Final answer:
Turn signal indicators became mandatory on all motor vehicles manufactured after January 1, 1968, as regulated by the National Highway Traffic Safety Administration to promote road safety.
Step-by-step explanation:
The mandate for turn signal indicators on motor vehicles is regulated by law, specifically within the domain of vehicle manufacturing standards and road safety regulations. In the United States, turn signal indicators became mandatory for all vehicles manufactured after January 1, 1968, according to the National Highway Traffic Safety Administration (NHTSA). This standard was set forth to increase safety on the roads by ensuring that other drivers and pedestrians are clearly aware of a vehicle's intended direction of travel, thereby reducing the likelihood of accidents.
Turn signal indicators are critical for safe driving practices, and they are part of the standard equipment on modern vehicles. Additionally, checking and maintaining proper functioning of these indicators is essential for vehicle inspection and roadworthiness, contributing to road safety. Failure to use turn signals properly can lead to traffic violations and accidents, underlining the importance of their continued use and functionality.