Reversing is strictly prohibited on **motorways and dual carriageways**. This is because these roads are designed for fast and smooth traffic, and any maneuver involving reversing could be extremely dangerous.

- In general, reversing is only allowed in very specific situations, such as when parking or when it is not possible to move forward or change direction, for example, when exiting a street blocked by construction. However, even in these cases, there are restrictions: you cannot reverse more than 15 meters or enter an intersection.

- On motorways and dual carriageways, these exceptions do not apply. The prohibition is absolute due to the high speed of vehicles and the structure of these roads, which are not designed for reversing maneuvers.

Therefore, the correct answer is that **it is always prohibited to reverse on motorways and dual carriageways**.