Which combination best reduces rollover risk in a combination vehicle?
Correct answer: Keep cargo low and slow down for turns
A low, centered load and reduced turn speed keep the center of gravity controlled.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.1 Rollover Risks, printed 6-1 (physical PDF page 107) ↗
What can the crack-the-whip effect do during a sudden lane change?
Correct answer: Cause the rear trailer to overturn
Rearward amplification makes steering movement more severe toward the back of a multi-unit combination.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.2 Steer Gently, printed 6-1 (physical PDF page 107) ↗
Why can an empty combination vehicle be harder to stop smoothly than a fully loaded one?
Correct answer: Light loading can cause wheel lock and poor traction
Light loads can provide poor traction, making wheel lock, trailer swing, and jackknife more likely.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.3 Brake Early, printed 6-1 (physical PDF page 107) ↗
What should a driver do first if a low-clearance combination becomes stuck on railroad tracks?
Correct answer: Leave the vehicle and move away from the tracks
Get everyone out of the vehicle and away from the tracks. From safety, call for emergency help and provide the crossing location, including its posted DOT identification when available.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.4 Railroad-Highway Crossings, printed 6-2 (physical PDF page 108) ↗
What is the earliest reliable sign that a trailer has started to skid?
Correct answer: Trailer misalignment appears in the mirrors
Frequent mirror checks during hard braking reveal trailer swing before a jackknife develops.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.5 Prevent Trailer Skids — Recognize the Skid, printed 6-3 (physical PDF page 109) ↗
A trailer without functioning ABS has locked wheels and starts swinging during braking. What action addresses the skid?
Correct answer: Release the brakes to restore traction
Release the service brakes so the locked trailer wheels can regain grip. Do not apply the trailer hand valve to try to straighten the rig; added trailer braking can worsen this skid. Follow the vehicle's ABS instructions when equipped.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.5 Prevent Trailer Skids — Stop Using the Brake, printed 6-3 (physical PDF page 109) ↗
What does off-tracking mean in a turn?
Correct answer: Rear wheels cut inside the front-wheel path
Long vehicles’ rear wheels cut inside the path of their front wheels, with the last trailer cutting in most.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.6 Turn Wide, printed 6-3 (physical PDF page 109) ↗
If straight-line trailer backing is not possible and a curved path is necessary, which side generally gives the driver a better view?
Correct answer: The driver’s side
Driver-side backing provides the clearest view; straight-line backing is preferable whenever possible.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.1.7 Backing with a Trailer, printed 6-4 (physical PDF page 110) ↗
What is the proper routine use of a trailer hand valve?
Correct answer: Testing the trailer brakes
The hand valve is for testing trailer brakes; normal braking should use the foot pedal.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.1 Trailer Hand Valve, printed 6-5 (physical PDF page 111) ↗
What is the tractor protection valve designed to do?
Correct answer: Preserve tractor air after a major trailer leak
The valve closes to protect the tractor air supply when trailer-side pressure is lost.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.2 Tractor Protection Valve, printed 6-5 (physical PDF page 111) ↗
On a newer tractor, what does pulling out the red eight-sided trailer-air-supply knob do?
Correct answer: Shuts off trailer air and applies trailer emergency brakes
Pulling the red knob closes the supply; pushing it in supplies air to the trailer.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.3 Trailer Air Supply Control, printed 6-5 (physical PDF page 111) ↗
What are the two air lines connecting the tractor and trailer in an air-braked combination?
Correct answer: Service and emergency
The service line carries brake-control pressure, while the emergency line supplies tanks and controls emergency braking.
Source: Pennsylvania Commercial Driver's License Manual PUB 223 (6-26), section 6.2.4 Trailer Air Lines, printed 6-5 (physical PDF page 111) ↗
