Controller and control method for rider assistance system

A control method for a rider assistance system (1) that is configured to assist a rider of a lean vehicle, the control method comprising:

• acquiring (S101), using a controller (20), a positional relationship information between a subject vehicle (100) and a target while the subject vehicle (100) is traveling, the target located around the subject vehicle (100), the controller (20) configured to acquire the positional relationship information based on a surrounding environment information relative to environment around the subject vehicle (100);
• executing (S102), using the controller (20), an assistance operation, in which the rider is assisted in driving the subject vehicle (100), based on the positional relationship information acquired by the controller (20), and
• in executing (S102) the assistance operation, change the assistance operation between a case where the subject vehicle (100) is in a group ride in which the subject vehicle (100) is traveling in a group of a plurality of lean vehicles including the subject vehicle (100) and a case where the subject vehicle (100) is not in the group ride,
• executing the assistance operation at a first degree when the subject vehicle (100) is in the group ride,
• executing the assistance operation at a second degree, which is higher than the first degree, when the subject vehicle (100) is not in the group ride, wherein the assistance operation includes a notification operation in which a warning is generated to the rider, and
• lowering the degree of perception of the rider to lower the first degree of the assistance operation performed when the subject vehicle (100) is in the group ride as compared to the second degree of the assistance operation performed when the subject vehicle (100) is not in the group ride.