O’Neill, president of IIHS, “In real world crashes of similar severity [to IIHS crash test] without side airbags, there would be a high likelihood of serious head injuries and rib fractures [for many of the vehicles tested]”.^[6]

     IIHS uses a qualitative rating system for side impact, similar to their offset frontal crash test rating system, consisting of 4 scores: GOOD, ACCEPTABLE, MARGINAL, and POOR. Each vehicle’s overall side impact rating is based on injury measurements, structural performance and head protection. So, unlike NHTSA’s side impact rating system, IIHS’s ratings do include head injury evaluation.  

Reviewing the distribution of these ratings for the 2004 vehicles shows that:   

Ø       Out of 382 vehicles, only 43 were rated for IIHS side impact, of which 33 received "POOR" ratings  

A "MARGINAL" rating  is the score incorporated into the SCORE for the “average” vehicle and assumed to be applicable to vehicles which have not been tested. 

  c.   Head-Protecting Side Impact Airbags

     IIHS side crash testing demonstrated that most vehicles tested did a poor job of protecting the dummy’s head during a side impact with an SUV-type vehicle.  However, when 4 2004 vehicles rated “POOR” were re-tested, after incorporating their optional equipment, head-protecting side airbags (also referred to as curtain airbags):   

Ø       3 improved from POOR to GOOD  

Ø       1 improved from POOR to ACCEPTABLE  

     For  2006 model-year, IIHS lists 18 vehicles tested for side-impact, both with and w/o side-curtain airbags. Of those 18

Ø       5 improved from POOR to GOOD  

Ø       6 improved from POOR to ACCEPTABLE  

     These results are dramatic. While head-protecting side airbags, alone, do not ensure good side impact crashworthiness, based on limited sampling, they appear to be very effective and, therefore, should be considered essential. Because of the substantial life-saving potential of these airbags a new federal regulation has been proposed.

     In April 2004 the U.S. Department of Transportation proposed a requirement that head protection for side impact be installed in all passenger vehicles^[7]. This proposed rule requires incorporation into all passenger vehicles by model year 2009. NHTSA estimates that this change, alone, will reduce side impact caused fatal head injuries by 45%, saving 700 to 1000 lives per year.

     An additional safety element for the SCORE is the availability of head-protecting airbags, consistent with the 2004 proposed NHTSA rule. Note that only 55% of the 2004 vehicles offer head-protecting airbags , as either optional or standard equipment, so it is critical that you determine their availability before making your vehicle selection.

     Another consideration for side impact head protection awareness is that side airbags in some vehicles are actually designed only for torso protection, and not head protection. Often, but not always, the manufacturer refers to head-protecting airbags as "side-curtain" airbags. Make certain the vehicle you are considering has side airbags that extend to the head level. Also, in some vehicles that do provide side head-protecting airbags, they are only available for front seat occupants, not rear; others for driver’s side, front only. Based on your planned usage of the vehicle, you should restrict your vehicle selection considering these additional limitations. 

     In order to combine the side impact ratings provided by NHTSA and IIHS the SCORE assumes equal weight to each agency's ratings. Therefore, since side impacts are responsible for 26% of all U.S. traffic fatalities (i.e., 26 points out of 100), the SCORE allocates 13 points to IIHS and 13 points to NHTSA (which is further subdivided into 6.5 points for NHTSA's front seat rating and 6.5 points for NHTSA's rear sear rating). Since NHTSA's star rating system does not include the effect of head injury, the availability of side-curtain airbags is assumed not to have any effect on NHTSA's ratings in the SCORE.   

     For purposes of quantifying the benefit of side-curtain airbags it is assumed that their availability reduces fatality risk by 45% and that this reduction applies only to the IIHS rating and only when side impact testing has not been performed. (When IIHS side impact ratings are available the SCORE presumes that the benefits of these airbags have already been included in these ratings.) For the purpose of defining  the "average" vehicle configuration, the SCORE presumes that the "average" vehicle receives only half benefit, (22.5% reduction in fatality risk)  i.e.,  the "average" vehicle has 77.5% x the risk of vehicles rated "MARGINAL" for side impact by IIHS.

(IV)   Rear Impact

     Although only 3% of all traffic fatalities are attributed to rear impact there are some fundamental design considerations that greatly reduce your risk of injury when you are involved in this type of collision.

     Whiplash neck injury occurs after a vehicle is struck from behind and when the occupant's head lags behind the accelerated motion of the torso.  Through proper seat and head restraint design it is possible to minimize the lag between the torso and head acceleration, thereby reducing the stretching of the neck.

     The IIHS evaluates rear crash protection based on the geometry of the head restraints and dynamic testing. Unlike IIHS frontal and side impact crash testing the dynamic rear impact testing does not crash test the entire vehicle. Instead, a dummy is placed on a mock-up of the seat, including head restraint, and the seat is mounted on a laboratory sled. The impact simulates a 20 m.p.h. rear-end collision between two vehicles of equal weight. A "MARGINAL" rating is assumed representative of the "average" car and when a vehicle has not been rated for rear impact a "MARGINAL" rating is therefore assigned.

(V)   Rollover 

a.   NHTSA Ratings

     With the increasing popularity of SUVs and pickups there has been a corresponding rise in deaths due to rollovers since these categories of vehicles tend to be relatively top-heavy, when compared with the typical passenger car. They ride higher off the ground and this high center-of-gravity position directly increases the tendency for rollover, either in sudden maneuvers or in wheel stubbing events, such as driving into a curb or onto soft grass at the side of the roadway, after the driver has lost control of the vehicle. Examination of rollover data reveals that compared with passenger cars, SUVs are approximately 2.8 times more likely to result in rollover; pickups approximately 2.0 times.