Vehicles that travel on the hard shoulder must do so in a single file, one behind the other. However, there is an exception for **bicycles**. Cyclists can ride in parallel, that is, two bicycles side by side in the same line. This exception does not apply to other vehicles that must also use the hard shoulder, such as mopeds or slow-moving vehicles.

It is important to note that, although bicycles can ride in parallel, they must do so in single file in sections with no visibility or when there is heavy traffic. This flexibility for bicycles is due to their smaller size and speed, which allows them to take up less space and not interfere as much with the flow of traffic.

Therefore, bicycles are the only category of vehicles that can ride in parallel on the hard shoulder, which makes the correct answer option 1: **Bicycles**.