**Priority vehicles** are those that are equipped to perform **emergency services**, such as police cars, fire trucks, civil protection vehicles, and ambulances. When they are on an urgent service, they have the right of way and may not follow certain traffic rules, as long as they do not endanger others.

However, when these vehicles **are not on an urgent service**, they must follow the same traffic rules as other vehicles. This includes not driving on the **hard shoulder of the motorway**, as the hard shoulder is reserved for emergencies and not for the regular use of vehicles, even if they are priority vehicles. Therefore, the correct answer is that they **cannot** drive on the hard shoulder if they are not on an urgent service.