The driver must be able to see a warning before pressure in the service air tanks falls below which PSI?

The correct answer is based on safety standards for air brake systems. A warning must activate for the driver when the pressure in the service air tanks drops below 55 PSI. This threshold is pivotal because maintaining adequate air pressure is essential for the effective functionality of air brakes. If the pressure falls below this level, it indicates a risk of brake failure or inadequate braking power, which can lead to dangerous driving conditions.

Understanding this pressure threshold helps ensure that drivers remain alert and can take corrective actions, such as safely stopping the vehicle or addressing the issue before it compromises safety. Drivers are trained to recognize these indicators as critical to the safety and operation of their air brake systems, thus enhancing overall road safety.