What color is the service line in combination vehicles?

The service line in combination vehicles is blue. This color coding is significant as it helps drivers and maintenance personnel quickly identify the function of the air lines connected between the tractor and the trailer. The blue line specifically is responsible for delivering the air supply to operate the trailer's brakes, which is crucial for safe stopping and maintaining control. Recognizing this color allows for straightforward checks and maintenance before operation, contributing to overall safety on the road. Understanding the color-coding system, including the blue service line, is an essential part of CDL training, helping drivers ensure their vehicles are correctly configured and safe for operation.