Final answer:
The driver must be alerted by a warning when service tank air pressure falls below 60 PSI, to ensure safety and proper vehicle functionality.
Step-by-step explanation:
The driver must be able to see a warning before air pressure in the service tanks fall below 60 PSI. This regulation is designed to ensure safety on the road by alerting the driver of potentially dangerous drops in air pressure which could impair braking performance and overall vehicle control. The air pressure warning is a critical feature because maintaining the proper air pressure is essential for safe vehicle operation; it impacts not just the braking system, but also affects the smoothness of the ride, fuel efficiency, and the rate at which tires wear.