Method of detecting use of a lap belt only routing of a combined lap belt/shoulder belt seat belt system in a vehicle

A method of detecting improper routing of a seat belt having a lap belt portion and a shoulder belt portion by an occupant of a vehicle, the method comprising:

• detecting whether the seat belt has been buckled;
• when the seat belt is detected as being buckled, detecting a first length of the seat belt unspooled from a retractor during a time duration, and determining a minimum seat belt length based on the first length;
• determining an over-the-head length threshold based on the minimum seat belt length and stored data indicating a first predetermined relationship between minimum seat belt length and over-the-head length threshold;
• determining a behind-the-back threshold based on the minimum seat belt length and stored data indicating a second predetermined relationship between minimum seat belt length and behind-the-back threshold;
• detecting a second length of the seat belt unspooled;
• determining whether the seat belt has been placed over the head of the occupant based on the second length and the over-the-head threshold;
• when determined that the seat belt has been placed over the head of the occupant, detecting a movement of the seat belt from the second length to a third length;
• determining whether the shoulder belt portion was placed behind a back of the occupant based on the third length and the behind-the-back threshold;
• and
• providing a warning indicting improper routing of the shoulder belt portion of the seat belt when determined that the shoulder belt portion was placed behind the back of the occupant.