A good reference for car safety are the Euro NCAP tests, these are more stringent than the US federal tests (NHTSA), and at least as stringent as the IIHS tests.
In the Euro NCAP tests the 2004 Prius with side airbags (std in Europe) got 5 stars, the highest ratings:
http://www.euroncap.com/content/safe...?id1=3&id2=193
The only test I know of where the Prius did not fare that well was in rear impact testing (IIHS), on account of poor seat and head restraint design:
http://www.iihs.org/ratings/ratingsbyseries.aspx?id=566
otherwise the PII with side bags has the highest ratings of good. Without side bags it has the lowest possible rating of Poor for side impact, which basically means:
get side airbags!