Braking and stopping distance

By the end of this topic you can

• You can explain the parts of total stopping distance.
• You can describe how speed, road surface and tires affect stopping.
• You can brake correctly in a car with anti-lock brakes (ABS).
• You can keep a following distance that gives you time to stop.

What you need to know and understand

• Total stopping distance is perception distance plus reaction distance plus braking distance.
• Braking distance rises with the square of speed: double your speed and the braking distance is roughly four times as long.
• Wet, icy, snowy or gravel roads all make braking distances longer.
• Worn tires and brakes increase stopping distance.
• California's handbook uses a three-second rule in good conditions (Pennsylvania teaches four seconds); leave more in poor conditions.
• With four-wheel ABS, press firmly and hold steady pressure; do not pump the pedal.
• Without ABS, avoid slamming the brakes; press and release them so the wheels do not lock and you can still steer.

Three parts of stopping

• Perception: the time it takes to see and recognize a hazard. Distraction, fatigue, alcohol and drugs make it longer.
• Reaction: the time it takes to move your foot from accelerator to brake. The car keeps traveling at full speed during this time.
• Braking: the distance the car travels once the brakes are applied, which depends on speed, road, tires and brakes.

Why speed matters so much

Because braking distance rises with the square of speed, a small increase in speed adds a lot of distance. The California handbook advises reducing speed by 5 to 10 mph on wet roads, cutting it by half on packed snow, and driving no faster than 5 mph on ice. Bad weather can multiply stopping distances many times over.

Using anti-lock brakes

ABS stops the wheels locking so you can keep steering during hard braking. In an emergency with four-wheel ABS, press the brake firmly, keep pressing and steer around the hazard if needed. You may feel the pedal pulse slightly and hear mechanical noises: this is normal, so keep your foot down. Check your owner's manual to see which type of ABS your car has.

Building in time

Pick a fixed object ahead. When the vehicle in front passes it, count. If you reach it before three seconds, drop back (some states, such as Pennsylvania, teach four seconds, so check your own manual). Increase the gap at night, in rain, behind motorcycles and large trucks, and in work zones. If someone is tailgating you, keep a steady speed and move to the right lane when it is safe so they can pass.

Common mistakes

• Pumping the brakes in a car with ABS.
• Following too closely to be able to stop.
• Keeping normal speed on wet or icy roads.
• Braking hard and late instead of early and progressively.

Test yourself: Braking and stopping distance

Answer each question, then read the explanation. Questions come in a random order each time.