Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Bike helmets are designed to withstand the speed at which your head hits the ground (ignoring Scoblephysics) - this would be the same if you are stationary, or traveling at 50kph (assuming you don't hit something other than the ground).

Ah right... I can see this makes sense, but imagine (in a similarly 100% theoretical way) your bike being knocked 90 degrees so you're tipped over, wouldn't you be flung towards the ground much faster at 50mph than 5mph? Stationary, it'd be a tin-whistle soundtracked slump.