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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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