After some changes to make the requirements more stringent, no U.S. models earned The Insurance Institute for Highway Safety's Top Safety Pick Award.

The winners of the awards, which are based on crash tests conducted by the Insurance Institute, were announced Tuesday.

Last year, the Ford Five Hundred large sedan and the closely related Mercury Montego made the list. Both earned Gold Top Safety Pick Awards.

The criteria for an award are more stringent than last year. Previously, there were two levels of Top Safety Pick Awards, Gold and Silver. This year, there is only one and it harder to earn than last year's Gold Top Safety Pick Award.

Gallery: Top Safety Picks--->>http://money.cnn.com/popups/2006/autos/iihs/index.html

To earn a 2007 Top Safety Pick award, a vehicle must earn the best possible rating of "Good" in front and side impact safety as well as in whiplash protection for rear impacts. Previously, a vehicle could earn a Silver Top Safety Pick Award with only an "Acceptable" rating, the second-best possible, for rear impact protection.

Also, vehicles must be available with electronic stability control, at least as an option, to earn the award.

For that reason, five vehicles - the Ford Five Hundred, Mercury Montego, Subaru Impreza, Saab 9-2X and Honda Civic - that won the top award last year were dropped from the list this year.

Electronic stability control, or ESC, helps drivers maintain control during abrupt emergency maneuvers. According to crash statistics gathered by the Insurance Institute and the government's National Highway Traffic Safety Administration, ESC could prevent more than 10,000 deaths annually if it were installed on every vehicle in the U.S.

The Ford Five Hundred and Mercury Montego were the only vehicles from U.S. brands to make the list last year. Both Ford Motor Co. and General Motors are represented on this year's list by their respective Swedish subsidiaries, Volvo and Saab.

Japanese brands Honda and Subaru, a subsidiary of Fuji Heavy Industries, have the most vehicles on the list, with three each. Audi, the luxury division of Volkswagen, has two vehicles on the list and Mercedes-Benz, the luxury division of DaimlerChrysler, has one.

The Insurance Institute's front and side impact tests are different from those performed by the government.

For front impact safety, the Institute uses an "offset" test in which the vehicle strikes a barrier with just part of its front bumper, concentrating impact forces.

In its side impact test, the Institute strikes the vehicle with a moving barrier that is raised off the ground to mimic the impact of an SUV or truck.

Rear impact safety is calculated by, first, measuring various features of the seat and headrest. If those are judged to seem reasonably safe, the seat is then tested using an actual impact test.

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