How does total stopping distance for an air-braked vehicle compare to a hydraulic-braked passenger car at the same speed?

Air-braked vehicles have an additional component to stopping distance that hydraulic-braked vehicles do not: brake lag. When you press the brake pedal, air pressure must travel from the valve through the lines to each brake chamber. This takes about half a second — short, but at 55 mph the truck travels approximately 40 feet in that time before the brakes even start to apply. So total stopping distance for a heavy truck is: perception distance + reaction distance + brake lag distance + actual braking distance. The brake lag is one reason a fully loaded tractor-trailer requires a stopping distance of around 300 feet at 55 mph on dry pavement, compared to maybe 150 feet for a passenger car. Anticipating stops well in advance is therefore much more important in a commercial vehicle.