Driving combination vehicles requires more skill than driving single commercial vehicles. Combinations are generally longer, generally heavier, and are more vulnerable to rollover.

The emergency air line (also known as the supply line) has two functions. The first is to supply air to the trailer air tanks, and the second is to control the emergency brakes on combination vehicles.

A tractor protection valve keeps air in a tractor or truck air brake system should the trailer break away or develop a bad leak. The valve will close automatically if the pressure drops to an unsafe level.

Driving combination vehicles requires more skill than driving single commercial vehicles. Combinations are generally longer, generally heavier, and are more vulnerable to rollover.

The earliest and best way to recognize the beginnings of a trailer skid is by seeing the trailer veer off-course in your mirrors.

To reduce the risk of a rollover when driving a combination vehicle, you should keep the weight of the cargo as low to the ground as possible and drive slowly around turns.

When driving a tractor-trailer combination equipped with an Anti-Lock Braking System (ABS), you should brake in the same manner that you would if operating a vehicle without ABS.

Before a trip, be sure to inspect all couplings. There should be no space between the upper and lower fifth wheel.

Landing gear, or trailer supports, should always be completely raised before a vehicle is driven. Landing gear that is not entirely raised could catch on railroad tracks or other hazards on the roadway.

The trailer hand valve (also referred to as the "trolley valve" or "Johnson bar") works the trailer brakes and should be used only when testing them. Using the trailer hand valve while driving could result in a skid.

Shut-off valves are used in air supply lines to control the passage of air from one trailer to another. The rear shut-off valve should always be closed to prevent air from leaving the braking system.

To lower the risk of a rollover, load your cargo as closely to the ground as possible. To further reduce this risk, take curves and corners slowly.

Combination vehicles require more space on the road than other vehicles. When entering or crossing traffic while driving a combination vehicle, it is especially important that there is a large enough gap in traffic for you to safely do so.

Combination vehicles are usually heavier and longer than single combination vehicles. Operating a combination vehicle requires a higher level of driving skill than operating a single commercial vehicle.

Combination vehicles are usually heavier and longer than single commercial vehicles. For these reasons, they usually require more driving skill than single commercial vehicles.

The trailer hand valve, also called the trolley valve or Johnson bar, is used to work the trailer brakes. It should be used only to test the brakes. Using it while a vehicle is in motion could cause a skid.

The trailer hand valve works the trailer brakes but should only be used when testing them. Using it while driving could create a skid. Never use the trailer hand valve while parking because doing so may release the air pressure from the braking system, releasing the brakes that are holding the vehicle in place.

A rollover happens as a result of a driver taking a turn at too high a speed. Be sure to take turns and curves safely and slowly.

If your trailer begins to drift while you are backing, you should correct the drift immediately by turning the top of the steering wheel in the direction of the drift.

Incorrect coupling and uncoupling of combination vehicles can have extremely dangerous consequences.