Brake check

CB warning that traffic ahead is suddenly slowing or stopping.

A 'brake check' on the CB warns everyone behind that traffic is bunching up and you're about to be on the brakes — critical info for a heavy load that needs a long distance to stop. (Used as a verb, 'brake-checking' someone means tapping your brakes to intimidate the driver behind you — reckless, and especially dangerous around oversize loads.)