CDL Vehicle Combination Practice Test

What does the trailer air supply control look like?

1. Red, circular knob

2. Blue, oval knob

3. Red, eight-sided knob

4. Yellow, square knob

The correct answer is: Red, eight-sided knob

The trailer air supply control is designed to be easily identifiable and functional, ensuring quick operation during critical situations. It is a red, eight-sided knob, which is a standardized design used in commercial vehicles. This distinct shape and color help prevent confusion with other controls, as the red coloring signifies an action related to air supply and trailer braking systems. The eight-sided design also provides a better grip, allowing drivers to operate the control effectively even in adverse conditions, such as low visibility or while wearing gloves. In addition to functionality, the shape and color are consistent with federal regulations that seek to improve safety through standardization across all commercial vehicles. The other options, while they do represent different kinds of knobs and controls found in vehicles, do not match the specific design and function of the trailer air supply control, which is crucial for the safe operation of combination vehicles.