How is the capacity of a highway typically measured?

Answer: B — In vehicles per hour.

The capacity of a highway is primarily measured in vehicles per hour. This measurement reflects the maximum number of vehicles that can effectively pass a given point on the highway within a specified timeframe, typically during peak travel periods. This metric is crucial for traffic engineering and planning, as it helps determine how well a highway can accommodate current and future traffic demands. In contrast, average speed per hour provides insight into how quickly vehicles are traveling but does not directly indicate the volume of traffic that can be handled. Tons of weight per day is more relevant for understanding freight capacity rather than passenger vehicle throughput. Lastly, passenger miles per year is a measure of travel intensity but does not specifically reflect the highway's ability to handle the flow of vehicles at a given moment. Thus, measuring capacity in vehicles per hour gives a more direct and useful insight into highway performance and is essential for traffic management and infrastructure planning.