What is the primary function of a Type D warning beacon?

A Type D warning beacon is used primarily to mark the outside edge of a curve. This function is crucial for improving visibility and alerting drivers to changes in the roadway alignment, especially in areas where curves might be sharp or difficult to see from a distance. By marking these curves, the beacon helps enhance driver awareness and reduces the risk of accidents that can result from drivers misjudging the road's trajectory.

The other choices present functions that are not aligned with the primary purpose of a Type D warning beacon. For instance, signaling traffic light changes, marking pedestrian crossings, and indicating speed limits have distinct safety devices and signage associated with them. These functions are handled by traffic lights, crosswalk signs, and speed limit signs, respectively, which are specifically designed for those particular purposes. Thus, the Type D warning beacon's role in marking the outside edge of a curve stands out as its primary function in ensuring roadway safety.