> How that works when you hit a car at 15 mph I cannot say. Bicycle and motorcycle helmets are primarily designed for the vertical drop, not the horizontal. If

> How that works when you hit a car at 15 mph I cannot say. Bicycle and motorcycle helmets are primarily designed for the vertical drop, not the horizontal. If we recall our basic physics, impacts have a vertical and horizontal component. The vertical drop from 6+ feet under gravity is the big problem. Most of the time, the horizontal force is the drag of someone slowing down and sliding. There's also the repeated impacts of tumbling to consider. A horizontal impact mitigation really isn't in the cards. A vertical impact from 6 feet is roughly 15mph. From 15 to 25MPH is almost 3 times the energy, so you can see how it can quickly get untenable to try to mitigate the horizontal impact energy of a rider striking an object. During a crash, most folks don't strike things horizontally, but they're pretty much guaranteed to hit the ground. Therefore, the helmets are designed to solve the more common problem and most common source of head trauma. Even though bicycle and motorcycle helmets ostensibly solve the same problem, i.e. melon -> concrete, motorcycle helmets are more robust to deal with the higher speed tumbling and abrasion issues that bicyclists don't really encounter. Plus there's a lot more potential creature comforts in a motorcycle helmet designed for 50MPH travel.