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attention to driving, or is not feeling
well physically or mentally. Perception
distance is how far a vehicle travels
during this time.
2 Driver reaction time is the amount of
time it takes between deciding to stop
and actually applying pressure to the
brake pedal. Normal driver reaction time
is about three quarters of a second.
Reaction time will be slower if the driver:
• is tired, or
• has been drinking alcohol or
using drugs
Reaction distance is how far a vehicle
travels during this time.
3 Lag time is the amount of time it takes
for the air brake system to respond after
the driver has applied pressure on the
brake pedal. Air brakes do not respond
immediately because it takes time for
the compressed air to flow through the
system and apply the brakes which
takes about 4/10 of a second. Lag time
distance is how far a vehicle travels
during this time.

4 Braking time is the amount of time
it takes for the vehicle to come to a
complete stop after the brakes have
been applied. Braking time depends on:
• the force with which the brakes
are applied
• the condition of the brake
linings and drums
• the traction of the tires on
the road surface
• the vehicle weight and speed

Braking distance is how far a vehicle
travels during this time.

Total stopping distance is the
sum of perception distance, reaction
distance, lag time distance and
braking distance.

Another factor involved in stopping
distances is the slope or grade of the road.
A vehicle travelling down a hill will need a
longer stopping distance than a vehicle
travelling at the same speed on a level
surface because of the effect of gravity.
A vehicle travelling up a hill will stop in a
shorter distance than a vehicle travelling
the same speed on a level surface, again
because of the effect of the grade.

General braking
information

• When applying the brakes, press down
the pedal using an even pressure and
then ease off the pedal as the vehicle
slows down. Just before the stop,
release the brakes to avoid a sudden
jerk or rebound. Then apply pressure
to the brake pedal again to hold the
vehicle while it is stopped.
• Do not pump (alternately applying and
then releasing) the air brakes as this will

42 A Commercial Driver’s Guide to Operation, Safety and Licensing

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