Passengers per hour per direction

Passengers per hour per direction or passengers per hour in peak direction (pphpd or p/h/d) is a measure of the capacity of a rapid transit or public transport system.

Directional flow

Three parallel escalators; the direction of the middle escalator can be changed to double capacity in one direction (↑↑↓ or ↑↓↓).

Many public transport systems handle a high directional flow of passengers— often traveling to work in a city in the morning rush hour and away from the said city in the late afternoon. To increase the passenger throughput, many systems can be reconfigured to change the direction of the optimized flow. A common example is a railway or metro station with more than two parallel escalators, where the majority of the escalators can be set to move in one direction. This gives rise to the measure of the peak-flow rather than a simple average of half of the total capacity.