The speed of your car affects the distance required to stop it. Stopping distance is determined by three factors:
- Perception distance. This is the length a vehicle travels from the time you see a hazard until your brain recognizes it. For an alert driver, this is approximately ¾ of a second.
- Reaction distance. This is the length a vehicle travels in the time it takes your brain to tell the foot to move from the gas pedal to the brake pedal and apply pressure. This takes approximately ¾ of a second.
- Braking distance. This is the length it takes to stop a vehicle once the brakes are applied.
Here's some food for thought. At 55 mph, your vehicle is traveling at about 80 feet per second. Feet-per-second is determined by multiplying speed in miles-per-hour by 1.47 (55 mph x 1.47 = 80 feet per second.) With this in mind, let's add the perception and reaction distance to the formula.
You're traveling at 80 feet per second and you see a hazard in the road ahead. It takes about ¾ of a second for your brain to acknowledge the hazard. During this fraction of a second, you've traveled an additional 60 feet. This is the perception distance.
Now that your brain has acknowledged the hazard ahead, it takes another ¾ of a second for it to tell the foot to move from the gas pedal to the brake pedal and apply pressure. During this reaction time, you've traveled another 60 feet.
So from the time you perceive the hazard until the time your foot is applying pressure to the brake pedal, you've traveled 120 feet but your car still isn't stopped. At 55 mph, on a dry road with good brakes, your vehicle will skid approximately 170 feet more before stopping. This distance, combined with the perception and reaction distances, means you need about 300 feet to stop a car traveling at 55 mph. As a point of reference, Lambeau Field is 360 feet long, end to end. Keep this in mind as you follow that other car on your way home tonight.