The IIHS Top Safety Pick list is already quite long, but it keeps growing, with five more vehicles added this month alone. The latest pair are the 2012 BMW 3 Series sedan and the 2013 Acura RDX. The BMW needs no introduction, as the new F30 chassis has already earned its right to sit alongside previous generations of the well-loved sport sedan, while the RDX is a refreshed version of the midsize Acura crossover.

Both vehicles earned their listed status by upgrading the strength of their roofs. The previous-generation 3 Series scored only acceptable on roof strength testing, while the 2007-2012 RDX earned a marginal rating. The newest models are now rated "good" for roof strength.

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Acura RDX and BMW 3 series earn IIHS Top Safety Pick award

The 2013 RDX and 2012 3 series, both redesigned models, earn the Institute's top award for affording occupants good protection in front, side, rollover and rear crashes.

The RDX, a midsize luxury SUV, and the 3 series, a midsize luxury car, earn the award with improved roof strength for protection in rollover crashes. Top Safety Pick only applies to the redesigned 4-door version of the 3 series.

To measure roof strength, a metal plate is pushed against one corner of a vehicle's roof at a constant speed. The maximum force sustained by the roof before 5 inches of crush is compared to the vehicle's weight to find the strength-to-weight ratio. This is a good assessment of occupant protection in rollovers.

The 2007-12 RDX was rated marginal for roof strength. The 2006-11 3 series was rated acceptable.