Why do trucks use engine brakes and aren’t they illegal?

Engine brakes, also known as “jake brakes” or “engine retarders” are an essential part of the braking system on heavy trucks. Most heavy trucks use an air-brake system for primary braking power, but with heavy loads it is safer to have the engine slow down to help the air-brakes. Local municipalities and counties are allowed by the Illinois Vehicle Code to enact ordinances regarding excessive noise from engine brakes. Many newer trucks are equipped with mufflers to reduce the sound emitted. The Village of Palatine does not have an ordinance specific to engine brake noise.  more