The 2018 Toyota Prius and Prius Prime Prime no longer earn the highest safety rating of Top Safety Pick +. The Rating has dropped to Top Safety Pick (without the +). The car has not changed, so how did its safety rating go down from last year, when the 2017 Prime earned the Plus designation?
The IIHS rating requirements are not static and the Prius has undergone a new test that it did not earn a score of Good on. But that isn't why it dropped. The new test measured the safety of the Prius in a passenger-side small frontal overlap test. Like the current RAV4, Toyota's Prius is not equally safe on both sides according to test results. The driver's side is safer.
What caused the car to drop down to the second-highest tier was its headlights. A score of Good is now required for the "Plus" ranking and the Prius has only Acceptable headlights.
What is more worrisome about the older Prius cars of this same generation, is that they are not equally safe as the 2018. IIHS clarifies why saying:
"The Toyota Prius was redesigned for the 2016 model year and the Prius Prime was introduced in the 2017 model year. Beginning with 2018 models of the Prius and Prius Prime built after May 2018, a structural brace was added to the passenger-side lower door hinge pillar to improve occupant protection in passenger-side small overlap frontal crashes. (Information about when a specific vehicle was manufactured is on the certification label typically affixed to the car on the driver door or adjacent B-pillar.)"
Toyota knew the passenger-side teste would be added, so it beefed up the structure. As to why the Prius Prime tested did not earn a score of Good on the new test, IIHS clarifies, "The dummy’s head barely contacted the frontal airbag before sliding off the right side, allowing the head to move far enough forward to hit the dashboard. The side curtain airbag deployed and has sufficient forward coverage to protect the head from contact with side structure and outside objects. The side torso airbag also deployed."
You can review details of the 2018 Toyota Prius Prime's IIHS testing here. Note that you can expand the information by clicking the topics along the lower left side of the summary page.