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Difficulty: Very hardSpeed on Curves and Downgrades

An 80,000-pound combination vehicle is about to travel down a 5-mile, 6% decline. The pavement is wet from recent showers. The driver has calculated a safe straight-line descent speed of 40 mph. Midway down the hill, there is a sharp bend with an advisory speed of 35 mph. How should the driver correctly manage the vehicle's speed and stability for this specific scenario?

  1. Ensure a low gear is engaged and the auxiliary braking system is turned off before starting down the hill. Apply the service brakes to reduce speed from 40 mph to 35 mph, release them until the speed naturally returns to 40 mph, and slow down to a safe speed well below 35 mph prior to entering the bend.Answer
  2. B
    Leave the auxiliary braking system on to reduce heat buildup in the foundation brakes, select a lower gear before the decline, and maintain a constant 40 mph using a continuous, light drag on the brake pedal until approaching the bend.
  3. C
    Turn off the auxiliary braking system and select a low gear prior to the decline. Apply light but continuous pressure to the service brakes to hold exactly 40 mph, then firmly apply the brakes while steering through the bend to achieve the 35 mph advisory limit.
  4. D
    Start down the hill in the standard cruising gear with the auxiliary braking system off. Employ the 5-mph reduction braking cycle on the straightaway, but wait until the tractor is actively turning inside the bend to brake down to the 35 mph advisory limit.

Answer

Engage a low gear and turn off the auxiliary brakes before the descent, use snub braking (40 mph down to 35 mph, then release) on the straight sections, and ensure the vehicle is slowed well below the advisory speed before entering the curve.
The correct sequence perfectly synthesizes three critical commercial driving safety rules: 1) The auxiliary braking system must be turned off on wet roads to prevent drive-wheel skids. 2) The proper service braking technique on a downgrade is snub braking (applying brakes to 5 mph below the safe speed, then releasing). 3) A driver must reduce speed to a safe level before entering a curve, as braking inside a curve significantly increases the risk of a skid or rollover.

Step-by-Step Solution

1
Evaluate the road conditions (wet pavement).
Determine that the auxiliary braking system (retarder) must be turned off.
Retarders apply braking force only to the drive wheels, which can quickly lose traction on slippery surfaces and cause a jackknife.
2
Determine the proper method for straight-line speed control on the steep grade.
Apply the snub braking method: brake from the 40 mph safe speed down to 35 mph, release the brakes, and repeat.
Continuous braking causes friction heat buildup leading to severe brake fade. Snub braking allows the brake drums to cool between applications.
3
Determine the correct approach for the mid-hill curve.
Brake to a speed significantly below the 35 mph advisory limit before entering the curve.
Braking inside a curve drastically increases the risk of skidding and rollover because the tires are already handling maximum lateral forces.

Key Concept

Comprehensive Downgrade and Curve Speed Management
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Topics

Speed on Curves and Downgrades

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