Which statement about the effectiveness of an engine brake retarder is true?

The effectiveness of an engine brake retarder is indeed limited on wet or icy roads. On these types of surfaces, the retarder can cause the wheels to lose traction, significantly reducing the ability to maintain control of the vehicle. Generally, engine brake retarders rely on the engine to create a braking force by reducing engine speed, which can be helpful in slowing down the vehicle, especially on downhill grades. However, on slippery surfaces such as wet or icy roads, this can lead to skidding, making it less effective and potentially dangerous.

While the statement regarding the retarder being equally effective on all types of road conditions is incorrect, as the performance varies based on road conditions, it's crucial to highlight that a retarder works best on dry surfaces where maximum traction can be maintained. The notion that it works better on dry roads than on snowy surfaces is also valid, as snow reduces the grip on the tires, also impacting the overall effectiveness of the engine brake. The claim that it is only useful on highways is misleading; while highway driving can benefit from engine braking, it is also useful in other settings, such as on hilly roads.