A small-overlap crash occurs when the front corner of a vehicle hits another vehicle or a fixed object. The IIHS passenger-side small overlap test is conducted at 40 mph with 25 percent overlap between the vehicle and the barrier it strikes.
"In our latest passenger-side tests, we didn't find any performance issues with safety belts or airbags like we did when we evaluated small and midsize SUVs earlier this year and midsize cars last year," IIHS Chief Research Officer David Zuby said in a statement Thursday. "Instead, we saw some structural deficiencies on the right side that still need addressing."
The Sienna rated poor, because it allowed structural intrusion, resulting in right thigh and lower leg injuries to the crash test dummy.
While Toyota has modified the structure of the vehicle to improve driver-side protection starting with 2015 models, the automaker did not make the same changes to the passenger side, IIHS said. The Sienna is the oldest of the models in IIHS' recent study.
The Pacifica -- introduced in the 2017 model year to replace the Chrysler Town & Country -- is rated acceptable because it has received upgraded protection in small overlap front crashes on both the driver and passenger sides, beginning with 2017 models built after August 2016.
The Pacifica also had intrusion issues, but sensors in the test dummies indicated a low risk of injury, offsetting the low structural rating. The Pacifica's passenger-side rating is based both on the IIHS crash test and a Fiat Chrysler test as part of the IIHS frontal crash test verification program.
Despite inconsistent ratings with overall structure, all three vehicles -- the Sienna, Odyssey and Pacifica -- rated good in passenger restraints and the head and neck and chest injury measures.
Both the Pacifica and Odyssey are 2018 Top Safety Picks.