[a humble lich]

A speed of 20 is closer to 40 mph. One yard per second is almost exactly 2 mph.

I like evileeyore's idea of using the speed table to get bonus damage and applying it to both parties. in this case, it means the attacker will take 6 damage to the hand, crippling it unless they have some kind of protection. Which makes sense, I think punching a street sign from the back of a moving truck sounds like a really really bad idea.

If the attacker is using a weapon, I might say that the weapon takes that damage and the attacker takes a fraction of it, because jousting from a motorcycle would hurt. Some kind of ST check would probably be required to not drop the weapon too.