In a combination vehicle with air brakes, the tractor protection valve keeps air in the tractor or truck brake system in the event of the trailer breaking away or developing a leak.

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.

When coupling, make sure you couple matching glad hands. They are often color-coded to help drivers avoid mistakes. Typically, blue is used for service lines and red is used for emergency lines.

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 service air line carries air and is controlled by either the foot brake or trailer hand brake. The service air line is attached to relay valves, which allow the trailer brakes to be applied quickly.

Bobtail tractors are tractors that are not attached to any semitrailers. When operating a bobtail, you should be aware that stopping can be difficult and that it will take a longer distance to bring the bobtail to a complete stop than it would a tractor attached to a loaded semitrailer.

Large combination vehicles take longer to stop when they are empty than when they are loaded. The decreased weight causes a vehicle's wheels to have decreased traction on the surface of the road.

When positioning trailers in a combination, the most heavily-loaded trailer should be the closest to the tractor. The lightest trailer should be positioned in the rear.

Correctly following the steps to couple and uncouple trailers is vital to safely operating a combination vehicle. Before backing your tractor under the trailer while coupling, make sure your trailer brakes are locked.

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

Having an Anti-Lock Braking System (ABS) on only the trailer, or even on just one axle, will still provide a driver with additional control over the trailer when braking.

Be sure to test the trailer emergency brakes before beginning a trip. After ensuring that the trailer rolls freely, you can test the emergency brakes by pulling out the trailer air supply control, or placing it in the "emergency" position. Pull forward slightly with the tractor and make sure the trailer does not move.

When backing with a trailer, turn the top of the steering wheel in the direction opposite of where you want to go. Once the trailer begins turning, you should then switch the direction of the steering wheel to follow the trailer.

Before a trip, you should ensure that air reaches all air brakes on all trailers by opening up the rear emergency line and service line shut-off valves to listen for escaping air. Close both shut-off valves before beginning to drive.

A rollover is more likely if cargo is unevenly loaded in a rig. To help prevent a dangerous rollover, it is important to keep a load as centered on the rig as possible.

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.

An empty trailer will require a longer stopping distance than a loaded trailer. Additionally, a trailer is most likely to swing out and strike other vehicles when it is lightly loaded or empty.

Knowing proper coupling and uncoupling technique is fundamental to the safe operation of combination vehicles. Improper coupling can be very dangerous.

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.