Speed bumps that are filled with water, made of rubber so if you drive slow the water has time to move to other places than under your tires. But if you drive fast you will feel the bump.

The engine could gain horse power by taking away the valve springs. In place the cam would be over the valves, the valve stem would hook to the cam lobe on the sides, contact on the face would be a roller. The engine could be reved very high and not float a valve.