What indicates that a driver is about to turn left?

The proper indication that a driver is preparing to turn left is when the left turn signal is flashing. Turn signals are specifically designed to inform other road users of a driver's intended maneuvers. By flashing the left turn signal, the driver communicates to pedestrians and vehicles that they plan to make a left turn, allowing others to anticipate and react accordingly. This enhances safety by reducing the chance of misunderstandings on the road.

In this scenario, the other options do not provide an accurate indication of a left turn. For example, a right turn signal flashing would suggest that the driver intends to turn right instead. Similarly, while illuminated brake lights signal that a driver is slowing down or stopping, they do not convey the intention to turn. Lastly, headlights being on simply indicate that it is either dark or that the driver requires increased visibility; this does not provide information about the direction the driver is planning to take. Therefore, the left turn signal is the clear indicator of a left turn.