YSK: A crash at 30mph is not twice as bad as 15mph, it's 4x as bad. Similarly, a crash at 60mph is 16x as bad. This is because kinetic energy is proportional to squared-velocity.
from /r/YouShouldKnow
YSK：時速30英里的車禍不是15英里的兩倍，是4倍。同樣地，以60英里/小時的速度撞車是16倍的糟糕。這是因爲動能與速度的平方成正比。