According to the safety organisation, it compiled crash reports for more than 400 frontal accidents across 14 states in the U.S.A and compared the injury risk in vehicles with knee airbags to vehicles without them. They then concluded that
“Knee airbags had only a small effect on injury measures recorded by dummies in IIHS driver-side small overlap front and moderate overlap front crash tests. In the small overlap test, knee airbags were associated with increased injury risk for lower leg injuries and right femur injuries, though head injury risk was slightly reduced. The airbags had no effect on injury measures in the moderate overlap test.”
Adding on, it stated that knee airbags only reduced the overall risk injury from 7.9% to 7.4%, a not very significant improvement statistically wise.
That said, it is possible knee airbags would be more effective in preventing injuries of those not using seat belts, but the IIHS study did not go into this area of research.