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Question

Difficulty: MediumStopping and Braking Procedures

An operator of a commercial motor vehicle equipped with air brakes identifies an obstruction ahead on the highway. Arrange the chronological phases that make up the total stopping distance from initial hazard recognition to the complete cessation of vehicle movement.

  1. 1The driver sees the hazard and mentally processes the need to initiate stopping.
  2. 2The driver moves their foot from the accelerator pedal to press down firmly on the brake pedal.
  3. 3Compressed air flows through the brake valves and lines to mechanically engage the brake shoes against the drums.
  4. 4Friction between the brake linings and drums dissipates kinetic energy until the vehicle comes to a complete standstill.

Answer

The correct chronological sequence begins with hazard perception, followed by driver reaction time, air brake lag delay, and concludes with effective friction braking to a stop.
Total stopping distance in an air-brake equipped commercial vehicle consists of four sequential stages: perception distance (seeing and recognizing the hazard), reaction distance (moving the foot from accelerator to brake pedal), brake lag distance (air flowing through the system to actuate mechanical components), and braking distance (friction slowing the vehicle to a complete stop).

Step-by-Step Solution

1
Identify the initial cognitive event in the stopping sequence.
Perception occurs first as the driver sees and recognizes the road hazard.
Vehicle deceleration cannot begin before the driver observes and acknowledges the obstruction.
2
Determine the physical response step taken by the operator.
Reaction follows perception as the driver moves their foot to apply the service brake pedal.
Physical pedal actuation occurs immediately after the brain processes the decision to stop.
3
Account for the mechanical delay inherent to pneumatic braking systems.
Air brake lag occurs next while air pressure charges the lines and moves the pushrods.
Air takes approximately one-half second to travel from the foot valve through the lines to the brake chambers.
4
Identify the final kinetic stage where the vehicle slows to a rest.
Braking distance concludes the sequence as friction stops wheel rotation.
Once brake linings contact the drums, vehicle momentum decreases until the commercial unit is fully stopped.

Key Concept

Total Stopping Distance Breakdown in Air Brake Systems
Estimated Time:1m 0s
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Topics

Stopping and Braking Procedures

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