Only a single American car earned the highest safety rating in the Insurance Institute for Highway Safety’s latest round of crash tests. The underlying message to automakers, as tougher criteria shut out many high-scoring 2015 models: Make automatic-braking available and keep strengthening those frames.

In total, 48 2016 models garnered the highest, Top Safety Pick+ rating. The Chrysler 200 was the only domestic car on that list, owing to its “good” (highest) score in the small front overlap, moderate front overlap, side, roof, and head-restraint tests, plus its offering an optional automatic-braking system that scored either an “advanced” or “superior” rating (the 200 scored “superior,” which indicates it can successfully prevent a crash from higher speeds). Those ratings qualify a car for the TSP+ slot. Last year, the criteria were easier. Cars that scored “acceptable” on the small front overlap test?which clips a quarter of the car’s frontal area and required automakers to stiffen their driver’s-side structures to minimize cabin intrusion and A-pillar buckling?still were allowed the TSP+ rating. The regular Top Safety Pick used to have the same requirements, but without any stipulations for collision avoidance. Now, a 2016 model earning TSP must offer frontal-collision warning (but not auto-braking) as optional or standard, and also must earn a “good” rating in the small overlap t...