In response to Tesla’s press release announcing the Model S’ performance, NHTSA provided a statement saying:

The agency’s 5-Star Safety Ratings program is designed to provide consumers with information about the crash protection and rollover safety of new vehicles beyond what is required by Federal standards. One star is the lowest rating; five stars is the highest. More stars equal safer cars. NHTSA does not rate vehicles beyond 5 stars and does not rank or order vehicles within the star ratings.

NHTSA also notes that side crash and rollover rating results (at the star level) can be compared across all classes; however, frontal crash rating results can only be compared to other vehicles in the same class and whose weight is plus or minus 250 pounds (113 kg) of the vehicle being rated, as the frontal crash rating into a fixed barrier represents a crash between two vehicles of the same weight.

US-NCAP. The purpose of the US-NCAP is to provide consumers with comparative vehicle safety information to aid in vehicle purchase decisions. The 5-Star Safety Ratings Program was initiated in 1978 to measure the level of increased safety for vehicle occupants in frontal crashes. Side crash rating results were added with 1997 model year vehicles and rollover assessments with 2001 models.

The most recent update to maintain the relevance and effectiveness of NCAP was in model year 2011. The new 5-Star Safety Ratings include, for the first time, the overall vehicle score and a combination of results from two side-impact crash tests. (In April 2013, NHTSA requested pubic comments on further proposed changes to US-NCAP, including crash avoidance technologies as well as crashworthiness.)

The new US-NCAP star rating system provides consumers with a single, overall Vehicle Safety Score (VSS), rendered as a star rating. The VSS rating is a combination of scores achieved in 3 areas of assessment:

• Frontal Crash (42%: Driver & Passenger scores evenly split);

• Side Impact Crashes (33%: MBD driver 40%, MBD passenger 50%, Pole driver 10%); and

• Rollover Risk – SSF only (25%).

Furthermore, there are now several significant changes to those test protocols, including an additional side-impact “pole test” which is already standard in EURO-NCAP testing. During the pole test, the vehicle is propelled sideways into 25-cm diameter pole. Since the pole is rigid and relatively narrow there is major penetration into the side of the vehicle. This test simulates a side-impact crash into a narrow fixed object like a tree or utility pole.

Each year, NHTSA tests a sample of new vehicles predicted to have high sales volume or vehicles that have been structurally redesigned. For example, in September 2012, NHTSA announced it would rate 54 vehicles for the 2013 model year, including 33 passenger cars, 16 sport utility vehicles, one van, and four pickups. Tested vehicles are purchased from dealerships across the country; the vehicles are not supplied directly to NHTSA by the manufacturer.

Broadly, the process is to use the crash data from the different tests to calculate injury probabilities for the passengers (test dummies) in the different tests. A relative risk score factor (RRS) is calculated as the quotient from combined injury probability and a statistical quantifying parameter.

The RRS values are then used to generate the “star rating”—separated into driver and passenger side. The VSS unites the weighted risk assessments from all the tests.

The statistical Relative Risk Score (RSS) of Model S compared with all other vehicles tested since the current NCAP standards went into effect for MY 2011 and following vehicles. Source: Tesla Motors. Click to enlarge.

Model S. To date, 132 MY 2013 models, including the Model S, have been tested, with the data available online. Of all vehicles tested, the Model S earned the lowest VSS, representing the lowest likelihood of injury to occupants. Since NHTSA selects vehicles for testing on the criteria noted above, there are, however, some competitive gaps in the data: no testing for Mercedes-Benz S-Class, Audi A8 or BMW 7-Series, for example.

The MY 2013 vehicle with the next-lowest VSS is the Buick Verano (FWD) with 0.50, and also with a 5-star overall rating.

The Model S has the advantage in the front of not having a large gasoline engine block, thus creating a much longer crumple zone to absorb a high speed impact. This is fundamentally a force over distance problem, the company noted: the longer the crumple zone, the more time there is to slow down occupants at g loads that do not cause injuries.

The Model S motor is only about a foot in diameter and is mounted close to the rear axle, and the front section that would normally contain a gasoline engine is used for a second trunk.

For the side pole intrusion test, considered one of the most difficult to pass, the Model S was the only car in the “good” category among the other top one percent of vehicles tested. Compared to the Volvo S60, which is also 5-star rated in all categories, the Model S preserved 63.5% of driver residual space vs. 7.8% for the Volvo.

Tesla achieved this outcome by nesting multiple deep aluminum extrusions in the side rail of the car that absorb the impact energy (a similar approach was used by the Apollo Lunar Lander) and transfer load to the rest of the vehicle. This causes the pole to be either sheared off or to stop the car before the pole hits an occupant.

The rear crash testing was particularly important, given the optional third row children’s seat. For this, Tesla factory installs a double bumper if the third row seat is ordered. This was needed in order to protect against a highway speed impact in the rear with no permanently disabling injury to the third row occupants. The third row is already the safest location in the car for frontal or side injuries.

The Model S was also substantially better in rollover risk, with an RRS_Roll of 0.38; the next closest was the Honda Accord, with an RRS_Roll of 0.66. During testing at an independent facility, the Model S refused to turn over via the normal methods and special means were needed to induce the car to roll. The reason for such a good outcome is that the battery pack is mounted below the floor pan, providing a very low center of gravity, which simultaneously ensures exceptional handling and safety.

Of note, during validation of Model S roof crush protection at an independent commercial facility, the testing machine failed at just above 4 g. While the exact number is uncertain due to Model S breaking the testing machine, what this means is that at least four additional fully loaded Model S vehicles could be placed on top of an owner’s car without the roof caving in. This is achieved primarily through a center (B) pillar reinforcement attached via aerospace-grade bolts.

Optimizing. It is possible to “game” the regulatory testing score to some degree, Tesla suggested, by strengthening a car at the exact locations used by the regulatory testing machines. After verifying through internal testing that the Model S would achieve a NHTSA 5-star rating, Tesla took somewhat of the opposite approach than in the gaming scenario it suggested—the company said it analyzed the Model S to determine the weakest points in the car and retested at those locations until the car achieved 5 stars no matter how the test equipment was configured.

The Model S lithium-ion battery did not catch fire at any time before, during or after the NHTSA testing. Tesla noted that no production Tesla lithium-ion battery has ever caught fire in the Model S or Roadster, despite several high speed impacts. While this is statistically unlikely to remain the case long term, the company added, Tesla is unaware of any Model S or Roadster occupant fatalities so far.

Resources

• Steffen Sohr, Axel Heym (2009) Benefit Of Adaptive Occupant Restraint Systems With Focus On The New US-NCAP Rating Requirements (21^st ESV Conference, Paper Number 09-0322)