Speed matters in a crash. A crash at higher speed contains more energy that translates directly into more damage to you and your vehicle. Speed matters when you need to brake. Your own reaction time (how quickly you perceive a threat and then decide what action to take) takes place before you put your foot on the brake. Then the mechanical actions of the vehicle, the conditions of the road, the weight of your vehicle, and the condition of your tires determine how long it takes to stop. Your car continues to cover ground – at speed. The higher the speed, the more ground you will cover during both your reaction time and the actual braking time of your vehicle.
A typical reaction time to perceive a threat such as a deer or a child running into the road is about 3/4 second, and then you add another 3/4 second to decide to act and move your foot to the brake pedal – that's 1.5 seconds so far. At 55 mph, the distance traveled is 121 feet. Only then does the car begin to slow. On dry pavement that takes 4 1/2 seconds, traveling another 144 feet, but if it's wet, you'll travel 183 feet. You can do the math – it has taken about as long as a football field to stop your car at 55 mph (265 and 303 feet), and that is assuming you were alert. At 30 mph, it is about half a football field.