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Difficulty: MediumSpeed on Curves and Downgrades

An operator driving a loaded semi-trailer on a wet highway approaches an unfamiliar off-ramp posted with a curve advisory speed limit. Which of the following actions represents the correct and safest procedure for negotiating this curve?

  1. A
    Enter the turn at highway speed and apply firm service brakes only after detecting the trailer beginning to lean.
  2. Decelerate to a safe speed below the posted advisory speed before entering the curve, then maintain a steady speed with gentle throttle through the turn.Answer
  3. C
    Coast through the turn in neutral while maintaining continuous light pedal pressure across the entire curve arc to control speed.
  4. D
    Match the posted curve advisory speed and engage maximum auxiliary retarder braking while actively steering through the wet curve.

Answer

Decelerate to a safe speed below the posted advisory speed before entering the curve, then maintain a steady speed with gentle throttle through the turn.
Because commercial vehicles possess a high center of gravity and carry heavy loads, posted curve advisory speeds are often too fast for safe operation. Drivers must slow down to a safe speed before entering the curve. Once inside the curve, maintaining a steady speed in gear and applying light throttle keeps the combination stable and prevents dangerous traction loss.

Step-by-Step Solution

1
Assess vehicle dynamics relative to the posted advisory speed
Recognize that posted advisory speeds are intended for passenger cars, meaning a high-center-of-gravity commercial vehicle must travel significantly slower.
Heavy combination vehicles roll over at speeds much lower than passenger vehicles on the same radius of curve.
2
Complete all required deceleration prior to entering the curve
The truck enters the curve at or below its safe calculated speed without requiring service brake application during the turn.
Braking while turning divides available tire traction between lateral cornering forces and longitudinal braking forces, drastically increasing skid and rollover risks.
3
Maintain proper gear engagement and apply light, steady throttle through the curve
Drive-wheel torque stabilizes the vehicle combination and maintains traction along the curved path.
Keeping the drivetrain engaged prevents loss of directional control and avoids brake overheating.

Key Concept

Speed reduction timing and curve negotiation dynamics for heavy commercial vehicles
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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