As Tesla Protests, What Exactly Is IIHS' Small Overlap Front Test?

The Tesla Model S and Model X are the safest cars in history... unless they're not. The variations in automotive crash testing are once again under the microscope, with Tesla protesting the latest batch of IIHS results after the Model S electric sedan failed again to ace the controversial small overlap frontal test. It's a frustrating blip for the EV company, which actually redesigned the car in the hope of performing better in the crash testing.

Tesla's previous Model S design scored "acceptable" in the small overlap frontal test, criticized at the time for its seatbelt allowing the crash dummy's torso to move too far forward. That meant the dummy's head could strike the steering wheel, even through the airbag. The January tweak aimed to reduce that forward movement.

However, the same problem occurred in IIHS' more recent crash tests, though for different reasons. This time around, there turned out to be less consistent movement of the left front wheel: it intruded 11-inches into the cabin in the lower part – versus under 2-inches in the first test – and 5-inches into the instrument panel. As such, it lost Tesla points overall.

"The first test resulted in a good rating for structural integrity, while the second test resulted in an acceptable structural rating," the IIHS concluded. "The two tests' structural ratings were combined, resulting in acceptable structure and an acceptable rating overall for the Model S."

Tesla, unsurprisingly, wasn't too impressed. In a statement, it highlighted that the small overlap frontal crash test was the only category in which the Model S was dinged by the IIHS, then went on compare the results to that of the NHTSA's testing. It also hinted at questions about the IIHS' subjectivity:

"Tesla's Model S received the highest rating in IIHS's crash testing in every category except for one, the small overlap front crash test, where it received the second highest rating available. While IIHS and dozens of other private industry groups around the world have methods and motivations that suit their own subjective purposes, the most objective and accurate independent testing of vehicle safety is currently done by the U.S. Government, which found Model S and Model X to be the two cars with the lowest probability of injury of any cars that it has ever tested, making them the safest cars in history."

The Insurance Institute for Highway Safety (IIHS) is a non-profit set up back in 1959 by a trio of some of the largest automotive insurers at the time. It evolved into an independent research organization, and in 1992 opened its Vehicle Research Center to perform crash testing on modern cars. It's work is funded entirely by auto insurers and insurance associations.

IIHS' tests focus on two areas. First, there's crashworthiness: effectively how well a vehicle manages to protect those inside it when a crash takes place. Second, though, is crash avoidance and mitigation: the technology that can try to prevent a crash from happening, or minimize it.

Cars are scored out of four – through good and acceptable, to marginal and finally poor – for crashworthiness, and out of three – basic, advanced, or superior – for crash avoidance and mitigation. IIHS also scores the headlamp performance, using the same ratings as for crashworthiness. Those with the best scores get TOP SAFETY PICK or TOP SAFETY PICK+ awards.

Added to the IIHS' test roster in 2012, the small overlap frontal test is one of two front-collision tests the organization performs. Initially, it started out with the moderate overlap frontal test, which examined how cars held up when 40-percent of the front of a car hits an aluminum honeycomb deformable barrier at 40 mph. That represents the forces involved when two similarly-weighted vehicles strike each other at just under 40 mph apiece.

The small overlap frontal test is a variation of the same. It keeps the 40 mph speed but uses a 5 foot tall, rigid barrier, which hits 25-percent of the front of the car, biased toward the driver's side. It simulates hitting either an oncoming vehicle or a tree or utility pole. IIHS examines what happens to a dummy in the driver's seat representing an average-sized man.

Primarily, because up until recently many cars simply weren't designed with this sort of crash in mind. Crush zones – which deform in predictable ways to manage how crash forces are passed through the structure of a vehicle – and the safety cage surrounding the car's occupants were focused on the middle 50-percent of the front of the vehicle. That makes most sense in a head-on collision, but not when the impact is offset.

"Crash forces go directly into the front wheel, suspension system and firewall," the IIHS explains. "It is not uncommon for the wheel to be forced rearward into the footwell, contributing to even more intrusion in the occupant compartment and resulting in serious leg and foot injuries."

It's important, the organization says, because a quarter of deaths from frontal crashes are down to small overlap crashes. In them, the driver's side front wheel gets hit with the majority of the impact forces, rather than them being absorbed by the crush zones. Meanwhile a study by the Medical College of Wisconsin found a greater likelihood of head, chest, spine, hip, and pelvis injuries were sustained during small overlap collisions.

The National Highway Traffic Safety Administration, or NHTSA, has been running crash tests since 1978. Its NCAP – New Car Assessment Program – ratings are now legally required to be included on the Monroney sticker, the pricing and specifications notice each new car bears. However, exactly what's tested is different from what the IIHS covers.

At the start, the NHTSA merely looked at what happened when a car had a front collision at a steady 35 mph. That expanded over time to encompass four different categories, including resilience to impacts from the side, a side pole crash where the car impacts sideways to a roughly 10-inch diameter pole, and a rollover at 55 mph. Each earns a star rating out of five.

The NHTSA's frontal crash test benefits from all the crush zone and safety cage protections. Success there is often down to how well the airbags and seatbelts perform, whereas in contrast the IIHS' tests look at structural design.

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Of course, no real-world crash is quite the same as a test carried out in lab conditions. Even if we shift entirely to autonomous vehicles, it seems unlikely that we'll ever get away from the occasional accident. The best advice, therefore, is to pick a vehicle that performs well in both NHTSA and IIHS tests.