When driving on main roads, which vehicles must use the hard shoulders where possible?

In Germany, specific traffic regulations require certain vehicles to use the hard shoulders of main roads when possible to ensure smoother traffic flow and safety. In this case, slow-moving agricultural tractors and machinery, as well as mopeds, are mandated to use hard shoulders due to their lower speeds and potential to impede faster-moving traffic.

Small-engine motorcycles, however, are generally not required to use hard shoulders, as they are capable of maneuvering with the flow of traffic more effectively. Therefore, including all three options in your answer is incorrect because it does not align with the regulations that define which vehicles must use the hard shoulder. Proper understanding of these rules is crucial for both safety and compliance while driving on main roads.