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Difficulty: MediumSpeed on Curves and Downgrades

A commercial driver operating a loaded freight vehicle is exiting an interstate highway onto a deceleration ramp posted with a sharp curve warning and a 30 mph30\text{ mph} advisory speed plate. Which of the following procedures should the driver follow to safely manage vehicle speed through the ramp curve?

  1. A
    Enter the curve at highway speed and apply continuous light pressure on the service brakes throughout the entire turn to control momentum.
  2. B
    Maintain highway travel speed until reaching the apex of the curve, then activate the engine retarder at its maximum setting while steering.
  3. Brake in a straight line before entering the curve to reduce speed below the posted advisory limit, then maintain slight power through the turn.Answer
  4. D
    Coast into the curve in neutral and apply firm service braking only if excessive body roll or tire squeal occurs mid-turn.

Answer

Brake in a straight line before entering the curve to reduce speed below the posted advisory limit, then maintain slight power through the turn.
The correct procedure requires reducing speed in a straight line before entering the curve to a speed below the posted advisory limit, followed by gentle acceleration through the curve. Posted advisory speeds are calculated for passenger vehicles; commercial vehicles have a higher center of gravity and must travel slower. Pre-braking preserves 100% of tire traction for directional steering.

Step-by-Step Solution

1
Assess the ramp advisory speed and vehicle configuration prior to entering the turn.
Recognize that advisory speeds are calibrated for passenger cars and loaded commercial vehicles require lower entry speeds due to a higher center of gravity.
Heavy commercial vehicles risk rollover or loss of directional control at speeds safe for passenger cars.
2
Complete all necessary service braking while the vehicle is still traveling in a straight line before curve entry.
The vehicle enters the curve at a controlled, safe speed without demanding simultaneous braking and turning traction from the tires.
Tires have a finite traction envelope; dividing grip between turning and hard braking reduces lateral stability.
3
Maintain a steady gear and apply light, smooth engine power throughout the curve.
Weight remains evenly distributed across axles, stabilizing the combination through the turn.
Gentle acceleration through the curve stabilizes the suspension and keeps the drive wheels engaged.

Key Concept

Safe curve negotiation requires straight-line deceleration prior to curve entry to preserve lateral tire traction and prevent rollover.
Estimated Time:1m 0s
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Topics

Speed on Curves and Downgrades

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