On a three-lane highway, one lane is designated for:

The correct answer highlights that one lane on a three-lane highway is designated for vehicles making left turns or passing. This specific designation allows for better traffic flow and safety by providing a dedicated lane for these maneuvers. When vehicles need to turn left or pass slower-moving traffic, having a separate lane helps minimize conflicts with other vehicles and reduces the likelihood of accidents caused by sudden lane changes or stops.

The distinction is important in traffic management, as it ensures that vehicles can execute these maneuvers without blocking the flow of traffic in the other lanes. By separating these actions from general traffic, it creates a more organized and predictable environment for drivers.

The other options relate to specific vehicle types or uses but do not address the general traffic flow management as effectively as the designated lane for turning and passing. Emergency vehicles, school buses, and heavy trucks have their own designated lanes or considerations, but they do not impact the standard operational flow of general traffic on a highway in the same way that a lane designated for turning or passing does.