The National Highway Traffic Safety Administration tells motorists that a vehicle with side airbags can be expected to get one more star than a comparable vehicle without side airbags in the government's five-star crash test program, said agency spokesman Tim Hurd.
That would mean an occupant in a vehicle equipped with a side airbag has a chance of serious injury that is 5 to 10 percentage points less than someone in a vehicles without side bags struck in the side at 38.5 mph.
But crash test results have not always borne out that rule of thumb, Hurd added.
Bolder claims of effectiveness for certain side airbags come from the Insurance Institute for Highway Safety, the research arm of auto insurers.
In a series of reports in the last couple of years, the institute says side airbags that provide head protection reduce the risk of serious head injuries in certain side crashes to a small fraction of what it would be otherwise.
The institute says federal research shows about 600 lives a year would be saved if all vehicles had side airbags with head protection.
In addition, automakers increasingly are looking at side airbag curtains as useful devices for keeping unbelted occupants from being ejected, especially in rollovers.
Still, the benefits of side airbags are less clear than the benefits of front airbags - and there are good reasons why, safety experts say.
Federal law requires the installation of front airbags that meet specific safety standards. By contrast, automakers are voluntarily offering a range of side airbag designs, as standard equipment on some models and as options on others. They may be mounted in doors, seat backs or roof rails, or some combination of those locations. They may be designed to protect the chest, the head or the head and chest.
And the designs are proliferating, adding to the disparity in performance.