It seems to have been “Drivers overtaking bicyclists: Objective data on the effects of riding position, helmet use, vehicle type and apparent gender”[1] (doi: 10.1016/j.aap.2006.08.010).
This study is cited in every discussion of helmet use, yet it was barely scientific -- the author was his own test subject, and there has been no attempt at replication.
[1] http://www.sciencedirect.com/science/article/pii/S0001457506...