Anything that is moving has energy due to its motion. You know this to be the case because if you try to stop something that is moving some effort is required to bring the item to a halt, energy has to be lost. Whether it's a ball, a car or a container ship they all have energy due to their motion, And if you were awake during physics lessons at school you will know that this energy is called
KINETIC ENERGY.
The thing about kinetic energy is that the amount of kinetic energy that a moving object has increases as the square of the speed. This means if you double the speed, the amount of kinetic energy has gone up not twice but FOUR times.
This kinetic energy in your moving object has to be got rid of to bring it to a stop. If you drive into a brick wall at 60mph you have not twice but four times the amount of energy to lose than if you were doing 30mph. And so the accident has the potential to be four times as bad at 60mph than it does at 30mph.
Here's a question to see if you've grasped the theory above. Two cars each weighing a ton and both doing 30mph have a head-on collision. If one car was stationary and the other doing 60mph would a head-on collision between them be the same, not so bad or worse?
If you said the same or not so bad, you'd be wrong. A few moments with your calculator would show that in the second case there is twice the amount of energy to be lost in the collision so it has the potential to be twice as bad.
And finally, what speed do you have to travel at to have twice the amount of energy than you have at 30mph? Answer is of course just over 42mph.