Don't know about studies, but while living in Germany in the 70's (in service), my wife and I were out for a walk one day. We were going up a fairly steep hill road when a guy on a bicycle came down the hill. He had a child riding on the handlebars. (yeah, stupid). The child must have stuck his foot in the spokes of the front wheel, because the bike came a near dead stop, did a 180 degree flip, and the guy went head over heels over the child, hit the pavement head first, knocking him out cold. I thought he was dead at first.
We ran over, got the kid out of the spokes (he wasn't hurt, just scared/crying), called the Polezei, who got an ambulance on scene, and they carted the guy off, still out of this world. I don't know that a helmet would have helped, but I suspect the guy had a serious concussion at a minimum.