Personally, I believe the "weak neck muscles contribute to Concussion risk" is bogus. It is based on faulty science. The laws of physics prove it to be wrong. But, weak neck muscles can increase the risk of the upper neck injuries that can magnify or mirror concussion symptoms.

A few years ago, someone posted a link to some neck exercises that were targeted at concussions.

But, regarding speed bumps, It helps to lean forward a bit and take your shoulders off the seat back as you go over the speed bump. Much of the vertical force is transmitted through the back of your shoulders. By keeping your shoulders off the seat back, you can minimize this force. Plus, if you roll your shoulders and head and neck forward a bit, the curve in your upper back is better able to absorb these forces.