Motor vehicles must be equipped with turn indicator lamps if:

Motor vehicles are required to be equipped with turn indicator lamps when the body or load extends 24 inches or more to the left of the vehicle. This regulation is in place to ensure safety on the road by providing adequate signals to other drivers regarding the intentions of the vehicle making a turn or lane change. A load that significantly extends beyond the left side can create hazards, and having turn indicators helps to improve visibility and communication to other motorists.

In contrast, the specific requirements related to the load exceeding 12 inches or the total vehicle length exceeding 20 feet do not necessarily trigger the same requirement for turn indicator lamps. The distinction is based specifically on how far the load extends laterally from the vehicle, as this poses a more direct risk when turning or changing lanes. Additionally, the option referring to lightweight models does not relate to a specific safety threshold regarding turn signals. Thus, the correct answer focuses on the critical measurement that ensures safe operation when a vehicle and its load exceed recognized limits.