The Insurance Institute for Highway Safety is considering whether to start a front passenger safety rating after tests determined that good driver protection doesn't always extend to all occupants at the front of the vehicle.

IIHS recently tested several small SUVs which received good ratings in driver side small overlap crash tests. When the same test was done on the passenger side, only one model performed at a level which would have merited a good rating.

"This is an important aspect of occupant protection that needs more attention," said Becky Mueller, a senior research engineer at IIHS and lead author of the study. "More than 1,600 right front passengers died in frontal crashes in 2014."

The small overlap test was introduced by IIHS in 2012, following the successful implementation of the moderate overlap test. While the moderate overlap test affects 40 percent of a vehicle's width, the small overlap test affects only 25 percent of this area. This test is meant to simulate crashes where the front corner of a vehicle collides with another vehicle or a stationary obstacle.

When assessing safety ratings for these frontal crashes, IIHS currently tests the left front corner of the vehicle. The test is done on this side because every vehicle on the road is occupied by a driver, but not all vehicles will have a passenger in the front seat.

Thirteen automakers made structural changes to a total of 97 vehicles after the small overlap test was introduced. Almost three-quarters of these vehicles earned a good rating after the redesign.

However, the recent IIHS tests suggest that some automakers may have focused on protection for the driver side while making little or no changes to the passenger side. In the tests, seven small SUVs with good driver side small overlap ratings were given small overlap tests on the passenger side.

The 2016 Hyundai Tucson was the only model whose performance would have earned it a good rating. Three other models performed at a level equal to a rating of acceptable. Two would have received marginal ratings, and one would have received a poor rating.

IIHS previously found that automakers commonly sought to improve a model's small overlap rating by strengthening the vehicle cabin. These methods including using a stronger material or slightly increasing the thickness of the structure. These changes may have only been made on the driver's side in some models.

"It's not surprising that automakers would focus their initial efforts to improve small overlap protection on the side of the vehicle that we conduct the tests on," said David Zuby, executive vice president and chief research officer at IIHS. "In fact, we encouraged them to do that in the short term if it meant they could quickly make driver side improvements to more vehicles. As time goes by, though, we would hope they ensure similar levels of protection on both sides."

While the front structural design of the Tucson was symmetrical and had similar performance on both the driver and passenger sides, other models did not perform as well on the passenger side test even if the structural design was symmetrical. The 2014 Nissan Rogue and 2015 Toyota RAV4 were the only vehicles with an asymmetrical front design and would have earned marginal and poor ratings, respectively.

The main safety issue identified by the test was intrusion. This factor determines how well the structure withstood the impact by measuring how far the structure intruded into the cabin. The risk of serious injuries increases with a greater amount of intrusion.

In the test of the RAV4, the intrusion on the passenger side was 13 inches more than in the small overlap test on the driver's side. The door also opened in the impact, a factor which would increase the possibility of an occupant being ejected from the vehicle in a real crash.

The intrusion was 10 inches more on the passenger side than on the driver's side in the Rogue test. The door hinge pillar was also torn off by the impact.

IIHS noted that two models whose frontal structural appeared symmetrical—the 2014 Subaru Forester and the 2015 Mazda CX-5—also allowed significantly more intrusion on the passenger side than on the driver's side.

Two moderate overlap tests were also conducted to see if there were any substantial differences in this type of crash. Researchers used a visually symmetrical vehicle, the 2015 Honda CR-V, as well as a visually asymmetrical vehicle, the RAV4. These tests determined that both vehicles would have received good ratings for the passenger side.

"We conducted the moderate overlap tests as a spot check, and we weren't surprised that both vehicles performed well," said Mueller. "Many of today's models are designed for the global market and are subject to driver side moderate overlap tests in right hand drive countries. With small overlap, there isn't the same incentive for symmetrical design because we're the only organization conducting the test."

IIHS says it could implement small overlap ratings in 2017 and make it them a requirement for their "Top Safety Pick" awards as early as 2018.