The IIHS has expanded its verification tests after finding that some automakers were only beefing up the driver-side structure.

The Insurance Institute for Highway Safety has expanded its 'gotcha' tests to identify vehicles that only received small-overlap crash reinforcement on the driver side.More than a dozen midsize cars were tested a second time to verify that a mirrored impact to the passenger side would result in similar safety performance. Most still received a 'good' overall rating in the follow-up trial, though the Volkswagen Passat and Chevrolet Malibu were both deemed 'marginal.'

"In both cars, the passenger dummy's head slid off the front airbag and contacted the dashboard," the IIHS report notes. "Measures taken from the dummy showed head injuries would be possible in a real-world crash of the same severity."

The small-overlap test was introduced in 2012 to address a common deadly crash scenario that concentrates the full impact force on a much smaller frontal cross-section than a moderate overlap or full-width connection.

Many automakers quickly revised models to better cope with the unique small-overlap forces, but IIHS researchers eventually noticed that some vehicles appeared to have received reinforcement only on the driver side. The observation immediately led to suspicion that some vehicles were narrowly engineered to ace the IIHS test without consideration for real-world crashes involving the other side.

The cars showed a much narrow discrepancy than a previous test of 2014-2016 model-year small SUVs. Out of seven high-riding models reviewed for passenger-side performance, only one maintained its 'good' rating and three were considered 'marginal' or 'poor.'

Interestingly, to alleviate suspicions of vehicles that have not been verified, the institute says it will accept automaker test data and assign a rating. 'Occasional audit tests' will still be performed in the future, however.

A 'good' or 'acceptable' passenger-side rating in the small-overlap test will be required for vehicles listed on the upcoming 2018 Top Safety Pick Plus honor roll.