Why should a driver turn off the vehicle's retarder when the road is wet, icy, or snow-covered?

49 CFR 383.111(a)(5) (Shifting); AAMVA CDL Manual Section 2.3.4, p.2-10 — Section 2.3.4 (Retarders) falls within the manual's '2.3 – Shifting Gears' grouping, consistent with the Shifting citation used for the manual's other 2.3.x subsections

Drivers who leave a retarder engaged on ice or snow can unexpectedly lose control of the drive wheels, since the retarder applies braking force without the driver pressing the brake pedal. Switching it off in poor traction conditions is a practical safety habit tested directly on the knowledge exam.