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.

Double and triple combinations are the most vulnerable to turning over as a result of the "crack-the-whip" effect.

If you are experiencing a trailer jackknife, you can help your tires regain traction by releasing the brakes. Using a trailer hand brake while experiencing a skid will only cause the skid to continue.

Never use the trailer hand valve while operating your vehicle. Using the trailer hand valve while driving can cause your trailer to skid.

Driving a combination vehicle usually requires more skill than driving a single vehicle. Combination vehicles are generally longer and heavier than single commercial vehicles.

Due to the "crack-the-whip" effect caused by rearward amplification, changing lanes too quickly in a combination vehicle can result in a rollover.

Making a sudden lane change can be dangerous when driving a combination vehicle. Avoid the need to make a sudden lane change by looking down the road for upcoming obstacles that will require you to change lanes.

Landing gear, or trailer supports, should always be completely raised before a vehicle is driven. When uncoupling, landing gear should be lowered to make firm contact with the ground. If a trailer is loaded when it is being uncoupled, you should lower the landing gear and turn the crank a few additional times to remove some weight from the trailer.

If your trailer begins to skid, you should release the brakes to help regain traction. Once the wheels grip the road again, the trailer will begin to straighten out and follow the path of the tractor.

If you need to back your trailer but cannot back up in a straight path, you should back on a curved path toward the driver's side of the vehicle. This will permit you the best possible visibility while backing.

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.

Rollovers happen when a vehicle is driven too fast. To reduce the risk of rollover, make sure the weight of cargo is loaded low to the ground, and that you take turns and curves slowly.

The trailer service air line is also referred to as the control line or signal line.

Every combination vehicle has two air lines: the service line and the emergency line.

Trailers and converter dollies built on or after March 1, 1998 are required to have Anti-Lock Braking Systems (ABS). This will be indicated by the presence of a yellow malfunction lamp. You can determine if older trailers have ABS by checking under the trailers for ECU and wheel speed sensor wires coming from the back of the brakes.

If your trailer starts to skid while you are braking, you should release the brakes and allow them to begin to regain traction. Once its wheels have regained their grip on the road, the trailer will begin to straighten out and follow the tractor.

Rollovers happen when an operator turns too fast. Drivers should be sure to slow down before entering turns and curves, especially when transporting a fully-loaded rig.

While inspecting a coupling, you should go under the trailer and look into the back of the fifth wheel. Ensure that the fifth wheel jaws are secured around the shank of the kingpin.

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

In vehicles equipped with an automatic tractor protection valve, the valve will pop out if air pressure drops to a level between 20 and 45 psi. When the valve pops out, the protection valve will close, stopping the flow of air.