Picking up hitchhikers on a **dual carriageway** is not allowed. This is because dual carriageways are high-speed roads designed for smooth and safe traffic flow, and stopping to pick up passengers could be dangerous.

- **Stopping the vehicle on a dual carriageway** is only permitted in emergency situations, such as an accident or a breakdown. In these cases, you must exit the road at the first available exit or stop on the right shoulder if the vehicle cannot continue driving.
- **Picking up passengers** is not considered an emergency, so stopping for this purpose is not allowed on dual carriageways.
- Additionally, safety regulations indicate that passengers should get in and out of the vehicle only when it is stopped and from the side closest to the pavement or shoulder, which is neither safe nor practical on a dual carriageway.

For these reasons, the correct answer is that **you cannot pick up hitchhikers on a dual carriageway**.