Louisiana Commercial Driver License
Doubles and Triples Exam Practice
Questions And Correct Answers (Verified
1. During the uncoupling of a set of doubles, the driver has released the dolly air lines
and electrical cord but cannot release the dolly from the rear trailer. What is the most
likely cause?
A. The dolly spring brakes are still engaged due to insufficient air pressure.
B. The dolly converter gear is still locked to the rear trailer's kingpin.
C. The rear trailer's landing gear is not fully raised.
D. The tractor protection valve is closed, trapping air in the dolly.
Answer: B. The dolly converter gear is still locked to the rear trailer's kingpin.
Rationale: The dolly converter gear locking mechanism must be manually released; if it
remains locked, the dolly cannot separate from the rear trailer. Spring brakes (A) affect
movement, not coupling. Landing gear (C) affects trailer support, not dolly release. Tractor
protection valve (D) controls air supply, not physical locking.
2. A driver is coupling a tractor to a set of doubles. After connecting the air lines and
electrical cord, the driver performs a tug test. Which of the following indicates a
proper connection?
A. The tractor moves forward slightly and stops when the trailer brakes are applied.
B. The tractor moves forward without resistance, indicating the trailer is free.
C. The trailer wheels lock and the tractor stalls.
D. The trailer moves backward, indicating the kingpin is engaged.
Answer: A. The tractor moves forward slightly and stops when the trailer brakes are
applied.
Rationale: A tug test involves gently pulling against the locked trailer brakes; if the
coupling is secure, the tractor will not move or will move only slightly. Free movement (B)
indicates a missed kingpin. Wheel lock or stall (C) suggests brake system issues. Backward
movement (D) is not a normal response.
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,3. When calculating the gross vehicle weight (GVW) of a triple-trailer combination,
which factor is most critical for maintaining compliance with Louisiana bridge laws?
A. The total weight of the cargo only.
B. The distribution of weight across all axles, including the dolly axles.
C. The weight of the tractor and first trailer only.
D. The weight of the heaviest trailer.
Answer: B. The distribution of weight across all axles, including the dolly axles.
Rationale: Bridge laws regulate weight per axle group, so proper distribution across all
axles, including dolly axles, is essential to avoid overloading any single axle. Cargo weight
alone (A) ignores axle limits. Partial weights (C, D) do not represent the full combination.
4. In a triple-trailer setup, the driver notices that the second trailer's brakes are not
releasing. Which component is the most likely culprit?
A. The relay valve on the second trailer.
B. The tractor protection valve.
C. The dolly's emergency brake valve.
D. The first trailer's spring brake control valve.
Answer: A. The relay valve on the second trailer.
Rationale: The relay valve on the second trailer controls the release of its brakes; a
malfunction can cause the brakes to stay engaged. The tractor protection valve (B) affects
the whole system, not just the second trailer. The dolly's emergency valve (C) affects the
dolly brakes. The first trailer's valve (D) is unrelated to the second trailer's brakes.
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,5. A driver is backing a set of doubles under a loading dock. Which of the following
techniques is most effective for maintaining control and avoiding jackknifing?
A. Back slowly and use the mirrors to monitor the rear trailer's movement.
B. Back quickly to minimize the time in the blind spot.
C. Use a spotter at all times, regardless of visibility.
D. Turn the steering wheel sharply to correct any misalignment.
Answer: A. Back slowly and use the mirrors to monitor the rear trailer's movement.
Rationale: Backing slowly while using mirrors allows the driver to monitor the rear trailer
and make gradual corrections, reducing jackknife risk. Backing quickly (B) increases
danger. A spotter (C) is helpful but not always available; technique is primary. Sharp
steering (D) can cause jackknifing.
6. When inspecting a double trailer combination, the driver must check the air lines
for proper connection. Which color air line is typically used for the emergency
brakes?
A. Red
B. Blue
C. Green
D. Black
Answer: A. Red
Rationale: In standard color coding, the red air line is for emergency brakes. Blue (B) is
for service brakes. Green (C) is often used for secondary or auxiliary lines. Black (D) is not
a standard emergency line color.
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, 7. A driver is driving a set of doubles on a curve. The rear trailer begins to swing
outward. What is the most appropriate immediate action?
A. Apply the brakes hard to stop the swing.
B. Accelerate slightly to pull the trailers straight.
C. Steer into the direction of the swing.
D. Release the accelerator and steer gently in the opposite direction.
Answer: D. Release the accelerator and steer gently in the opposite direction.
Rationale: If the rear trailer swings out, releasing the accelerator and steering gently
opposite can help straighten the combination. Hard braking (A) can worsen the swing.
Accelerating (B) may increase the swing. Steering into the swing (C) can cause jackknifing.
8. Which of the following is a key difference between the coupling procedures for a
single trailer versus a set of doubles?
A. Doubles require the use of a dolly converter gear.
B. Doubles require a fifth wheel on the rear trailer.
C. Doubles do not require air line connections.
D. Doubles require the tractor to be equipped with a pintle hook.
Answer: A. Doubles require the use of a dolly converter gear.
Rationale: Doubles require a dolly converter gear to connect the first trailer to the second.
A fifth wheel (B) is on the tractor, not the rear trailer. Air lines (C) are required for all
trailers. A pintle hook (D) is used for towing, not for doubles coupling.
