The driver must be able to see a low air pressure warning which comes on before pressure in the service air tanks falls below ____ psi.

A low air pressure warning signal is required on vehicles with air brakes. A warning signal you can see must come on when the air pressure in the tanks falls between 55 and 75 psi (or 1/2 the compressor governor cutout pressure on older vehicles). The warning is usually a red light. A buzzer may also come on. [Commercial Driver Handbook]