Yeah, your "horizontal" speed is a potential factor in the severity of a helmet impact, but the critical factor is the distance your head is falling, just on account of the acceleration of gravity. Think of it this way: imagine putting something about the weight of a human head (3-4 kg?) into a bike helmet, and simply drop it on a concrete floor from the height of a driver of a DF bicycle. How would you guess the helmet would fare? This is the speed and energy at which your head and helmet contact the pavement in a fall, regardless of how fast you're going horizontally.