What must drivers do when entering a roundabout or traffic circle?

When entering a roundabout or traffic circle, drivers are required to yield to vehicles that are already in the roundabout. This is crucial for maintaining a smooth flow of traffic and preventing accidents. By yielding, drivers give priority to those who are already navigating the circle, which helps to reduce confusion and potential collisions.

In contrast, the other options do not align with the proper protocol for navigating roundabouts. For example, accelerating to match the speed of vehicles in the roundabout could lead to dangerous situations, as it may not allow adequate time to assess and merge safely. Stopping and waiting for a signal is not applicable in roundabouts, where traffic is usually unregulated and flows continuously. Turning on headlights may be a necessary safety practice in some conditions but does not relate directly to the specific actions required when entering a roundabout. Yielding is fundamental for ensuring safety and efficiency in these traffic patterns.