What is one of the purposes of using photocells in traffic systems?

Answer: B — Control illumination levels

The use of photocells in traffic systems primarily serves to control illumination levels. Photocells are sensors that detect light levels in their environment. In traffic systems, they are commonly used to automatically adjust street lighting based on the availability of natural light. For instance, as dusk settles and natural light diminishes, photocells signal streetlights to turn on, ensuring adequate visibility for drivers and pedestrians. Similarly, during daylight, the photocells can turn off these lights to conserve energy. This adaptive control of illumination enhances safety by ensuring that roadways are properly lit when needed, while also contributing to energy efficiency by reducing unnecessary lighting during daytime hours. The other choices, while related to traffic management, do not align with the specific functionality of photocells. Detecting vehicle speed, monitoring traffic flow, and providing real-time data typically rely on different types of sensors and technologies designed for those specific purposes.