Angular Impact Mitigation System for Bicycle Helmets to Reduce Head Acceleration and Risk of Traumatic Brain Injury
Accid Anal Prev. 2013 October ; 59: 109–117. doi:10.1016/j.aap.2013.05.019

There are some real gems in this study, such as:

--for standard cycling speeds, the typical bicycle helmet reduces the risk of concussion by 1% in oblique impacts.
--a speed of 10 mph corresponds "to to a high probability of TBI when wearing a conventional EPS helmet, which is confirmed by the finite element and IRF analyses showing a risk of TBI greater than 97%." (Before you start screaming: This does not imply that there is no risk of TBI without a helmet, only that a helmet does virtually nothing to prevent it).
--conclusion: "Contemporary bicycle helmets with standard EPS liners may not be optimized to mitigate head angular acceleration caused by oblique impacts representative of a realistic bicycle crash."