Page 34 - Driving Commercial Vehicles Manual

chapter 2 — heavy vehicle braking

Stopping distance and stopping time
definition
To stop a vehicle, you need to see-think-do. Total stopping distance is the
Driver’s perception time distance your vehicle will travel from the moment you:
+ reaction time • see — a hazard
+ brake lag time • think — decide to stop
+ braking time
• do — place your foot on the brake pedal until you stop.
= total stopping time
This distance can also be expressed as the time it takes to stop.
When you see a problem ahead while driving, it’ll take you about / 4 of a
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driving tip second of perception time (see-think) and another / 4 of a second of reaction
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time (do). Only then will your vehicle begin to slow.
Total stopping time will A car’s brakes begin to work almost instantly when you press on the brake
be even longer if brakes pedal. This is not the case with air brakes because there’s a brake lag time of
are incorrectly adjusted or approximately / 10 of a second between when you apply the brake and when
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vehicle is on a downgrade.
the brakes begin to function. Add to this the braking time, which is the time
The most common air — or distance — the vehicle travels before it stops.
brake system defect found
during a commercial vehicle This is why it’s so important to allow enough distance when following
inspection is brakes that are other vehicles.
out of adjustment.
Passenger cars and light truck drivers should use the two-second rule. Watch
the vehicle ahead pass some checkpoint on the roadway, such as an overpass
or sign post. Start to count: “One thousand and one, one thousand and two.”
That’s two seconds.
Drivers of buses, trucks and other large heavy vehicles should never be less
driving tip than five seconds behind the vehicle ahead at highway speeds.

Refer to chapter 3, basic driving skills, for more information on following
Total stopping distance or
time depends on the ability distance.
of the brake linings or pads Stay even further back when conditions are less than ideal.
to produce friction, the brake
drums to dissipate heat and
the tires to grip the road.

Total stopping distance
includes perception time,
reaction time, brake lag
time and braking time.
Always allow enough
following distance or time
and choose a speed that will
allow you to stop safely.

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