If a car weighing 2,000 pounds going 100 mph hit another car going the same speed and weighing the same, the potential energy released follows the formula E=1/2mv^2. (And yes, I know the difference between mass and weight and that this formula is for point energy and all that.)

So:

(2000 x (146.333fps^2))/2 = you work out the foot-pounds of energy released. Then multiply it by 2. The answer, in either foot-pounds or joules, is "An awful lot."

I'm pretty sure I've got this right. My days of calculatin' physics is long behind me, except when I really, really gotta, and I don't have a either a calculator OR a slide rule handy.