I get asked from time to time why I am always telling people to slow down. My answer is always the same...the slower you are travelling, the more time you have to see what is happening around you and the more time you will have to react to changing situations.
But what that means in the end is simple...as speed doubles, stopping distances increase exponentially. As speed decreases, stopping distances decrease in the same manner.
When you are moving you are covering distance which is relative to the speed or velocity that you are travelling. The faster you are going, the more distance you travel in a relative time.
At 50 km/h you are travelling at a velocity 13.89 meters per second.
At 100 km/h you are travelling at a velocity of 27.78 meters per second.
So lets assume that 10 meters (32 feet) from where you are you see a child run into the street in front of you. I won't even get into perception and reaction time, just a simple "hammer the brakes" to avoid the child. You are travelling in a 40 km/h school zone so you thing that going just a little over the speed limit is ok...well lets look at that.
At 50 km/h on dry asphalt with 100% braking it will take you 13.1 meters (42 feet) to stop. So 10 feet after you struck the child you stopped.
So, why slow down? Lets take 10% off the speed.
At 45 km/h on dry asphalt with 100% braking it will take you 10.62 meters (34.84 feet) to stop. So 2 feet after you struck the child you stopped.
Considering that approximately half your speed gets cleared in the last five meters of braking, the impact speed is far greater when the child gets hit.
Now here is what happens when you travel the speed limit.
At 40 km/h on dry asphalt with 100% braking it will take you 8.39 meters (27.52 feet) to stop.
Congratulations! The child now looks at the front of your car, grabs his ball and runs back to the sidewalk. You get home and the we never have to go tell the child's parents that because you were going just a little over the speed limit they need to go to the hospital to see their child...or worse.
The above is best case scenario...you want the reality? Add distance for perception and reaction time, add distance for poor tires, insufficient brakes, distraction and wet roads....so, slow down and drive safe.
Don't take it from me, here are a couple of things that support what I am saying.