Imagine a helmet that will only appear when you needed it. I saw this concept some time ago. Now, they have become a reality and you can purchase at a price. Check out Hövding.
The idea of Hovding is like an airbag function. By using sensors, the “helmet” will be deployed once it detects a crash or a fall. This Hovding helmet can only be used once. You will need to purchase it again if it has been deployed.
BTW, it does not come cheap. At US$600 a piece, you will get a piece of technology not seen before and envy from other fellow cyclists. However, you have to ensure that you charge the battery and to turn it on before you ride. No battery equals no protection.
Here is a video of how it works.
BTW, the project started at 2005 as an industrial design master thesis at Lund University and has progressed until the product that you see today. Hope that the price could be lower to allow mass adoption.
(source, Thanks to Alex)