At what point must the low air pressure warning signal (light and/or buzzer) activate on a vehicle with air brakes?

AAMVA CDL Manual, Section 5, subsection 5.1.11 (p. 5-3), 'Low Air Pressure Warning'; 49 CFR 383.111(b)(4), 'implications of low air pressure readings'

Confusing the warning threshold with the compressor's cut-in/cut-out pressures is a common exam trap because all of these numbers appear in the same section. Knowing the 55 psi warning threshold specifically tells you how much safety margin you have before spring brakes start to apply and you lose control of when braking happens.