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Question

Difficulty: MediumSpeed on Curves and Downgrades

An articulated commercial vehicle is approaching a curved highway connector ramp. Why must the driver reduce the vehicle to a safe speed in a straight line before entering the turn rather than braking during the curve?

  1. A
    Braking should be delayed until the apex of the curve so that centrifugal force can help stabilize the trailer track.
  2. Braking while turning divides tire traction between steering and decelerating, which can cause the wheels to lock and induce a skid.Answer
  3. C
    Straight-line pre-braking is required solely to keep the auxiliary retarder continuously active across wet or slippery pavements.
  4. D
    Braking during a turn is prohibited because turning the steering wheel eliminates perception and reaction distance from total stopping distance.

Answer

Braking while turning divides tire traction between steering and decelerating, which can cause the wheels to lock and induce a skid.
Tires possess a limited amount of frictional adhesion with the road surface. When a driver brakes while simultaneously turning, available traction is divided between lateral cornering forces and longitudinal braking forces. If the combined demand exceeds tire grip, the wheels will lock, leading to loss of directional control, jackknifing, or rollover. Therefore, all necessary deceleration must occur in a straight line before steering into the curve.

Step-by-Step Solution

1
Analyze tire traction limits during vehicle maneuvering
Recognize that tire contact patches provide a finite amount of total grip that must be shared among braking, accelerating, and lateral steering forces.
Applying heavy service brakes while cornering demands both longitudinal braking friction and lateral cornering friction simultaneously.
2
Evaluate the risk of braking while actively turning
Determine that demanding both braking and cornering from tires in a curve easily exceeds available adhesion, causing tire slip, jackknife, or trailer rollover.
Commercial vehicles have a higher center of gravity and articulated pivot points, making them especially vulnerable to traction loss inside turns.
3
Identify the standard CDL speed management procedure for curves
Slow down to a safe speed in a straight line prior to entering the curve, then maintain gentle power through the curve.
Completing all deceleration before the turn preserves 100% of tire traction for steering stability throughout the curve.

Key Concept

Speed Management and Traction Allocation on Curves
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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