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Doubles and Triples Exam Practice
Questions And Correct Answers (Verified
1. During the uncoupling of a set of doubles, the driver has released the dolly air lines
and electrical cord but cannot release the dolly from the rear trailer. What is the most
likely cause?
A. The dolly spring brakes are still engaged due to insufficient air pressure.
B. The dolly converter gear is still locked to the rear trailer's kingpin.
C. The rear trailer's landing gear is not fully raised.
D. The tractor protection valve is closed, trapping air in the dolly.
Answer: B. The dolly converter gear is still locked to the rear trailer's kingpin.
Rationale: The dolly converter gear locking mechanism must be manually released; if it
remains locked, the dolly cannot separate from the rear trailer. Spring brakes (A) affect
movement, not coupling. Landing gear (C) affects trailer support, not dolly release. Tractor
protection valve (D) controls air supply, not physical locking.
2. A driver is coupling a tractor to a set of doubles. After connecting the air lines and
electrical cord, the driver performs a tug test. Which of the following indicates a
proper connection?
A. The tractor moves forward slightly and stops when the trailer brakes are applied.
B. The tractor moves forward without resistance, indicating the trailer is free.
C. The trailer wheels lock and the tractor stalls.
D. The trailer moves backward, indicating the kingpin is engaged.
Answer: A. The tractor moves forward slightly and stops when the trailer brakes are
applied.
Rationale: A tug test involves gently pulling against the locked trailer brakes; if the
coupling is secure, the tractor will not move or will move only slightly. Free movement (B)
indicates a missed kingpin. Wheel lock or stall (C) suggests brake system issues. Backward
movement (D) is not a normal response.
Page 1 of 55
,3. When calculating the gross vehicle weight (GVW) of a triple-trailer combination,
which factor is most critical for maintaining compliance with Louisiana bridge laws?
A. The total weight of the cargo only.
B. The distribution of weight across all axles, including the dolly axles.
C. The weight of the tractor and first trailer only.
D. The weight of the heaviest trailer.
Answer: B. The distribution of weight across all axles, including the dolly axles.
Rationale: Bridge laws regulate weight per axle group, so proper distribution across all
axles, including dolly axles, is essential to avoid overloading any single axle. Cargo weight
alone (A) ignores axle limits. Partial weights (C, D) do not represent the full combination.
4. In a triple-trailer setup, the driver notices that the second trailer's brakes are not
releasing. Which component is the most likely culprit?
A. The relay valve on the second trailer.
B. The tractor protection valve.
C. The dolly's emergency brake valve.
D. The first trailer's spring brake control valve.
Answer: A. The relay valve on the second trailer.
Rationale: The relay valve on the second trailer controls the release of its brakes; a
malfunction can cause the brakes to stay engaged. The tractor protection valve (B) affects
the whole system, not just the second trailer. The dolly's emergency valve (C) affects the
dolly brakes. The first trailer's valve (D) is unrelated to the second trailer's brakes.
Page 2 of 55
,5. A driver is backing a set of doubles under a loading dock. Which of the following
techniques is most effective for maintaining control and avoiding jackknifing?
A. Back slowly and use the mirrors to monitor the rear trailer's movement.
B. Back quickly to minimize the time in the blind spot.
C. Use a spotter at all times, regardless of visibility.
D. Turn the steering wheel sharply to correct any misalignment.
Answer: A. Back slowly and use the mirrors to monitor the rear trailer's movement.
Rationale: Backing slowly while using mirrors allows the driver to monitor the rear trailer
and make gradual corrections, reducing jackknife risk. Backing quickly (B) increases
danger. A spotter (C) is helpful but not always available; technique is primary. Sharp
steering (D) can cause jackknifing.
6. When inspecting a double trailer combination, the driver must check the air lines
for proper connection. Which color air line is typically used for the emergency
brakes?
A. Red
B. Blue
C. Green
D. Black
Answer: A. Red
Rationale: In standard color coding, the red air line is for emergency brakes. Blue (B) is
for service brakes. Green (C) is often used for secondary or auxiliary lines. Black (D) is not
a standard emergency line color.
Page 3 of 55
, 7. A driver is driving a set of doubles on a curve. The rear trailer begins to swing
outward. What is the most appropriate immediate action?
A. Apply the brakes hard to stop the swing.
B. Accelerate slightly to pull the trailers straight.
C. Steer into the direction of the swing.
D. Release the accelerator and steer gently in the opposite direction.
Answer: D. Release the accelerator and steer gently in the opposite direction.
Rationale: If the rear trailer swings out, releasing the accelerator and steering gently
opposite can help straighten the combination. Hard braking (A) can worsen the swing.
Accelerating (B) may increase the swing. Steering into the swing (C) can cause jackknifing.
8. Which of the following is a key difference between the coupling procedures for a
single trailer versus a set of doubles?
A. Doubles require the use of a dolly converter gear.
B. Doubles require a fifth wheel on the rear trailer.
C. Doubles do not require air line connections.
D. Doubles require the tractor to be equipped with a pintle hook.
Answer: A. Doubles require the use of a dolly converter gear.
Rationale: Doubles require a dolly converter gear to connect the first trailer to the second.
A fifth wheel (B) is on the tractor, not the rear trailer. Air lines (C) are required for all
trailers. A pintle hook (D) is used for towing, not for doubles coupling.
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