Final answer:
The low air pressure warning in a vehicle's service air tank is required by law to activate before the pressure drops below 60 PSI, ensuring safety and ample time for corrective measures.
Step-by-step explanation:
The driver must be able to see a low air pressure warning which comes on before pressure in the service air tank falls below 60 PSI. This is an essential safety feature in vehicles to prevent brake failure due to insufficient air pressure in the system. The correct federal regulations for commercial vehicles in the United States stipulate this requirement to ensure that drivers are warned well in advance to take corrective action and maintain safe operating conditions.