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：时速30英里的车祸不是15英里的两倍，是4倍。同样地，以60英里/小时的速度撞车是16倍的糟糕。这是因为动能与速度的平方成正比。