"Passenger miles" is a measurement used in transportation to quantify the distance traveled by passengers. It is calculated by multiplying the number of passengers by the distance they travel. For example, if 100 passengers travel 10 miles, that results in 1,000 passenger miles. This metric is important for understanding the efficiency and usage of various transportation modes, such as airplanes, trains, and buses. It helps transportation agencies and companies assess demand, plan services, and evaluate environmental impacts, making it a key factor in transportation planning and policy.