Final answer:
The question about when turn signal indicators were required is not addressed by the provided information, which focuses on the Clean Air Act of 1970 and emission standards. This act led to the use of catalytic converters and lead-free gasoline but does not detail the regulations surrounding turn signals.
Step-by-step explanation:
While the question relates to turn signal indicators, it does not provide a specific year when they were made mandatory, and as a tutor, I must be careful not to provide incorrect information. The historical context provided in the question relates to a different aspect of vehicle regulations - the Clean Air Act of 1970 signed by President Nixon, which had significant effects on automobile manufacturing. This act required a 90 percent reduction in vehicle emissions and the introduction of catalytic converters on new cars. These environmental regulations led to the use of lead-free gasoline and slightly increased the cost of new automobiles, affecting consumer habits and market trends.
The impact on safety features like turn signal indicators is not directly related to the Clean Air Act and thus, information specific to this question is not provided in the reference material. Nevertheless, it is important to recognize that turn signal indicators are critical for road safety, as they help in preventing accidents by signalling the driver's intentions to others on the road.