What does "brake lag" mean in the context of an air brake system's total stopping distance?

Explanation

Brake lag is the time it takes compressed air to travel through the lines to the brakes after the driver pushes the pedal, roughly half a second or more, and it adds distance to an air-braked vehicle's total stopping distance that a hydraulic brake system, which acts instantly, does not have. 49 CFR 383.111(b)(7) requires CDL applicants to know general operating practices and procedures for air brakes, the knowledge area this stopping-distance concept falls under; the specific perception-plus-reaction-plus-brake-lag-plus-braking-distance formula and the roughly 32-foot figure at 55 mph are standard driver-training content from the manual, not numbers stated in the CFR paragraph itself. Confusing brake lag with the driver's perception and reaction time is wrong because those are separate, earlier stages in the same total stopping distance formula. Calling it a defect found only in out-of-adjustment brakes is wrong because brake lag is a normal, unavoidable characteristic of every air brake system, not a sign of a problem. Attributing it to ABS is wrong because brake lag is caused by air traveling through the lines, a mechanical property of air brakes generally, not a function of the antilock system.

Source: 49 CFR 383.111(b)(7)